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Factorial experiments for iron removal from kaolin by using single and two-step leaching with sulfuric acid

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Abstract

In the present study, to investigate the effects of acidic leaching on the iron removal from kaolin clay supplied by Cam-Ser Madencilik A.S., the 24 full factorial design of experiments and analysis of variance (ANOVA) were performed. The aim of the initial phase of research was to determine the effects of the main and interaction factors which can potentially influence the iron removal process. The factors were; pulp density, sulfuric acid concentration, temperature, leaching duration and reducing agent (oxalic acid, citric acid and hydrogen peroxide) concentration. The highest iron extraction yield (IEY) (26.3%) was obtained after 120 min of treatment at 90 °C, 20% pulp density, 3 M H2SO4 and with 0.50 M H2O2 as a reducing agent. For the purpose of increasing the IEY, in the second phase of research, initially the particle size of the kaolin sample was decreased to − 106 μm providing the liberation of iron oxides. Then, two-step leaching experiments were carried out with the optimum leaching conditions of full factorial design tests. The maximum IEY was 37.0% after two-step leaching with the optimum conditions of H2SO4 and H2SO4 with oxalic acid leaching. Iron oxide content of kaolin clay was decreased from 2.40% to 1.51%.

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... Thus, pure kaolinite (Al 2 O 3 .2SiO 2 .2H 2 O) with chemical composition 46.54% SiO 2 , 39.50% Al 2 O 3 and 13.96% H 2 O is an important industrial mineral with wide span of applications [3,4]. Millions of tons of this ore are widely used annually for a large variety of applications such as paper filling and coating (45%), refractory and plastics (58%), paint (3%) and others (4%) [5,6]. Of the aforementioned uses, paper has the largest market value for high-grade kaolin at a price equivalent to $200-$1200 [5,6]. ...
... Millions of tons of this ore are widely used annually for a large variety of applications such as paper filling and coating (45%), refractory and plastics (58%), paint (3%) and others (4%) [5,6]. Of the aforementioned uses, paper has the largest market value for high-grade kaolin at a price equivalent to $200-$1200 [5,6]. The whiteness and brightness properties are one of the most important factors considered in determining the beneficiation and economic value of the kaolinite ore especially for its use in pure alumina production. ...
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Increasing demands for aluminium and aluminium oxide with diverse industrial applications have prompted the development of a low-cost and eco-friendly technique as a substitute for conventional ore treatments by reduction-roasting route, requiring high energy consumption. For example, the demand for high-grade industrial alumina (Al 2 O 3 ) as valuable materials in refractories, pigments, adsorbents, catalysis, water purification, aluminium production and metallurgical applications cannot be over emphasized. Thus, the upgrading of a Nigerian biotite-rich kaolinite ore containing admixture of kaolinite (Al 2.00 Si 2.00 O 9.00 : 96-900-9231), biotite (Mg 6.55 Fe 3.46 Al 5.29 Ti 1.34 Si 11.36 K 4.00 O 48.00 : 96-900-0844) and quartz (Si 3.00 O 6.00 : 96-900-9667) impurities by hydrometallurgical route was investigated in phosphoric acid media. During leaching, parameters such as leachant concentration, particle size and reaction temperature on the extent of ore dissolution were accordingly examined. At optimal leaching conditions (2.0 mol/L H 3 PO 4 , 75 °C), 87.0% of the initial 10 g/L ore reacted within 120 min. The activation energy of 16.6 kJ/mol supported the diffusion control reaction mechanism. The unreacted product (∼13.0%) analyzed by XRD was found to contain siliceous impurities including iron silicate and manganese silicate which could serve as valuable by-products for some defined industries. The leach liquor at optimal leaching conditions was accordingly treated to obtain pure aluminium solution which was further beneficiated to obtain high-grade alumina (α-Al 2 O 3 ) of industrial value with 96.3% purity.
... The effectiveness of several organic acids (sulfuric acid with oxalic acid or critic acid) to dissolve iron from iron compounds was evaluated. Sulfuric acid with oxalic acid was found the most promising because of its acid strength, good complexing characteristics and high reducing power, compared to other organic acids [6]. Leaching of iron oxides with oxalic acid takes place via the following equations [7,8]. ...
... A 3 factor 3 level Box-Behnken design (BBD) for (Response Surface Method) RSM was used to study the correlation between the combined effect of individual parameters and two responses over three levels. The experimental design matrix and the responses based on experimental runs proposed by BBD are given in Table ( 6). No transformation was chosen while analyzing both the responses. ...
... The plasticity of the pastes is defined as the capacity of anelastic deformation without cracking [1][2][3], which is the result of a complex mechanical behavior of highly concentrated dispersions of particles in an aqueous medium that has dissolved and complexed ions in solution. Plasticity is a property of clays, which is determined mainly by the size of the particles in the form of plates, therefore, the smaller the planar particles of the clay, the more plasticity it can achieve [1,4,5]. This is associated with a physicochemical behavior of the surface of clays, so that the superficial electric potential also affects plasticity in principle [1,[6][7][8]. ...
... The results of the X-ray diffraction (see Fig. 2) show in the ore 69% of Kaolinite Al 2 Si 2 O 5 -(OH) 4 and 31% of quartz (SiO 2 ). The sample does not report the contaminants of the kaolin that generate the change of the coloration as the Hematite and the Goethite, this is due to the low concentration that it has in the kaolin, that is why an atomic absorption 'spectrophotometry was performed, determining a percentage initial iron of 0.19% by weight. ...
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The mechanical behavior of mineral pastes is controlled by minerals of clay origin since their laminar type morphology allows a special configuration of the particles, which allows them to dissipate mechanical forces applied by means of high plastic deformations. In this research, a clay used in the ceramics industry was evaluated and two granulometries called MG 270 (53um) and MG 325 (43um)were studied. Simple compression testswere performed to evaluate the mechanical performance of each of these materials. The simple compression data allowed to determine the radial flow of the particles within the control volume (specimens evaluated), the random organization of the clayeymaterial demonstrates a rapid energy dissipation, translated into rapid advances fromelastic state to plastic states of deformation. Later, the finite element method (FEM) was used to simulate the possible states of stress and deformation that are produced in ceramic pastes constituted by this type of clay. This allowed to obtain a graphic idea of the distribution of stresses and deformations, areas of stress concentration are evidenced, where it is possible that the start of microcracks occurs, which are generally the cause of the generalized fracture and sudden faults of the ceramic paste.
... Several studies have evaluated the kaolin processing by different methods (Guo et al., 2010;Tuncuk et al., 2013;Zegeye et al., 2013) especially with oxalic acid and oxalic acid-producing strains of A. niger, to reduce its iron impurities and increase its quality (De Mesquita et al., 1996;Raghavan et al., 1997;Taxiarchou et al., 1997;Ambikadevi and Lalithambika, 2000;Cameselle et al., 2003;Mulligan et al., 2004;Calderon et al., 2005;González and Ruiz, 2006;Lee et al., 2007;Du et al., 2011;He et al., 2011;Martínez-Luévanos et al., 2011;Xia et al., 2012). In the previous studies (Hosseini et al., 2007;Aghaie et al., 2009), firstly, bioleaching of iron from a highly contaminated kaolin sample using two different strains of A. niger was carried out on laboratory scale and the effects of strain type, pulp density and time of clay addition on the iron removal were investigated by employing a 2 3 full factorial design. ...
... The most widely applied physical beneficiation techniques are magnetic [4], flotation [5-7], gravity separation [7], and selective flocculation [8]. Chemical beneficiation methods involve the removal of mineral impurities from kaolin clay by organic and inorganic acids such as oxalic acid [9-15], citric acid [16, 17], thiourea dioxide [18], sodium dithionite [19], sulfuric acid [ [20][21][22], and hydrochloric acid [13]. The effect of different organic acids and their concentration in iron removal from kaolin ore has been reported by Saikia et al [17]. ...
