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Abstract
Upward capillary leaching is a method of extracting useful components from solutions by filtering them through a column. This method is based on the natural movement of solutions in the column aeration zone. The vadose zone is a geochemical evaporation barrier.
The solution moves upward from the water table to the surface via capillaries in the vadose zone, and the water evaporates during its upward movement, resulting in the concentration and deposition of metal salts. In this study, laboratory experiments using a special set-up show that water-soluble metal compounds can be directionally redistributed in a column. Flotation tailings from the Norilsk industrial region were packed into the laboratory column, and drinking water was employed as the leaching agent. The kinetics of the upward solution flow through the column capillaries were investigated. Elemental redistribution in the column aeration zone was confirmed by geochemical analysis.
... Due to hydrophilic nature of substrates, water can move upward through capillary effect. The capillary rise occurs all the way from water table level to soil surface (Mikhailov, Vashlaev, Kharitonova, & Sviridova, 2018;Xing, Li, & Ma, 2019) ...
... Capillary rise reveals the mecanism involved in groundwater evaporation. During the upward movement, water soluble minerals were carried and distributed with the growing substrate (Mikhailov, Vashlaev, Kharitonova, & Sviridova, 2018;Zhao et al., 2019). The distribution of the soluble minerals can be beneficial or toxic to crops, depending on the characteristic and concentration of the minerals. ...
A successful development of low cost floating culture system (FCS) has opened an opportunity for local farmers to cultivate vegetables during prolonged flooding at riparian wetlands. Research was aimed to identify the optimal depth of water-substrate interface (WSI) and optimal rate of NPK fertilizer application (RFA) in cultivating green apple eggplant using the FCS. Depths of WSI were adjusted to 0, 1, 3, and 6 cm and substrate was enriched with 8.4, 12.6, and 16.8 g NPK per pot. The enriched substrates were contained in pots and placed on floating rafts with variable WSI depths. Experiment was arranged in Split Plot Design. WSI was assigned as main plot and RFA was as subplot. The results exhibited that direct contact between water surface and substrate significantly increased water moisture content, enhanced shoot growth, and increased fruit yield, regardless the WSI depths. However, WSI treatments decreased SPAD value and restricted root elongation when WSI depth was more than 3 cm. The RFA treatments up to 16.8 g per pot could enhance shoot and root growth, and increased the fruit yield. This yield increase was associated with the increasing number of fruits, not the fruit size.
... However, only a few reports the capillary penetration. Yin 31 studied the influence of particles and accumulation pores on lateral capillary permeation, and Mikhailov 32 reported on the capillary rise during column leaching. These studies focussed on the height evolution of the wetting line on the column surface and capillary water absorption. ...
Capillary penetration is widely existed in stope leaching, both the rate of liquid wetting ore and flow out of ore are affected by it. Stope leaching is carried out in a high-temperature environment when mining minerals with large burial depth. The mechanism of intra-particle liquid capillary penetration mechanisms at high-temperature have not been revealed. In this paper, samples with a size of Φ50 mm × 100 mm were selected for quantitative analysis. The capillary rise behaviour inside samples with different porosity were detected at 30 °C, 40 °C and 50 °C by using magnetic resonance imaging (MRI). In most cases, capillary rise height is underestimated when the outside wetting line is used as an indicator, because the rise height inside the sample is greater. The liquid capillary rise height increased slightly with the temperature, whereas the wetting surface profile remained unchanged. The capillary rise rate increased significantly with porosity, mainly due to the increase of internal effective porosity. The results help to understand the liquid penetration behaviour under high-temperature stope leaching condition, and lay a theoretical foundation for improving the liquid permeability.
