Hydrothermal solidification behavior of municipal solid waste incineration bottom ash without any additives

Municipal solid waste incineration (MSWI) bottom ash could be solidified with and without slaked lime (calcium hydroxide) addition by a hydrothermal method under steam pressure of 1.56 MPa at 200 °C for up to 72 h. Experimental results showed that CSH gel or tobermorite exerted a main influence on strength development, and without any additives CSH gel was easy to form, while slaked lime addition favored to form tobermorite. Tobermorite seemed to exert a larger effect on the strength development than CSH gel. Leaching results showed that the concentrations of heavy metals dissolved from the solidified specimens were effectively reduced after hydrothermal processing. The immobilization was mainly due to the tobermorite or CSH gel formation, and Pb²⁺ and Zn²⁺ seemed to be fixed more readily than Cr⁶⁺, which might be the reason that the structural Ca²⁺ within tobermorite or CSH gel was exchanged by Pb²⁺ and Zn²⁺ more easily than Cr⁶⁺. In addition, there existed a close relationship between leaching concentration and strength enhancement, and a higher strength seemed to exert a larger effect on immobilization of heavy metals.     

Solidification/stabilization of ash from medical waste incineration into geopolymers

In the present work, bottom and fly ash, generated from incinerated medical waste, was used as a raw material for the production of geopolymers. The stabilization (S/S) process studied in this paper has been evaluated by means of the leaching and mechanical properties of the S/S solids obtained. Hospital waste ash, sodium hydroxide, sodium silicate solution and metakaolin were mixed. Geopolymers were cured at 50 °C for 24 h. After a certain aging time of 7 and 28 days, the strength of the geopolymer specimens, the leachability of heavy metals and the mineralogical phase of the produced geopolymers were studied. The effects of the additions of fly ash and calcium compounds were also investigated. The results showed that hospital waste ash can be utilized as source material for the production of geopolymers. The addition of fly ash and calcium compounds considerably improves the strength of the geopolymer specimens (2–8 MPa). Finally, the solidified matrices indicated that geopolymerization process is able to reduce the amount of the heavy metals found in the leachate of the hospital waste ash.     

Stabilization of heavy metals in MSWI fly ash using silica fume

The objective of this work was to investigate the feasibility and effectiveness of silica fume on stabilizing heavy metals in municipal solid waste incineration (MSWI) fly ash. In addition to compressive strength measurements, hydrated pastes were characterized by X-ray diffraction (XRD), thermal-analyses (DTA/TG), and MAS NMR (²⁷Al and ²⁹Si) techniques. It was found that silica fume additions could effectively reduce the leaching of toxic heavy metals. At the addition of 20% silica fume, leaching concentrations for Cu, Pb and Zn of the hydrated paste cured for 7 days decreased from 0.32 mg/L to 0.05 mg/L, 40.99 mg/L to 4.40 mg/L, and 6.96 mg/L to 0.21 mg/L compared with the MSWI fly ash. After curing for 135 days, Cd and Pb in the leachates were not detected, while Cu and Zn concentrations decreased to 0.02 mg/L and 0.03 mg/L. The speciation of Pb and Cd by the modified version of the European Community Bureau of Reference (BCR) extractions showed that these metals converted into more stable state in hydrated pastes of MSWI fly ash in the presence of silica fume. Although exchangeable and weak-acid soluble fractions of Cu and Zn increased with hydration time, silica fume addition of 10% can satisfy the requirement of detoxification for heavy metals investigated in terms of the identification standard of hazardous waste of China.     

Water washing effects on metals emission reduction during municipal solid waste incinerator (MSWI) fly ash melting process

This study investigated that water washing effects on the metals emission reduction in melting of municipal solid waste incinerator (MSWI) fly ash. Experimental conditions were conducted at liquid-to-solid (L/S) ratio 10, 20, and 100 for water-washing process and its subsequent melting treatment at 1450 °C for 2 h. The simple water-washing process as a pre-treatment for MSWI fly ash can remove most of the chlorides, leachable salts, and amphoteric heavy metals from the MSWI fly ash, resulting in the washed ash having lowered chlorine content. MSWI fly ashes washed by L/S ratio 10 and above that were melted at 1450 °C produced slag containing relatively high vitrificaton ratio of Cu and Pb. Besides, the vitrification ratios of Na, K, Ca, and Mg in washed MSWI fly ash were also higher than that of MSWI fly ash. The results indicated that washed MSWI fly ash can reduce the emission of metallic chlorides during its subsequent melting treatment.     