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Kaolin (china clay) is a rock material that is very rich in kaolinite. A kaolin ore from Debre Tabor, Ethiopia containing 59.2 wt% SiO 2 , 24.9 wt% Al 2 O 3 , 2.4 wt% Fe 2 O 3, and 8.22 wt% loss on ignition (LOI) was physically beneficiated, chemically leached, and thermally treated for possible industrial use, especially for ceramic membrane fabrication. The leaching experiments were carried out using oxalic acid solutions as leaching reagents for the iron extraction process. The effect of acid concentration, reaction temperature, and contact time on iron leaching was investigated. It was determined that the rate of iron extraction increased with the oxalic acid concentration, leaching temperature, and contact time. A substantial reduction of iron oxide (2.4 to 0.36 wt%) from the raw kaolin was observed at operating conditions of 2.0 M oxalic acid, the temperature of 120 °C, and contact time of 120 min. A maximum kaolin whiteness index of 81.4% was achieved through this leaching process. Finally, the physically beneficiated, chemically leached, and thermally treated kaolin raw material was used to fabricate a low - cost kaolin - based ceramic membrane. After firing at 1100 °C the ceramic membrane was found to have a mass loss of 11.04 ± 0.05%, water absorption of 8.9 ± 0.4%, linear shrinkage of 14.5 ± 0.05%. It was demonstrated to be chemically stable, having less than 3% mass loss in acid solution, and less than 1% mass loss in alkali solution. The newly developed membranes have thus properties comparable to commercial ceramic membranes.
... The main chemical methods are treatment in gaseous media [3] or in a solution of mineral [4] and organic [5] acids or a two-step treatment with the addition of reducing agents [6]. ...
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A two-step technological scheme for removing iron from porcelain stone is presented: a combination of biological and chemical reduction of iron in stagnant and washing water regimes. The whiteness of porcelain stone increases after aging as well as after subsequent iron-removal operations with differing contributions: washing with ammonia oxalate > magnetic separation > washing with water. Large amounts of clayey materials (CM) are used in the ceramic industry. The commercial cost of this raw material largely depends on its whiteness. However, because of the content of mineral coloring impurities, primarily iron minerals , much of the CM is not white and is not used in the production of porcelain articles. The aim of the present work is to determine the possibility of used and optimizing a technological scheme for biological enrichment of CM and studying their coloristic properties before and after heat-treatment. The problem of increasing the effectiveness of iron removal from clayey materials involves the solution of a number of problems: the determination of the coloring power of iron compounds differing by state and structural organization in the initial CM before and after their partial removal, including the effect of heat-treatment on the color characteristics of CM before and after enrichment, and the identification of iron compounds in CM, which makes it possible to pick a method for removing them and evaluate its effectiveness. Coloring of Clayey Materials. The color of CM and the effectiveness of iron removal by one means or another depend not only on the total content, physical-chemical properties and the state and structural organization of the iron compounds but also on the transformation of the iron compounds at different steps of enrichment. The variations of the state of iron in CM are due to valence, coordination, character of the arrangement of the ligands, particle size and so on. The iron compounds present in CM are usually found at four levels of the structural organization: magnetically dilute (in silicates — 'silicate' iron, in organic compounds and paramagnetic minerals), molecular-cluster, superparamagnetic and crystalline magnetically ordered. The theory of the color of minerals is quite well developed. However, it is not always clear which iron compounds in any particular case determine the color of CM. As total iron in CM increases, it is the iron content mainly in second and third levels of the structural organization that increases. This is completely understandable since clayey minerals have a limited capacity to incorporate chromophoric ions in their structure (for example, kaolinite to 0.5 – 0.6 wt.%). For this reason the color of CM depends mostly on the iron compounds in the second and third levels of the structural organization and less on the coarse-crystalline, magnetically ordered particles (fourth level). But, at the same time, the amorphous and weakly crystallized iron (hydr)oxides are strongly bound with clayey particles and their magnetic susceptibility is low. In this connection the effectiveness of their removal by conventional methods is, as a rule, very low. Choice of Technological Scheme for Removing Iron from CM. There are three methods for removing iron: physical , chemical and biochemical, the latter being new [1, 2]. The main chemical methods are treatment in gaseous media [3] or in a solution of mineral [4] and organic [5] acids or a two-step treatment with the addition of reducing agents [6]. Some components used for treating CM, for example, treatment in a chlorine gas stream [3], have an adverse effect on the environment.
... Magnetic separation, froth flotation, selective flocculation, and chemical and biological leaching are some of the commonly practiced techniques for the removal of ferric iron oxides or hydroxides in the silica raw materials. However, physical separation techniques, such as flotation or magnetic separation, are generally less effective for iron removal than chemical leaching, aiming at dissolving iron compounds bonded to the surface or existing as distinct mineral grains [8]. Some of the previous work and results in these field are presented in Table I. ...
... Especially iron oxides are the main contributors of pigmentation. The main techniques used today are: leaching [2][3][4][5], high-intensity magnetic separation [6,7], flotation and flocculation [1,8]. The application of these techniques is expensive and in Mexico is not yet systematically applied to benefit kaolin clay [9]. ...
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n the present research work was carried out the study of the electrochemical behavior of iron species in solutions prepared from kaolin and different industrial dispersants. The iron species present different behavior in relation with pH, because tha t one solution pH was adjusted in order to study electro activity of the species in acid pH. To carry out this study was used a diagram type Pourbaix in order to establish the thermodynamic potentials of oxidation and reduction. These results were compared with kinetic studies by means of cyclic voltammetry. These studies revealed that the species of iron are more electro active in the solutions prepared with sodium hexameta phosphate and sodium polyacrylate and that setting to an acid pH, the electro activ ity of iron species increases
... But, this process suffers from the existence of substantial minerals impurities in quartz sand such as Al 2 O 3 , K 2 O, Na 2 O, Li 2 O, Fe 2 O 3 and FeO. [2][3][4] These impurities are chemically bonded to the surface and located in the interstitial positions of the quartz lattice itself. Therefore, purification steps should be carried out before the use of quartz in order to increase the chances of obtaining silica of higher purity and surface area suitable for nanotechnology and heterogeneous catalysis fields. ...
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Tunisian industrial phosphoric acid H 3 PO 4 was supported on silica gel SiO 2 (SIPA) to catalyze the hydrolysis reaction of aqueous alkaline sodium borohydride (NaBH 4). The SiO 2 was produced from purified quartz sand using alkali fusion-acidification chemical process. The BET surface area results indicate that the prepared silica gel could reach a specific surface area up to 585 m 2 /g. The addition of PO 3 H 2 functional groups resulted in an increase of surface acidity of SiO 2 catalyst as shown by FT-IR and DTA-DTG spectra. The total acidity of SIPA catalyst was determined by titra-tion to be 2.8 mmol H þ /g. SEM/EDS maps reveal the distribution of heavy metals on the silica surface. The effect of supported PO 3 H 2 functional groups and heavy metals on the NaBH 4 hydrolysis reaction was studied for different ratios of SIPA catalyst to NaBH 4. The sample 12SIPA/NaBH 4 leads to a very high hydrogen generation rate (up to 90%). The activation energy of hydrogen generation by NaBH 4 hydrolysis was 25.7 kJ mol À1. GRAPHICAL ABSTRACT ARTICLE HISTORY
... But, this process suffers from the existence of substantial minerals impurities in quartz sand such as Al 2 O 3 , K 2 O, Na 2 O, Li 2 O, Fe 2 O 3 and FeO. [2][3][4] These impurities are chemically bonded to the surface and located in the interstitial positions of the quartz lattice itself. Therefore, purification steps should be carried out before the use of quartz in order to increase the chances of obtaining silica of higher purity and surface area suitable for nanotechnology and heterogeneous catalysis fields. ...