Heap leaching is a well-established extractive metallurgical technology enabling the economical processing of various kinds of low-grade ores, which could not otherwise be exploited. However, despite much progress since it was first applied in recent times, the process remains limited by low recoveries and long extraction times. It is becoming increasingly clear that the choice of heap leaching as a suitable technology to process a particular mineral resource, which is both environmentally sound and economically viable, very much depends on having a comprehensive understanding of the underlying fundamental mechanisms of the processes and how they interact with the particular mineralogy of the ore body under consideration. This paper provides an introduction to the theoretical background of various heap leach processes, offers a scientific and patent literature overview on technology developments in commercial heap leaching operations around the world, identifies factors that drive the selection of heap leaching as a processing technology, describes challenges to exploiting these innovations, and concludes with a discussion on the future of heap leaching.
From its first implementation for the recovery of gold from low-grade ores by cyanidation in the early 1970s, heap leaching has developed into a key hydrometallurgical technology, in conjunction with solvent extraction and electrowinning, for the recovery of base metals, including most notably copper from both oxides and secondary sulphides, and more recently, nickel and zinc. And, given recent advances in bioleaching with thermophilic microbes, the successful application of heap leaching technology to low-grade primary copper ores seems only a matter of time. However, with each new development, it becomes increasingly apparent that the successful application of heap leaching technology will ultimately depend on our having an ever more complete understanding of the fundamental processes underlying it. Much work has been done toward the development of this understanding, on many different fronts. In this paper, a critical overview of heap leach modeling work to date is presented in terms of the most important phenomena which occur during heap leaching, and potential directions for future work are discussed.
The evolution of the field saturated hydraulic conductivity of four covers located on a reclaimed saline-sodic shale overburden from oil sands mining is presented. Three covers consisted of a surface layer of peat/glacial topsoil over a mineral, soil. and one cover was a single layer of mixed peat and mineral soil. Measurements of the field saturated hydraulic conductivity of the cover and shale materials were made with a Guelph permeameter between 2000 and 2004. The hydraulic conductivity of the cover materials in the multilayered covers increased by one to two orders of magnitude over the first few monitoring seasons. The hydraulic conductivity of the single-layer cover system, which was placed three years before the multilayered covers, marginally increased from 2000 to 2002 and then remained relatively unchanged. The hydraulic conductivity of the shale underlying all four covers increased approximately one order of magnitude. Soil temperature measurements indicated that one freeze/thaw cycle occurred each year within all cover soils and the surficial overburden. This suggests that freeze/thaw effects were the cause of the observed increases in hydraulic conductivity, as previously observed by other researchers working on compacted clays.
Air entrapment in soil is common in cases of farmland flood irrigation or intense rain. A simple, physically based model would be more useful than the complex two-phase (gaseous and liquid phase) flow model to describe water infiltration in layered soils with air entrap-ment. This study proposed a modified Green-Ampt model (MGAM) to simulate water infiltration in layered soils with consideration of entrapped air. A saturation coefficient S a was introduced in MGAM to account for the resistance effect of air entrapment on infiltration. S a had robust physical meaning, and was approximately equal to one minus the plus of the residual air and residual water saturation degree that could be determined from the soil water retention curve equation. In MGAM, the actual water content and hydraulic conductivity of the wetted zone were determined by multiplying S a with the saturated values. Infiltration experiments in a 300-cm-long five-layered soil column and a 280-cm-deep eight-layered field soil profile were conducted to test the applicability of MGAM. For comparison, the infiltration process was also simulated by the traditional Green-Ampt model (TGAM), in which the wetted zone was assumed to be fully saturated, and the Bouwer Green-Ampt model (BGAM), in which the hydraulic conductivity of the wetted zone was half that of the saturated hydraulic con-ductivity. The estimated S a values were very close to the measured saturation degree of soil layers at the termination of the experiment. The simulation results indicated that the TGAM overestimated the infiltration rate and cumulative infiltration, whereas the BGAM underestimated the infiltration rate and cumulative infiltration. Furthermore, the depths of the wetting fronts simulated by TGAM and BGAM were con-siderably smaller than those measured. The MGAM provided satisfactory simulation results and adequately described the infiltration process in both the laboratory soil column and the field soil profile.