Article
Full-text available
Tunisian industrial phosphoric acid H 3 PO 4 was supported on silica gel SiO 2 (SIPA) to catalyze the hydrolysis reaction of aqueous alkaline sodium borohydride (NaBH 4). The SiO 2 was produced from purified quartz sand using alkali fusion-acidification chemical process. The BET surface area results indicate that the prepared silica gel could reach a specific surface area up to 585 m 2 /g. The addition of PO 3 H 2 functional groups resulted in an increase of surface acidity of SiO 2 catalyst as shown by FT-IR and DTA-DTG spectra. The total acidity of SIPA catalyst was determined by titra-tion to be 2.8 mmol H þ /g. SEM/EDS maps reveal the distribution of heavy metals on the silica surface. The effect of supported PO 3 H 2 functional groups and heavy metals on the NaBH 4 hydrolysis reaction was studied for different ratios of SIPA catalyst to NaBH 4. The sample 12SIPA/NaBH 4 leads to a very high hydrogen generation rate (up to 90%). The activation energy of hydrogen generation by NaBH 4 hydrolysis was 25.7 kJ mol À1.
... For instance, microorganisms have been used to bioleach and bio-remediate solutions to achieve mineral separation. Gold extraction from sulfide ores (pyrite (FeS 2 ), chalcopyrite (CuFeS 2 ), arsenopyrite (FeAsS)), carbon desulfurization, phosphorus, copper, zinc and nickel extraction, kaolinite bleaching, and cyanide treatment [21,22,[28][29][30][31][32][33], are examples of such biotechnologies. Some bacteria swim by the rotation of flagella, which are distributed through the cell body. ...
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To explore engineering platforms towards “active bacterial baths”, we grow and characterize native and commercial strains of Acidithiobacillus ferrooxidans to promote swimming locomotion. Three different energy sources are used, namely elemental sulfur, ferrous sulfate and pyrite. The characteristics of the culture, such as pH, Eh and the concentration of cells and ions, are monitored to seek correlations between the oxidation route and the transport mechanism. We found that only elemental sulfur induces swimming mobility in the commercial DSMZ – 24419 strain, while ferrous sulfate or the sulphide mineral, pyrite, did not activate swimming on any strain. The bacterial mean squared displacement and the mean velocity are measured to provide a quantitative description of the bacterial mobility. We found that, even if the A. ferrooxidans strain is grown in a sulfur-rich environment, it preferentially oxidizes iron when an iron-based material is included in the media. Similar to other species, once the culture pH decrease below 1.2 the active locomotion is inhibited. The engineering control and activation of swimming in bacterial cultures offer fertile grounds towards applications of active suspensions such as energy-efficient bioleaching, mixing, drug delivery and bio-sensing.
... In order to examine the effect of stirring speed on the reduction process, the experiments were performed at 90, 150 and 200rpm using an orbital shaker. Despite the references of the kaolin reducing process (Da Silva et al., 2011;Martínez-Luévanos et al., 2011;Tuncuk et al., 2013), the stirring speed had no influence on the reduction process in this study and there was no difference in percentage of chemical reduction. ...
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A mechanism to convert Fe(III) to Fe(II) by a reduction process could be a solution to the problem of selective separation of metals in extraction processes by using the ion-exchange process. The aim of this research was to reduce Fe(III) to Fe(II) present in leachate from nickel mining waste. Reducing agents used were sodium dithionite and sodium metabisulfite. The potential was decreased in order to quantify the minimum reducing agent that was necessary for conversion. Reaction time, pH, stirring speed and temperature were also studied. Results indicated dithionite was a better reducing agent than metabisulfite and complete conversion was possible by reducing the potential to 590mV at pH 0.5-2 and 240mV at pH 2.5. Stirring speed had no influence on the reduction process at 2 hours; however, conversion decreased after 24 hours, perhaps due to oxygen present in air. The analyses were conducted by ion chromatography and voltammetry.
... The actual determined concentration of iron in biomass was lower, thus, the iron recovery based on the mass balance in biomass, medium and kaolin after cultivation was 94.2 ± 2.1%. Nevertheless, the final content of iron in kaolin after bioextraction decreased to approximately 0.34% (w/w) which is desired for its commercial and industrial applications (Hosseini and Ahmadi 2015), including paper and ceramic industries, where the Fe content is expected to be below 0.5% (Tuncuk et al. 2013). Furthermore, the final extraction efficiency of silicon and aluminium was negligible, achieving only approximate values of 0.07% and 0.15%, respectively. ...
Article
Elevated iron content in low-grade kaolin is one of the chemical factors detrimental for its industrial applications. Therefore, various physical and chemical methods have been applied to decrease iron concentration in kaolin. One of the more feasible and environmentally friendly method is biobenefication via microbially-induced bioextraction. Therefore, this paper provides data on fungal bioextraction of iron from kaolin sample when incubated with fungal Aspergillus niger strain. After 19-day cultivation, approximately 45% of iron impurities have been bioextracted by fungus and distributed into fungal biomass or culture medium. The bioextraction was triggered by both acidification of culture medium by fungus (below pH of 1.5) and organic acids production, which are biologically conversed from glucose. However, bioextraction of silicon and aluminium, which form building blocks of clay minerals in kaolin, was reasonably low and did not overcome 0.15%. Thus, our data indicate that the bioextraction of iron using A. niger strain for kaolin processing is a viable alternative method for increasing quality of the low-grade kaolin.
... Liç işlemleri, kaolen ve kuvars gibi cam, kağıt ve seramik endüstrileri için önemli olan minerallerden ve diğer endüstriyel hammaddelerden demirin uzaklaştırılmasında büyük öneme sahiptir [9-12, 16, 26-33]. Günümüzde en çok kullanılan yöntemler kimyasal yöntemler olup, düşük pH'da yüksek seviyelerde demir uzaklaştırılması ve beyaz kuvars elde edilmesi amacıyla güçlü indirgeyici ortamlardan yararlanılmasına dayanmaktadır [27,34,35]. Kuvarsda demir ve diğer kirletici bileşenler ne kadar yüksek ise reaktif tüketimi ve kimyasal yöntemlerin maliyeti de o derece yüksek olacaktır. ...
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Removal of impurities (iron in particular) from quartz using aqueous chemical processes is important for its potential use in many industries including detergent, ceramics, paint, refractory, metallurgy and, in particular, glass and electronic industries to fulfill the desired requirements by these industries for high quality quartz as raw material. In this study the removal of impurities, iron in particular, from the quartz ore by physical and chemical methods with the aim of producing high quality quartz with desired specifications were investigated on laboratory scale. Impurities with magnetic properties were removed by high field intense magnetic separator and in leaching tests, the influence of various factors including the performance of acids, reagent type/concentration, temperature, leaching time and solid/liquid ratio and extent of the removal of iron were studied. After the experimental studies with quartz sample, Fe2O3 content was decreased to 37,8 ppm from 400 ppm, whiteness index was also increased to 96,81 from 87,43.
... Liç işlemleri, kaolen ve kuvars gibi cam, kağıt ve seramik endüstrileri için önemli olan minerallerden ve diğer endüstriyel hammaddelerden demirin uzaklaştırılmasında büyük öneme sahiptir [9-12, 16, 26-33]. Günümüzde en çok kullanılan yöntemler kimyasal yöntemler olup, düşük pH'da yüksek seviyelerde demir uzaklaştırılması ve beyaz kuvars elde edilmesi amacıyla güçlü indirgeyici ortamlardan yararlanılmasına dayanmaktadır [27,34,35]. Kuvarsda demir ve diğer kirletici bileşenler ne kadar yüksek ise reaktif tüketimi ve kimyasal yöntemlerin maliyeti de o derece yüksek olacaktır. ...
... Uzun and Gulfen achieved up to 97.46% iron extraction using 6 mol/L sulfuric acid at 105°C (Uzun and Gülfen, 2007). Tuncuk et al. reported aluminum recovery as high as 96.82% using 4 mol/L sulfuric acid and a solid-to-liquid ratio of 0.04 at 90°C for 4 h (Tuncuk et al., 2013). ...