Water conservation and prevention of salt accumulation are important issues in arid and semi-arid regions. Mulching is the most frequently used method for water conservation and prevention of salt accumulation. A mulch with various layers, hence named layered mulch (LM), was applied for reclamation of saline soil. The LM comprises of a light-colored mineral, farmyard manure, and common reed (Arundo donax L.). Two treatments, one with LM and the other without LM (NLM), were carried out on saline soil for 2 years. Soil temperature, water content, and electrical conductivity (EC) were measured during March and September at the depths of 1 to 120 in 4 cm intervals. The results showed that LM soil had a lower temperature and EC than NLM soils. The LM soil recorded higher soil water content than the NLM soil. The LM soil had a lower temperature possibly because it reflected the sunlight, had high heat capacity, and was less permeable to heat conduction. Consequently, the lower temperature in LM soil resulted in higher soil water content and this can be a soil condition, that enables enhanced salt leaching by subsequent rains.
Global synthesis of the findings from ∼140 recharge study areas in semiarid and arid regions provides important information on recharge rates, controls, and processes, which are critical for sustainable water development. Water resource evaluation, dryland salinity assessment (Australia), and radioactive waste disposal (US) are among the primary goals of many of these recharge studies. The chloride mass balance (CMB) technique is widely used to estimate recharge. Average recharge rates estimated over large areas (40–374 000 km2) range from 0·2 to 35 mm year−1, representing 0·1–5% of long-term average annual precipitation. Extreme local variability in recharge, with rates up to ∼720 m year−1, results from focussed recharge beneath ephemeral streams and lakes and preferential flow mostly in fractured systems. System response to climate variability and land use/land cover (LU/LC) changes is archived in unsaturated zone tracer profiles and in groundwater level fluctuations. Inter-annual climate variability related to El Niño Southern Oscillation (ENSO) results in up to three times higher recharge in regions within the SW US during periods of frequent El Niños (1977–1998) relative to periods dominated by La Niñas (1941–1957). Enhanced recharge related to ENSO is also documented in Argentina. Climate variability at decadal to century scales recorded in chloride profiles in Africa results in recharge rates of 30 mm year−1 during the Sahel drought (1970–1986) to 150 mm year−1 during non-drought periods. Variations in climate at millennial scales in the SW US changed systems from recharge during the Pleistocene glacial period (≥10 000 years ago) to discharge during the Holocene semiarid period. LU/LC changes such as deforestation in Australia increased recharge up to about 2 orders of magnitude. Changes from natural grassland and shrublands to dryland (rain-fed) agriculture altered systems from discharge (evapotranspiration, ET) to recharge in the SW US. The impact of LU change was much greater than climate variability in Niger (Africa), where replacement of savanna by crops increased recharge by about an order of magnitude even during severe droughts. Sensitivity of recharge to LU/LC changes suggests that recharge may be controlled through management of LU. In irrigated areas, recharge varies from 10 to 485 mm year−1, representing 1–25% of irrigation plus precipitation. However, irrigation pumpage in groundwater-fed irrigated areas greatly exceeds recharge rates, resulting in groundwater mining. Increased recharge related to cultivation has mobilized salts that accumulated in the unsaturated zone over millennia, resulting in widespread groundwater and surface water contamination, particularly in Australia. The synthesis of recharge rates provided in this study contains valuable information for developing sustainable groundwater resource programmes within the context of climate variability and LU/LC change. Copyright © 2006 John Wiley & Sons, Ltd.