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Dealkalization is a major limiting factor for the comprehensive utilization of red mud, and acid leaching is considered an effective and promising way to achieve it. Most studies have focused on the recovery of metals in this extreme alkaline waste by strong acid leaching, ignoring the dealkalization and stabilization of red mud with low concentrations of acid. In this study, the leaching behavior of the major elements in red mud was carefully investigated by the treatment with a range of H⁺ concentrations from 10⁻⁶ to 1.8 mol/L. Analysis of both leachate and residue confirmed a stepwise sequential leaching behavior of Na, Al, and Ca, which was important for dealkalization of red mud. Treatment with an acidic solution at pH 1 removed 17.84% of Na, but almost no other metals. Greater than 60% Na was leached by an acidic solution with H⁺ concentration of 0.5 mol/L, but the dissolution efficiencies of Al, Si, and Ca were less than 10%, resulting in a relatively simple leaching liquid treatment process. And the pH of the leaching residue was 8.5 which slightly alkaline so the residual RM could be used as soil aggregates. When using an acidic solution with an H⁺ concentration of 1.6 mol/L, the dissolution efficiency of Na reached at 94.70%, leaving only 0.47% Na in the residue, which makes it possible to meet the demand of application for construction materials. From the results of the phase analysis, it was found that the sequential leaching behavior was due to the dissolution, decomposition, and transformation of the particles containing the main elements, and was compatible with the SEM-EDS inspection. Based on these results, a new multi-stage acid leaching dealkalization process has been proposed to provide an effective and economical way to dealkalize red mud.
... In order to remove impurities, various processing methods were proposed, including magnetic separation, froth flotation, selective flocculation, acidic leaching, and bioleaching [6]. However, the above techniques could improve the removal of physically associated impurities but are not effective in removing chemically bonded impurities, occluded impurities, or particularly in removing interstitial impurities which are part of the quartz lattice itself. ...
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Separation and identification of minerals composing the Tunisian silica sands have been conducted by acid etching and hot filtration/sedimentation (HFS) without affecting the quartz component. It was found that the hot acid etching process is an effective method to separate quartz and clay minerals in the silica sand. The SEM micrographs of the silica sand show rough surface and some cracks with irregular shapes. The surface becomes clean and smooth after acid etching process. After chemical dissolution, the clay minerals were recovered by hot filtration following by sedimentation at low temperature. FT-IR, EDS maps, and secondary electron (SE) analyses of separated clay minerals reveal the presence of Fe2O3, montmorillonite, and kaolinite minerals. The LIBS analysis of clay minerals shows the presence of Al, Fe, Ca, Al, and Mg. The sequential etching process can be introduced in the purification methods as an effective way to separate quartz from clay minerals.
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Silicon powder waste (SPW) from diamond-wire saw cutting process has a very low content of boron and phosphorus. Therefore, it is a promising and precious raw material for producing SoG-Si. The SPW also contains iron impurities which were from the worn of cutting wire during the silicon wafer producing process. Since the presence of iron will affect the quality of the silicon products, it is essential to remove the iron from SPW. In this paper, the removal of iron from SPW by acid leaching was researched and its kinetics were investigated by the heterogeneous and homogeneous reaction methods. Diluted sulfuric acid was selected as the leaching agent. The leaching parameters were optimized, i.e., temperature 60 °C, acid concentration 12%, liquid-solid ratio 10 mL/g and stirring speed 200 rpm. Under the optimized conditions, the removal rate of iron could reach 94.34%. It was found that the iron removing process can be divided into two stages. In both of the stages, the leaching process was controlled by second order rate of homogeneous reaction model, and the activation energy are 10.78 kJ/mol and 35.97 kJ/mol, respectively.
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Coated abrasive disks are supported by vulcanized fiber backing. The disks are normally used for various material removal processes, at the end of which the remaining backing is dumped as waste. Being vulcanized fiber, it is difficult to dispose of. It is non-biodegradable and causes environmental pollution. The fiber backing itself is costly and will be of value, if recovered. In this research work, chemical separation technique is opted to remove the available resin and grains from the fiber backing. The present work highlights and reduces pollution by addressing recovery of fiber backing from used fiber disks. Coated abrasive disks of grit sizes 36, 60, and 120 were chosen for the optimization process. The process parameters such as normality (2.5, 3, and 3.5 N), solution quantity (300, 400, and 500 ml), number of flexing cycles (0, 5, and 10), and immersion time (8, 10, and 12 h) were selected for the fiber recovery. Taguchi method was employed for the optimization process. Recovery duration was selected as a response parameter. Signal to noise (S/N) ratio was calculated from the experimental results, to optimize the quality and recovery duration of the fiber backing. The results indicate that increase in normality, solution quantity, and immersion time facilitate quick and easy recovery of the fiber backing. Apart from that, the disks of medium and fine grits are possible to reuse compared with coarser grit. The effectiveness of resin removal from the backing, is traced through optical microscopy.
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This investigation reports the silver dissolution kinetics from a ore-type sulfosalt identified as miargyrite, which prevails in the chemical concentration process of polymetallic concentrates, in the S2O3²⁻-Ca(OH)2 system as well as a statistical study of the design of experiments concerning the effect of temperature (T), calcium hydroxide concentration ([Ca(OH)2]), thiosulfate concentration ([S2O3²⁻]), particle size (d0) and stirring speed (RPM). The results show that temperature is the variable with the most significant effect on the dissolution rate of silver and obtains an activation energy (Ea) of 57.19 kJ mol⁻¹, followed by particle size and hydroxide concentration. Furthermore, it was observed that stirring speed does not affect the dissolution rates in basic medium. These results are consistent and demonstrate that the dissolution reaction is controlled by the chemical reaction. The study of the factorial design was relevant to elucidate the behavior of the variables with the greatest impact in the experimental kinetic study. Finally, kinetic and statistical models were proposed to describe the silver dissolution process in an alkaline medium. The highest silver dissolution was achieved using the following conditions V = 0.5 L, mineral = 40 g L⁻¹, PO2 = 1 atm, t = 360 min, d0 = −105 + 74 μm, [S2O3²⁻] = 0.5 mol L⁻¹, [Ca(OH)2] = 0.05 mol L⁻¹, T = 338 K and RPM = 670 min⁻¹.
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In this work, a process is established for recovering cubic boron nitride (cBN) grains from fired vitrified cBN products. The abrasive grains are recovered by an acid washing technique. Two methods were used to recover the abrasive grains, namely with and without pre-heating. Shape and surface characteristics of the recovered grains were analysed through optical microscopy and Scanning Electron Microscopy (SEM). The recovery yield of abrasive grain and bond percentage present in the abrasive tool was measured experimentally. From the experimental results, the maximum recovery yield of 81 percentage was obtained in the pre-heated sample. Bond free abrasive grains are also obtained from the pre-heated samples and confirmed through elemental analysis.
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Kaolin (china clay) is a rock material that is very rich in kaolinite. A kaolin ore from Debre Tabor, Ethiopia containing 59.2 wt.% SiO 2 , 24.9 wt.% Al 2 O 3 , 2.4 wt.% Fe 2 O 3, and 8.22 wt.% loss on ignition (LOI) was physically beneficiated, chemically leached, and thermally treated for possible industrial use, especially for ceramic membrane fabrication. The leaching experiments were carried out using oxalic acid solutions as leaching reagents for the iron extraction process. The effect of acid concentration, reaction temperature, and contact time on iron leaching was investigated. It was determined that the rate of iron extraction increased with the oxalic acid concentration, leaching temperature, and contact time. A substantial reduction of iron oxide (2.4 to 0.36 wt. %) from the raw kaolin was observed at operating conditions of 2.0 M oxalic acid, the temperature of 120°C, and contact time of 120 min. A maximum kaolin whiteness index of 81.4 % was achieved through this leaching process. Finally, the physically beneficiated, chemically leached, and thermally treated kaolin raw material was used to fabricate a low - cost kaolin - based ceramic membrane. After firing at 1100°C the ceramic membrane was found to have a mass loss of 11.04 ± 0.05%, water absorption of 8.9 ± 0.4 %, linear shrinkage of 14.5 ± 0.05%. It was demonstrated to be chemically stable, having less than 3 % mass loss in acid solution, and less than 1 % mass loss in alkali solution. The newly developed membranes have thus properties comparable to commercial ceramic membranes.