Heap leaching is a widely used extraction method for low-grade minerals, including copper, gold, silver, and uranium. This method has new applications in nonmetallic minerals such as saltpeter and soil remediation. Although a number of experimental studies have been carried out as well as modeling, which allows better understanding of the phenomena and its operation, few studies have been carried out with the objective of optimizing the process. Most of the studies which consider optimization, either experimentally or through the use of models, have been done from a technical perspective. The aim of this work is to develop a methodology for the design and planning of the heap leaching circuit. A superstructure which includes a number of alternative circuits is proposed. Then a mathematical model is developed that represents the constraints of the system and maximizes the profits. An example is considered to validate the proposed methodology. The results show that the current mode of operation of these systems can be improved using this methodology.
Minespoil sodicity has the potential to limit plant growth and impede reclamation success on surface-mined land. A joint government/coal industry experiment was established near Highvale, AB, to determine suitable subsoil thicknesses (0, 55, 95, 135, 185, and 345 cm underlying 15 cm topsoil) for reclaiming sodic minespoil and maximizing production of an annual barley (Hordeum vulgare L.) cereal crop or a perennial alfalfa-smooth bromegrass (Medicago sativa L., Bromus inermis Leyss.) forage mixture. Barley and forage yields were lower on the 0-cm subsoil treatment than all other treatments. Yields for both crops increased as subsoil thickness increased to 55 cm. There was a consistent trend toward optimum yields on the 95-cm subsoil treatment, but the difference between 55 and 95 cm was not significant. The replacement of 55 to 95 cm subsoil plus 15 cm topsoil appeared sufficient to restore post-mine productivity to the potential achieved on surrounding agricultural land. Root depth under the forage mixture increased as total soil thickness increased, while increases under the cereal were not generally significant. The average effective root zone extended to about 85 cm under barley and 185 cm under alfalfa-smooth bromegrass. Average seasonal soil water within the effective subsoil root zone generally increased under barley and decreased under alfalfa-smooth bromegrass over time. Lower consumptive use of available soil water and a shallower effective root zone under barley contributed to accumulations of soil water above the subsoil/minespoil interface. Perennial forages appeared to be more effective in reducing soil water accumulations above the interface and promoting reclamation success of sodic minespoil.
A comprehensive modelling study of the HydroZinc™ heap bioleach process, using the HeapSim modelling tool, is described. The model was calibrated on the basis of a small number of column leach experiments and compared against pilot heap test results. The model calibration thus confirmed, a detailed sensitivity study was conducted in order to establish the key parameters that determine the overall rate of Zn extraction. In the present case these were found to be oxygen gas–liquid mass transfer, various factors affecting the delivery of acid into the heap, and factors affecting the temperature distribution within the heap. A set of improved design parameters are proposed that would almost double the zinc conversion rate measured in the pilot plant – from 83% in 740 days to 78% in 383 days – and increase zinc production rate from 1.77 to 4.35 kg/m2/day. However, this improvement must be evaluated in the context of various implications for the downstream process.
Evaporation from porous media in the presence of a water table is influenced by the water table depth and the hydraulic characteristics of the medium, among other factors. For shallow water tables, the upward capillary liquid flow maintains continuous liquid pathways connecting the water table to the surface. For deeper water tables, however, the hydraulic continuity may be disrupted due to the opposing gravity and viscous forces. The main objective of this study was to demonstrate the limitation of the common assumption of liquid continuity through the unsaturated zone across large distances above a water table. The concept of the evaporation characteristic length was used to predict the maximum depth of the water table hydraulically connected to the surface. When the water table depth exceeded this characteristic length, the evaporative water loss was significantly suppressed due to the loss of liquid continuity. For model comparison, experimental data from the literature and data from laboratory evaporation experiments were used. Experimental results from cylindrical columns packed with sand with different particle size distributions connected to water tables fixed at various depths below the surface confirmed the existence of a finite characteristic length indicating the maximum depth of the water table hydraulically connected to the surface. The experimentally determined characteristic lengths were in a good agreement with the theoretical predictions. The obtained results also provided new insights regarding the solute transport and deposition patterns during evaporation in the presence of a water table.