Article
The dissolution kinetics and mechanisms of reaction of Batagbon Kaolin in sulphuric and fluosilicic acids were studied. Leaching temperature, acid concentration, particle size, solid-to-liquid ratio, and stirring speed were selected as process parameters. It is observed that the dissolution rate increases with decreasing particle size and solid-to-liquid ratio, and increases with stirring speed, acid concentration, and leaching temperature. The experimental results indicate that the dissolution rate is of mixed control via hydrogen ion [H⁺] action, with reaction order of 0.813 and the reaction kinetics can be expressed as Kmt=[1-(1-x)1/3+y/6[(1-x)1/3+ 1-2(1-x)2/3]. The activation energy of the process is determined to be 21.6 kJ/mol. The level of the product quality is also evaluated.
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In this study oxalic, citric, and glycolic acids were used in order to reduce Fe and Ti (colored impurities) from the slimes (-75 µm) feldspar samples using organic acids. The results showed that removal ratios of the colored impurities from the feldspar samples were 67.9% for Fe and 43.75% for Ti using oxalic acid and the agitated leaching (AL) method. The influence of main parameters (temperature, pulp density, leaching time, and acid concentration) were examined by using full the factorial design (2 4) ANOVA-Yates test technique. Next, the removal ratios of Fe% and Ti% in the tests were determined to be 80.44% and 45.39%, respectively. Additionally, the main parameters which were obtained from the best results of AL were optimized for the-500+75 µm feldspar sample. Finally, the microwave-assisted pressure leaching (MAPL) method was apply to determine the effect of pressure on leaching. The obtained results indicated that the optimum removal ratios obtained were 95.74% for Fe and 70.88% for Ti by using the MAPL tests with oxalic acid. Furthermore, the measured whiteness (L) values were observed to be over 90%. This is a suitable purification ratio for the ceramic and glass industry.
Article
The quality of the kaolin is always measured by iron contents since this element gives undesirable reddish color which limits the usage of this type of minerals. Reducing the iron contents to increase the value of kaolin by bleaching process is investigated. The effects of factors which can improve the whiteness of kaolin have been studied. The dosage of sodium dithionite, pH value, solid-to-liquid ratio and reaction time were chose as factors based on mono-factor experimental results. Orthogonal experiments were carried out and the optimum processing conditions of the reductive bleaching were obtained as the dosage of sodium dithionite 3%, pH 2, solid-to-liquid ratio 1:3 and reaction time 45min. After bleaching process described above, we obtained a great improvement in the whiteness from 69.93% to 81.31% and a decrease of Fe2O3 content from 0.52% to 0.40% of the kaolin.
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Dealkalization is the necessary step for the multipurpose use of red mud (RM), and acid leaching is a productive method to realize the dealkalization of RM. Most researches focus on recovering metals from the highly alkaline waste by pure acid leaching or stabilization by dealkalization. In this study, according to the strong alkalinity of RM and strong acidity of the waste acid from titanium dioxide production, the waste acid was used for the dealkalization of RM. The effects of leaching temperature, reaction time, the concentration of waste acid, liquid–solid ratio (L/S), and stirring rate on the dealkalization of RM were investigated, and the main metal ions in the dealkalization solution were analyzed. The results show that the leaching ratio of sodium can reach 92.3591% when the leaching temperature is 30 °C, the reaction time is 10 min, the concentration of waste acid is 0.6238 mol/L, the L/S is 4:1, and the stirring rate is 300 rpm. The residual alkali content in the treated RM is 0.2674%, which is a reduction to less than 1%. The phase analysis results show that the sodalite and cancrinite in RM are dissolved, decomposed, and transformed after acid leaching. Therefore, RM meets the requirements of building materials after dealkalization, which provides further development as building material products.
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Purification of quartz (iron in particular) with chemical prosesses is extremely important for many industries including glass, electronic, detergent, ceramics, paint, refractory and metallurgy. The iron content of quartz is required to be in the range of 0.01-0.1% for its industrial use. Turkish quartz reserves have not been exploited effectively in the glass and electronical industries mainly due to the limitations in the quality and quantity of these resources. There appears to be different physical and chemical methods in industrial scale available for the beneficiation of quartz to reduce its iron content down to the desired levels. Although the physical methods including conventional magnetic separation, new and more expensive super conductive magnetic separation and flotation are commonly employed, the chemical methods have been gaining importance particularly due to the low efficiency of physical methods for iron removal. Iron as the most significant impurity present in quartz ores can be substantially removed by aqueous chemical processes using organic and inorganic acids. In this respect, oxalic acid appears to be one of the most significant leaching reagents for the removal of iron. In this article, the industrial use of quartz, area specific requirements for product quality and chemical methods (with particular reference to oxalic acid systems) used in the purification of quartz are critically reviewed.
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Kaolin available in Deopani, Assam of northeastern region of India was characterized by FTIR, XRD, DTA and by wet chemical analysis methods. The major impurities in the clay are quartz and siderite. Wet sieving of the clay by −53-μm sieve removes almost all the quartz and a major amount of Fe bearing impurities. The iron content of the clay may be reduced further by treating with wet high intensity magnetic separator (WHIMS) or leaching with organic acids. The effectiveness of various leachants follows the order: Oxalic>Oxalic+EDTA>Malonic>Citric acid. Almost all the nonstructural iron may be removed at room temperature by using 0.4 M oxalic acid solution. The leached clay may find use as a ceramic raw material and filler material for paper and other materials.
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Model for predictive analysis of the quantity of heat absorbed by oxalic acid solution during leaching of iron oxide ore has been derived. It was observed that the validity of the model is rooted in the expression (lnQ)/N = √T where both sides of the relationship are correspondingly almost equal. The model was found to depend on the value of the final solution temperature measured during the experiment. The respective deviation of the model-predicted Q values from the corresponding experimental values was found to be less than 21% which is quite within the acceptable range of deviation limit of experimental results. The positive values of heat absorbed as obtained from experiment and model indicate and agree that the leaching process is endothermic in nature.
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ZET Kaolenlerin safsızlaştırılması, içeriğindeki demirin kimyasal ve biyolojik yöntemlerle uzaklaştırılıp öncelikli olarak kâğıt, seramik, boya, refrakter, plastik sanayilerinde kullanılabilecek hale getirilmesi açısından önemlidir. Kaolenler seramik ve kâğıt sanayinin en önemli hammaddesidir. Kaolen demir içeriğinin endüstriyel proseslerde kullanılabilmesi için belirli oranlarda (en fazla %0,4-0,5) olması gerekmektedir. Özellikle ülkemiz kaolenlerinin demir içeriğinin yüksek olması (%1'in üstünde) nedeniyle, %90'ın üzerinde beyazlık indeksi istenen kâğıt ve seramik sanayinde kullanılamamaktadır. ABSTRACT Purification of kaolins i.e. removal of iron with physical, chemical and biological methods is important for their use mainly in paper, ceramics, paint, refractory, and plastic industries. Kaolin is the most important raw material of ceramics and paper industries where its iron content must be less than 0.4-0.5%. Especially, because of the high iron content of Turkish kaolins (>1%), they cannot be used in the paper and ceramics industries for which >90% whiteness index is required. By using organic acids such as oxalic, gluconic, citric, asetic, formic, ascorbic, succinic, tartaric and malonic acids, iron found in kaolin as an impurity is chemically removed. Biological methods may be used as an alternative to the chemical methods. Purification of kaolin using the biological methods are based on the activities of Aspergillus nigeras fungus and Bacillus and Pseudomonas as bacteria to produce high level of organic acids. In this article, the importance of chemical and biological methods and the application areas of kaolin are critically reviewed.