The pyrite heap biooxidation model discussed in the first paper of this two-part series was expanded to model the effects of heat generation, conduction, and convection within the heap and at its boundaries. The new sub-routine utilized parameters whose values are known to the user; the other sub-routines utilized the same parameters as in the first paper. The inclusion of the heat balance sub-routine significantly changes the pyrite oxidation profile, which is uniform in the vertical and lateral planes in an isothermal system, such as a column, but highly segregated in the vertical plane of a non-isothermal system, such as a heap. There are numerous phenomena at play in a non-isothermal system: at the top, faster cell growth and self-inhibition of the fines oxidation; at the bottom, evaporative cooling and oxygen depletion. A model sensitivity study revealed that, for a given sulfide head grade and particle size, the magnitude and duration of these phenomena can best be controlled by inoculating the heap with mesophiles, moderate thermophiles, and extreme thermophiles, increasing the aeration rate, and keeping the heap height as short as possible to achieve the fastest and most uniform oxidation throughout the heap.
Nickel may be extracted with partial selectivity over magnesium from laterites containing serpentine by reduction roasting followed by sulphuric acid leaching. This paper describes the results of a kinetic study of the sulphuric acid leaching of nickel and magnesium from the reduction roasted serpentine component of a typical laterite. The serpentine used in this work analyzed 1.65% nickel, 20.2% magnesium and 6.10% iron.Initially, leaches were carried out at temperatures of 30, 50 and 70° C to determine the acid requirement for complete nickel extraction using practical leaching conditions (25% solids) under which the acidity drops to a low level. A minimum acid addition of 60 g/l was needed, which gave 80% to 83% nickel extraction from material in which 85% of the nickel was reduced using hydrogen at 700° C. Under these conditions, about 17% of the magnesium was leached at each of the temperatures studied.To facilitate an understanding of leaching kinetics, leaches were then performed using constant acidities (0.1% solids) of 60, 30 and 15 g/l acid at temperatures of 30, 50 and 70° C. Closely sized particles (−65 + 100 mesh) were leached so that magnesium dissolution rates could be tested against established “shrinking core” models.The main conclusions are that, under the experimental conditions, nickel dissolution rates were chemically controlled by either or 2 H+ + 2e → H2 occurring at the surfaces of the 40% nickel/iron alloy platelets formed during reduction. The rate controlling process had an activation energy of 11kcal/mole.During extraction of metallic nickel, the dissolution of magnesium follows a “shrinking core” kinetic model, which assumes the reaction is unimpeded by a surface layer of silica - a reaction product. Rates of magnesium dissolution during this stage of the reaction (up to 25% dissolved) were chemically controlled with an activation energy of 12 kcal/mole.At magnesium extractions above 25%, at which point all the metallic nickel had been extracted, the rate of silicate attack was limited by diffusion through a silica coating attached to particle surfaces.The results indicate that selectivity for nickel dissolution over that of magnesium does not depend strongly on acidity and temperature at the levels investigated.
The application of heap leach technology to recovery of economically important metals, notably copper, gold, silver, and uranium, is wide-spread in the mining industry. Unique to heap leaching is the relatively coarse particle size, typically 12–25 mm top size for crushed and agglomerated ores and larger for run-of-mine dump leaching operations. Leaching from such large particles is commonly assumed to follow shrinking core type behaviour, although little evidence for the validity of this assumption exists. This review investigates the current state of knowledge with respect to the understanding of the characteristics and mineralogy of large particles and how these influence leaching in a heap context and the tools to characterize these. This includes the study of ore and particle properties, visualization techniques for ore characterization, the connection between comminution and leaching behaviour, as well as particle models within heap leach modelling. We contend that the economics of heap leaching are strongly governed by the trade-off between the slow rate and limited extent of leaching from large particles and the cost of crushing finer. A sound understanding of the underlying large particle effects will therefore greatly inform future technology choices in the area of heap leaching.