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The removal of iron from silica sand with oxalic acid has been studied under various experimental conditions in order to optimise the process parameters and reach a high degree of iron removal at minimum operating cost. The parameters studied were: temperature, pH of the solution, oxalate concentration, Ar purging, and ferrous ions addition to the solution. For the specific silica sand sample used, at temperatures varying between 90–100°C the maximum iron extraction that can be achieved is approximately 40%. At temperatures lower than 80°C this extraction is decreased to 30%. At these temperatures purging of the oxalate solution with Ar and ferrous ions addition has no effect on the iron extraction, while at temperatures lower than 25°C iron dissolution is accelerated with the addition of ferrous ions. Iron dissolution is significantly affected by the pH, while it is practically independent of the oxalate concentration and the pulp density. Without the addition of bivalent iron, iron extraction is optimised in high acid solutions; when ferrous ions are added in the oxalate solution, best results are achieved at pH 3.
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The oxidation state of Fe in the crystal structure of clay minerals plays an important role in determining the chemical and physical properties of the clay. These properties, among which are swelling, cation exchange capacity, and hydraulic conductivity, affect the fertility, strength, and porosity of agricultural soils and influence the fate and behavior of pollutant chemicals such as heavy metals and pesticides in soils and sediments. In natural systems, bacteria may be the most significant cause of Fe reduction in clay minerals.
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To reduce the iron content of kaolin to the desired levels, a variety of industrial physical and chemical processes are employed in practice. Though the physical methods (classical magnetic separation, new and more expensive super conductive magnetic separation in particular) are extensively used, generally low efficiency of these methods has led to the search for alternative chemical methods for the removal of iron, which have gained importance recently. In this study, the chemical removal of the iron impurity present in kaolin supplied by ÇİMSA Inc. was investigated by using organic acids. The effect of parameters including the type of organic acids, temperature, pulp density, leaching time and reagent concentration on the extent of iron removal were investigated applying factorial design ANOVA-Yates test technique for the statistical analysis of the results. Under optimum condition (10% pulp ratio, 40 g/L of oxalic acid concentration, 90°C temperature and 120 minutes of leaching (reaction) time), the removal of Fe2O3 was obtained as 34.45% for oxalic acid leaching.
Article
Heterotrophic acid-producing bacteria and fungi were used to remove iron from quartz sands. The best results were achieved by a strain of Aspergillus niger, which produced culture solutions in which the oxalic and citric acids were the main components. A two-stage process was developed in which sands were leached at 90 degree C with lixiviant produced as a result of the cultivation of the strain at 30 degree C. The study aims at elucidating possibilities to decrease, by means of bio-leaching, the iron content of some sands to a level which will make them suitable for the manufacture of high-quality glass.
Article
In the present study a bleaching process of a kaolin of industrial interest was carried out using thiourea as the leachant agent in the iron removal process. The beneficiation of kaolin, by iron removal, improves its economic value, and it is then possible to use it in the ceramic and paper industries. The effect of thiourea was investigated together with other factors, such as sulphuric acid concentration, temperature and treatment time. Full factorial experiments were performed to determine the main and interaction effects on the iron dissolution from the mineral: the largest iron extraction yield (94%) was obtained after 150 min. of treatment at 90°C, 0.25 M H2SO4 and with 10 g/l of thiourea. At the same temperature the iron extraction yield was 68% after 120 min. of treatment with 0.1 M H2SO4 and 1 g/l thiourea. An empirical model of data fitting was used in order to evaluate the optimal conditions of the process, considering as factors the treatment time and the thiourea concentration. The experimental results show the technical feasibility of this process for the removal of iron from kaolin used in the experimental tests.
Article
Kaolin is mostly associated with minor quantities of ancillary minerals containing transition elements such as iron and titanium. These ions impart color to the white kaolin which adversely affects its application in paper and paint industries. Hence their removal is of prime importance in the optimum utilization of kaolin. The coloring effect as well as the mode of removal of these impurities depends on the “species” of the ion and/or the type of mineral. The present paper deals with the investigation on two Indian kaolins of different geological origin, one from Gujarat state at the western part of India and the other from Kerala State at the southern most part. Detailed physical, chemical and mineralogical characterization of the samples was carried out. The product clays after beneficiation by size classification, high gradient magnetic separation and chemical leaching were found to be of acceptable grade for paper industry with respect to optical properties and particle size. The impurity minerals were concentrated by different methods so that their identification was easier. Attempts were made to study the Fe species by correlating the XRD, chemical assay, DCB treatment and EPR spectral information of the clay samples before and after beneficiation. Iron stained anatase was found to be the major impurity in the Gujarat clay whereas iron was present as oxide/hydroxide in the Kerala sample. The beneficiated products from the Kerala clay were found to have better optical properties.
Article
The effect of various organic acids viz. acetic, formic, citric, ascorbic, succinic, tartaric and oxalic acids, on the iron removal and the resulting brightness improvement of an iron-stained kaolinitic clay from Kalliyur, Thiruvananthapuram, South India, has been investigated. Oxalic acid was found to give the best results both at room temperature as well as at high temperatures because of its high acid strength, good complexing capacity and reducing power. The reaction parameters such as time, temperature and reagent concentration were optimised. The optimum conditions required for achieving brightness ≥80% were: temperature — 100°C, oxalic acid concentration — 0.1 M and reaction time — 90 min. The leaching tests at room temperature for 30 days improved the brightness from 66.3 to 83.5% ISO. The corresponding iron oxide removal was of the order of ∼80%. The addition of ferrous ions and protons improved the reaction kinetics. The leaching tests carried out on previously beneficiated samples using magnetic separation showed only a slight improvement in brightness indicating that brightness depended more on the surface coated iron oxides rather than on the discrete particles. The effect of acid leaching on the physical properties of the clay such as brightness, plasticity, viscosity, specific surface area and pore volume were compared. The slight increase in the specific surface area and the pore volume are suggestive of removal of the cementing non-crystalline alumina, silica and iron oxides from the clay surface and also due to the resulting delamination to a limited extent. No marked change was observed in the viscous as well as the plastic properties due to the deferration treatments.
Article
Goethite dissolution studies were carried out using a statistical design of experiments, and process parameters were further optimized. A regression equation for the dissolution of iron was developed as a function of four parameters (acid concentration, temperature, ascorbic acid concentration and time). All the parameters were varied at two levels for designing experiments and six base level experiments were also carried out to estimate error and standard deviation. The dissolution process was further optimized by the method of steepest ascent using the results of statistical design of experiments.
Article
Bleaching of kaolin with a high content of iron oxides by means of three organic acids (citric, gluconic and oxalic) was studied. The first two release small quantities of iron oxides from the kaolin, which means they will not be adequate for large-scale processes. Oxalic acid is capable of dissolving all the leachable iron in kaolin, reaching a whiteness index of 80%, thus making the kaolin suitable for its basic industrial applications. ©1997 SCI
Article
The removal of iron from kaolin of industrial interest by acid leaching under pressure, using sucrose as reductive substance, has been studied. Several factors have been taken into consideration in the experimental runs planned, using an experimental design technique: acid concentration (H2SO4), sucrose concentration (C12H22O11), temperature and time of treatment. The experimental tests were performed using industrial mineral concentration (30% w/v) in an autoclave device. The main effects and the significant interactions involved in the leaching process were found by ANOVA; a significant improvement in iron removal was obtained due to the presence of sucrose. Some considerations about the mechanism of the process have been made considering the behaviour of the carbohydrates in hot acid solutions. Furthermore, sequential leaching tests were carried out in order to examine the possibility of recycling leach solutions for the treatment of non-treated mineral. A methodological approach is shown in the study of leaching processes using factorial experiments instead of experiments changing “one factor at a time”. The experimental results demonstrate the possibility of using carbohydrates in the bleaching process of minerals of industrial interest.
Article
The purpose of this study was to investigate the impact of redox-induced changes in the organization of the clay fraction of a bulk vertisol using transmission electron microscopy. Chemical and X-ray powder diffraction (XRD) analyses indicated that the oxidized clay was composed of 32% kaolinite and 68% non-pure smectitic material, mostly a dioctahedral beidellite with octahedral Fe, according to Quantarg2 and DecompXR models. The cation exchange capacity of the soil increased from 26.1 to 65 cmol c ⁺ kg ⁻¹ due to structural iron (Fe Str ) reduction and dissolution of oxide coatings. Transmission electron micrographs revealed dramatic changes upon reduction. Oxides were dissolved and the smectite increased in particle darkness, lateral extension, thickness, compactness and stacking order. These changes were interpreted to be a consequence of sorption of ferrous Fe and reduction of Fe Str , as found in previous studies on pure Fe-bearing smectites.