Reclaimed landscapes after oil sands mining have saline soils; yet, they are required to have similar biodiversity and productivity as the predisturbance nonsaline landscape. Given that many species in the boreal forest are not tolerant of salinity, we studied the effects of soil salinity on plant communities in natural saline landscapes to understand potential plant responses during the reclamation process. Vegetation–soil relationships were measured along transects from flooded wetlands to upland forest vegetation in strongly saline, slightly saline, nonsaline, and reclaimed boreal landscapes. In strongly saline landscapes, surface soil salinity was high (>10 dS/m) in flooded, wet-meadow, and dry-meadow vegetation zones as compared to slightly saline (<5 dS/m) and nonsaline (<2 dS/m) landscapes. Plant communities in these vegetation zones were quite different from nonsaline boreal landscapes and were dominated by halophytes common to saline habitats of the Great Plains. In the shrub and forest vegetation zones, surface soil salinity was similar between saline and nonsaline landscapes, resulting in similar plant communities. In strongly saline landscapes, soils remained saline at depth through the shrub and forest vegetation zones (>10 dS/m), suggesting that forest vegetation can establish over saline soils as long as the salts are below the rooting zone. The reclaimed landscape was intermediate between slightly saline and nonsaline landscapes in terms of soil salinity but more similar to nonsaline habitats with respect to species composition. Results from this study suggest it may be unrealistic to expect that plant communities similar to those found on the predisturbance landscape can be established on all reclaimed landscapes after oil sands mining.
Preferential flow in dump leaching of low-grade ores was investigated in this paper. The production of the dump leaching plant at Dexing Copper Mining in China was significantly influenced by preferential flow due to the heterogeneity of the ores in dump. Surface tension, negative pore water pressure, matrix suction and permeability of dump were found to be correlated to particle size. Preferential flow, which occurred in the fine and coarse region, was determined by solution application rate. Experiments showed that preferential flow happened in the fine region at an application rate below 1070 L/m2 min. Experiment of solute transportation under preferential flow was conducted by selecting NaCl as solute. The NaCl concentration of the outflow from the coarse region was greater than that of the fine region. The leaching rate increased slowly as the leaching process proceeded because NaCl dissolved in the immobile water could only be leached through diffusion. The percentage recovery of coarse region surpassed 100%. The preferential flow within the fine region under the condition of small application rate was confirmed.
Although heap leaching has become established as the technology for treatment of some copper, gold, and zinc minerals, as well as its extension to the treatment of other types of minerals such as saltpeter and mine tailings, little study has been made on the optimization of this technology. Usually the operation of heap leaching is carried out until the maximum recovery has been obtained, or until observing that the concentration in the output solution of the heap exhibits no further extraction. The present study makes an analysis to determine if these criteria present the best conditions from the economic standpoint. Two variables are examined including, (1) leaching time, and (2) height of the heap, at a copper mineral treatment plant. The results of the study showed that the design (height of the heap), and planning of the operation (operational time) were interactive factors, and that maximum recovery was not necessarily the best measure of operational efficiency based on economic considerations.
A two-dimensional dynamic model for bioleaching of secondary copper minerals from a pile has been developed. In the model. aeration of the pile is considered to be due to natural convection caused by the density gradient in the air within the bed. The rate of sulphide mineral dissolution is modelled according to the unreacted core model. The transport of ferric ions from the particle surface to the reaction zone is calculated considering film diffusion, diffusion within the particle and reaction kinetics. The rate of oxidation of the ferrous ion by bacteria attached to the ore surface is modelled using the Michaelis-Menten relationship. The influences of temperature, dissolved ferric iron and dissolved oxygen in the leaching solution are considered in the kinetic formulation. The set of partial differential equations is solved using the FEMLAB(R) software. The model was used to study the influence of process variables on copper recovery in the bed with time. This model is a useful tool to aid the design and optimisation of industrial operations.