Article
The aim of the work was to decrease the iron content of ferrous quartz sands by fixed-bed column leaching with recycling of the leaching solutions in order to attain a product suitable for industrial use. Dissolution of iron was achieved by treating the sands in an acid medium with a reducing agent (oxalic acid) to convert FeIII into FeII.The factors assumed to affect dissolution of iron, such as temperature, oxalic acid concentration, pH and flow-rate, were studied with a 24 full factorial design in order to assess the main effects and the interactions among the factors.Removal of 46.1% iron gives a product containing 0.0163% Fe2O3 which is fit for industrial applications.
Article
In the present work the removal of iron, present as impurity in a kaolin of industrial interest, was performed. An effective iron reduction using microbial mixed cultures was attained (up to 81%) while a significant iron reduction in the presence of single microbial cultures was achieved (up to 37%).The active microorganisms belong to the genera Bacillus and Agrobacter. The enzymatic and un-enzymatic iron reduction was evaluated.Finally the molasses as carbon source for heterotrophic bacteria were succesfully tested.
Article
In view of an industrial application of hydrometallurgical processing of kaolins of industrial interest, samples of this mineral have been treated in an acid medium with a reductive agent (sucrose) in order to improve its iron extraction. The basis of this process is the ferric iron solubilization and its reduction to ferrous iron to improve the iron removal in acid medium. In this manner it is possible to exploit the commercial value of the iron-containing kaolins. In this work the main factors that can influence the iron extraction, in order to obtain iron-free kaolins using a treatment at atmospheric pressure, have been tested using factorial experiments. Best results have been obtained using the following experimental conditions: iron removal of 98% at 90°C in 100 minutes with 1 M H2SO4, 4 g/l sucrose hydrolysed and 200 rpm.
Article
In the present work a preliminary study of iron removal from a yellow kaolin supplied by ECC International (Europe) Ltd. was performed using mainly oxalic and ascorbic acids in sulfuric acid solutions. The aim of the initial phase of research was to determine the effects of the main factors which can potentially influence the iron-removal process. Two-level fractional factorial experimentation was thus utilized to establish the main factors having an influence on the iron removal and to define a subsequent experimental test design. The factors involved in this study were:  temperature, concentration of oxalic and ascorbic acids, mineral concentration, mixing conditions, and concentration of the sulfuric acid. The experimental results shown that the effect of oxalic and ascorbic acids and of the temperature are the most important. In the investigated conditions, the maximum iron extraction yield was 43−45% after 3−4 h of treatment indicating a limit of iron removal for the mineral. Further study is in progress to define the best operative conditions considering the economic point of view.
Article
Reactions between kaolin and inorganic salts, such as sodium hydrogen sulfate and ammonium sulfate, or inorganic acids, such as hydrochloric acid and sulfuric acid, can be focused either on the production of Al or on the adjustment of the silica-to-alumina ratio of the kaolin. On the basis of adjusting the silica-to-alumina ratio, kaolin and ammonium sulfate [(NH4)2SO4] are mixed and heated in a furnace at temperatures between 200 and 1000 °C. The parameters studied were the reaction temperature and time, the proton-to-alumina molar ratio, the calcination temperature and time, and the reaction atmosphere. The products of the reaction between kaolin and (NH4)2SO4 were characterized by means of X-ray diffraction (XRD). The Al reaction yield grew until reaching a maximum at 600 °C and started to decrease sharply beyond this temperature. The Ti reaction yield values were lower than the Al reaction yield values, showing a maximum at 600 °C and decreasing smoothly as the reaction temperature was increased. The Fe reaction yield exhibited the same tendencies as the Al reaction yield. With regard to the proton-to-alumina molar ratio, a maximum in Al, Ti, and Fe reaction yields was observed at rH = 3. Precalcination of kaolin produced only slight decreases in the Al and Ti reaction yields and a slight increase in the Fe reaction yield. The products of the reaction were triammonium hydrogen disulfate [(NH4)3H(SO4)2], ammonium aluminum sulfate [NH4Al(SO4)2], aluminum sulfate [Al2(SO4)3], and alumina [Al2O3]. Brunauer−Emmett−Teller (BET) specific surface area tests indicated an increase of the BET specific surface area for short reaction times, reaching values above 200 m2/g.
Article
The dissolution of metal oxides is a process of importance in several fields such as hydrometallurgy, passivity of metals, and cleaning of boilers and metal surfaces in general. Oxalic acid is one of the most effective reagents for dissolution of magnetite under mild acid conditions. Magnetite is the oxide that confers passivity to steel surfaces. In the present communication, the more salient features of the mechanism of dissolution of magnetite by oxalic acid solutions are discussed with special focus on the role played by ferrous ions in the process. Oxalate plays an unique role among complexing carboxylic ligands in the dissolution of magnetite; it not only facilitates the electron-transfer reaction but also mediates in a relatively fast dissolution during the initial induction period (the induction period is much shorter than in the case of the dissolution of magnetite by ethylenediaminetetraacetic or nitrilotriacetic acid). This unique role has been used in the development of a very efficient scale removal formulation used in the decontamination of nuclear power plants.
Article
Four metakaolins were prepared by calcination of a natural Spanish kaolin (Navalacruz deposit, Zamora province) at 600, 700, 800, and 900 °C. These four metakaolins, more reactive than the parent kaolin, were submitted to acid and alkaline activations. Acid activation was carried out with 6 M HCl at room temperature and at 90 °C under reflux conditions, with times of treatment of 6 and 24 h. Treatment at room temperature did not produce any alteration in the structure or in the properties of the metakaolins. Treatment under reflux conditions for 6 h led to the removal of most of the octahedral Al3+ cations, and the formation of an amorphous silica phase, with surface area up to 219 m2/g, because of a high development of both the internal and the external surface of the solids. Acid treatment under stronger conditions (reflux, 24 h) also removed the octahedral cations, but provoked an amorphization of the silica obtained (S ≈ 23 m2/g). The metakaolin prepared by calcination at 900 °C showed a lower reactivity than those synthesized at the other temperatures considered. The alkaline treatments were carried out with 1 M and 5 M KOH solutions, the treatments with the concentrated solution, both at room temperature and, especially, under reflux, led to the dissolution of the metakaolins and to the formation of well-crystallized K−F zeolite. The solids obtained by acid treatments are promising as adsorbents and catalyst supports, while treatment in alkaline conditions may be a good method for preparing K−F zeolite.
Article
Second derivative diffuse reflectance spectroscopy (DRS) in the visible range has been used to characterize changes in colour and identify the nature of Fe oxides which withstand reduction during experimental yellowing of reddish materials. It is accepted that haematite dissolves preferentially and faster than goethite, and Al-substitution controls the dissolution kinetics of Fe oxides. However, DRS has shown that haematite is more resistant than predicted and that some Fe-oxides, probably trapped within kaolinite particles, are inaccessible to solvents. DRS allows the nature of dissolved phases at each deferration step to be determined and changes in Al-content of residual phases throughout deferration to be followed. It also demonstrated that Helmholtz coordinates correlate very well with changes in Fe-oxide mineralogy and are preferable to redness ratings when monitoring differential dissolution of Fe oxides through colour measurements. DRS is a powerful and sensitive technique for monitoring the dissolution of Fe oxides in soils.
Article
A study has been made of a leaching process to remove iron from quartziferous industrial minerals using oxalic and sulphuric acid in a drum reactor. The experimental work was necessary because of the paucity of literature on the use of oxalic acid in the treatment of industrial minerals containing iron as an impurity. The positive effect of oxalic acid on the iron extraction yield is clearly observed during leaching in a drum reactor. Iron extraction yields of 35–45% are obtained on treating the quartz with 3 kg/t oxalic acid and 2 kg/t sulphuric acid at 90°C for 4–5 h. Under the same conditions but without oxalic acid the iron extraction yield ranges from 3 to 9%, depending on the sulphuric acid content. Chemical and mineralogical analyses were run on the ore to ascertain where the iron compounds occur on the different mineral components. These analyses were carried out on an as-is sample and on three other samples obtained by magnetic separation. The experimental results indicate that 52% of the iron is found in the mica fraction. These results explain why the maximum iron extraction yield is only 35–45%. A flow-sheet of the process is proposed together with a rough material balance in order to estimate oxalic acid, sulphuric acid and water consumption.
Article
China clay is an important mineral, which is used in the manufacture of ceramics and refractory, as also in other industries. Mined China clay contains iron oxides and silicates as impurity; if present in excess of a threshold level, the impurities affect the commercial value of the products. Currently available processes for lowering the iron content in China clay to the desired level (<0.8%) are energy- and cost-intensive, not sufficiently flexible, and may cause environmental pollution. An alternative approach for iron removal consists in the development of a biotechnological process which is expected to be cost-effective, less complex and eco-friendly. We reported earlier that several fungi, especially Aspergillus niger, and their culture filtrates could leach sufficient amount of iron from a China clay sample; oxalic acid was found to be the most active component of the culture filtrate (Trans. Indian Inst. Met. 55 (2002) 1). We now report the rates of iron leaching from another China clay sample by oxalic acid and by the culture filtrate of A. niger NCIM 548 that was found to be the most active strain in our previous study (Trans. Indian Inst. Met. 55 (2002) 1). The iron-leaching rates increased with temperature (T) and followed biphasic kinetics. The effect of oxalic acid concentration (C), pH (H), solids concentration or pulp density (P), time and mode of agitation on the rate of iron leaching is described. The rate of leaching with oxalic acid (Rox) can be calculated theoretically from the following relationship: Rox∼(C)0.76(T)1.76(H)0.80(P)0.20 under the specified set of conditions. Using the same concentration of oxalic acid in A. niger culture filtrate, the relationship of the rate differed; this may be due to the influence of other metabolites present in the culture filtrate on the rate.
Article
The presence of iron oxides in clay or silica raw materials is detrimental to the manufacturing of high quality ceramics. Although iron has been traditionally removed by physical mineral processing, acid washing has been tested as it is more effective, especially for extremely low iron (of less than 0.1% w/w). However, inorganic acids such as sulphuric or hydrochloric acids easily contaminate the clay products with SO42− and Cl−, and therefore should be avoided as much as possible. On the other hand, if oxalic acid is used, any acid left behind will be destroyed during the firing of the ceramic products. The characteristics of dissolution of iron oxides were therefore investigated in this study.The dissolution of iron oxides in oxalic acid was found to be very slow at temperatures within the range 25–60 °C, but its rate increases rapidly above 90 °C. The dissolution rate also increases with increasing oxalate concentration at the constant pH values set within the optimum range of pH2.5–3.0. At this optimum pH, the dissolution of fine pure hematite (Fe2O3) (105–140 μm) follows a diffusion-controlled shrinking core model. The rate expression expressed as 1 − (2 / 3)x − (1 − x)2 / 3 where x is a fraction of iron dissolution was found to be proportional to [oxalate]1.5.The addition of magnetite to the leach liquor at 10% w/w hematite was found to enhance the dissolution rate dramatically. Such addition of magnetite allows coarser hematite in the range 0.5–1.4 mm to be leached at a reasonable rate.
Article
Kaolin is the most extensively used particulate mineral in the filling and coating of paper. It improves paper appearance, which is characterized by gloss, smoothness, brightness and opacity, and of greatest significance, it improves printability. Paper is also filled with kaolin to extend fiber.The characteristics of kaolin that have the greatest influence on the quality of paper are the purity, rheology and particle geometry of the processed mineral. Generally, the most deleterious impurities for brightness of kaolin are iron oxide and titanium oxide minerals. The rheology of kaolin-binder-water suspensions at high solid: liquid ratios (as much as 65%) must be approximately Newtonian for efficient blade coating at speeds as great as 1300 meter/min. Particle geometry, defined as particle size, particle size distribution, particle shape and aggregate structure, has a dominant influence over the rheological character of kaolin slurries, as well as on the properties imparted by the kaolin to filled and coated paper.Increase in the aspect ratio of kaolin, as opposed to increase in surface area, exerts the dominant influence on the increase in low-shear viscosity. Although low-shear viscosities of undelaminated coating grades of kaolin show good correlation with surface area, the relationship breaks down for delaminated grades. Particle packing is believed to be the controlling parameter for viscosity at high rates of shear.Opacity, gloss, printability and, to a lesser extent, brightness of paper imparted by coating and filling with kaolin, are largely functions of particle size and particle size distribution. The strength of coated and filled paper generally decrease with decrease in particle size. An increase in coating void volume generally has a deleterious effect on strength.The principal commercial printing systems today are rotogravure and offset. Although other parameters are important, coating structures containing numerous voids generally give superior rotogravure printing, whereas smooth, relatively ink-impermeable surfaces are generally most favorable for offset printing.
Article
Leaching studies of low-grade Joda manganese ore containing 24.7% Mn and 28.4% Fe were carried out at high temperature and atmospheric pressure using oxalic acid as reductant in sulphuric acid medium. The experiments were designed according to 24 full factorial design, and regression equations for extraction of manganese, iron and aluminum were determined from the data. All the significant main and interaction effects on extraction of Mn, Fe and Al have positive effect, except oxalic acid concentration and time interaction for extraction of Al. Oxalic acid concentration has strongest effect on extraction of Mn, whereas temperature and time have strongest effect on extraction of Fe and Al, respectively. 98.4% Mn and 8.7% Fe were extracted from −150+105 μm ore with 30.6 g/l oxalic acid, 0.543 M sulphuric acid concentration at 85 °C in 105 min.
Article
The light-induced dissolution of the iron oxide, lepidocrocite (γ-FeOOH), has been investigated and found to be greatly enhanced in the presence of citrate. A conceptual model of the dissolution process is presented and validated through studies of citrate adsorption to lepidocrocite, net iron oxide dissolution under a variety of conditions, and solution phase redox reactions. The initial dissolution rate is directly related to the concentration of the surface bound ferric citrate and the first order rate constant for the photodissolution process is very similar to that found for the photodissociation of soluble ferric citrate. Dissolution most likely occurs through direct excitation of charge transfer bands of the surface bound ferric citrate. At low pH (pH 4.0), a constant rate of dissolution is observed while at higher pH (pH 6.5 and 8.2), the dissolution rate decreases on continued photolysis. This decrease is due to (1) an oxygen-dependent deactivation process occurring at the surface and (2) loss of photo-produced iron from solution by “ligand-like” adsorption of soluble iron citrate complexes by the colloidal iron oxide. Superimposed on the dissolution process at these higher pH is a rapid oxidation-reduction cycle involving solution phase iron species with the reduction step induced by photodissociation of ferric citrate complexes and the oxidation step controlled by the formation of ferrous citrate.
Article
We report a novel protocol to prepare titania hollow nanospheres of size about 28 ± 1 nm with micelles of asymmetric triblock copolymers. The hollow particles exhibit unique electrochemical properties in lithium ion rechargeable batteries such as high capacity, very low irreversible capacity loss, and high cycling performance.
Article
Low- and high-defect kaolinites mechanochemically activated for different periods of time have been treated with sulfuric acid solution. These modified materials were analyzed using a combination of X-ray diffraction, thermogravimetry, chemical analysis, diffuse reflectance Fourier transform infrared spectroscopy, as well as specific surface area and pore size distribution measurements. In addition to the mechanochemically amorphized part, the disordered and the adequately distorted phases also reacted with sulfuric acid. The specific surface areas of the leached samples of the partially or the completely amorphized materials were found to be greater than those of the thermally amorphized ones. The acid treatment results in a greater total pore volume for the partially amorphized materials than for the totally amorphized mineral. The partially amorphized high-defect kaolinite was proved to be more soluble than the low-defect kaolinite under similar conditions.
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