The effects of the mechanical–chemical stabilization process for municipal solid waste incinerator fly ash on the chemical reactions in cement paste

A water extraction process can remove the soluble salts present in municipal solid waste incinerator (MSWI) fly ash, which will help to increase the stability of the synthetic materials produced from the MSWI fly ash. A milling process can be used to stabilize the heavy metals found in the extracted MSWI fly ash (EA) leading to the formation of a non-hazardous material. This milled extracted MSWI fly ash (MEA) was added to an ordinary Portland cement (OPC) paste to induce pozzolanic reactions. The experimental parameters included the milling time (96 h), water to binder ratios (0.38, 0.45, and 0.55), and curing time (1, 3, 7 and 28 days). The analysis procedures included inductively coupled plasma atomic emission spectroscopy (ICP/AES), BET, mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), and nuclear magnetic resonance (NMR) imaging. The results of the analyses indicate that the milling process helped to stabilize the heavy metals in the MEA, with an increase in the specific surface area of about 50 times over that of OPC. The addition of the MEA to the OPC paste decreased the amount of Ca(OH)₂ and led to the generation of calcium–silicate–hydrates (C–S–H) which in turned increased the amount of gel pores and middle sized pores in the cement. Furthermore, a comparison shows an increase in the early and later strength over that of OPC paste without the addition of the milled extracted ash. In other words, the milling process could stabilize the heavy metals in the MEA and had an activating effect on the MEA, allowing it to partly substitute OPC in OPC paste.     

Municipal incineration bottom ash treatment using hydrothermal solidification

Solidification of municipal incineration bottom ash (MIBA) has been carried out using a hydrothermal processing method, in which the MIBA was first compacted in a mold at 5-20 MPa, and then hydrothermally cured in an autoclave under saturated steam pressure at 150-250 degrees C for 10-72 h. Experimental results showed that the tensile strength of the solidified body was greatly influenced by the addition of NaOH solution and fresh cement in the MIBA. The hydrothermal curing temperature and time exerted a significant influence on the development of tensile strength of solidified body. The strength development is speculated to be due primarily to the formation of 1.1 nm tobermorite. Laboratory leaching tests were conducted to determine the amount of heavy metals dissolved from the solidified bodies and the results showed that under the hydrothermal conditions of this study the leaching of heavy metals was very low. As such, the hydrothermal processing method may have a high potential for recycling MIBA.     

Geotechnical characterization of a Municipal Solid Waste Incineration Ash from a Michigan monofill

A field and laboratory geotechnical characterization study of a Municipal Solid Waste Incineration Ash disposed of at the Carleton Farms monofill in Michigan was performed. Field characterization consisted of field observations, collection of four bulk samples and performance of shear wave velocity measurements at two locations. Laboratory characterization consisted of basic geotechnical characterization, i.e., grain size distribution, Atterberg limits, specific gravity tests, compaction tests as well as moisture and organic content assessment followed by direct shear and triaxial shear testing. The test results of this investigation are compared to results in the literature. The grain size distribution of the samples was found to be very similar and consistent with the grain size distribution data available in the literature, but the compaction characteristics were found to vary significantly. Specific gravities were also lower than specific gravities of silicic soils. Shear strengths were higher than typically reported for sandy soils, even for MSWI ash specimens at a loose state. Strain rate was not found to impact the shear resistance. Significant differences in triaxial shear were observed between a dry and a saturated specimen not only in terms of peak shear resistance, but also in terms of stress–strain response. In situ shear wave velocities ranged from 500 to 800 m/s at a depth of about 8 m, to 1100–1200 m/s at a depth of 50 m. These high shear wave velocities are consistent with field observations indicating the formation of cemented blocks of ash with time, but this “ageing” process in MSWI ash is still not well understood and additional research is needed. An improved understanding of the long-term behavior of MSWI ash, including the effects of moisture and ash chemical composition on the ageing process, as well as the leaching characteristics of the material, may promote unbound utilization of the ash in civil infrastructure.     

Porous materials produced from incineration ash using thermal plasma technology

This study presents a novel thermal plasma melting technique for neutralizing and recycling municipal solid waste incinerator (MSWI) ash residues. MSWI ash residues were converted into water-quenched vitrified slag using plasma vitrification, which is environmentally benign. Slag is adopted as a raw material in producing porous materials for architectural and decorative applications, eliminating the problem of its disposal. Porous materials are produced using water-quenched vitrified slag with Portland cement and foaming agent. The true density, bulk density, porosity and water absorption ratio of the foamed specimens are studied here by varying the size of the slag particles, the water-to-solid ratio, and the ratio of the weights of the core materials, including the water-quenched vitrified slag and cement. The thermal conductivity and flexural strength of porous panels are also determined. The experimental results show the bulk density and the porosity of the porous materials are 0.9–1.2 g cm^?3 and 50–60%, respectively, and the pore structure has a closed form. The thermal conductivity of the porous material is 0.1946 W m^?1 K^?1. Therefore, the slag composite materials are lightweight and thermal insulators having considerable potential for building applications.     

Effect of carbonation on the leaching of organic carbon and of copper from MSWI bottom ash

In Flanders, the northern part of Belgium, about 31% of the produced amount of MSWI bottom ash is recycled as secondary raw material. In view of recycling a higher percentage of bottom ash, a particular bottom ash fraction (Ø 0.1–2 mm) was studied. As the leaching of this bottom ash fraction exceeds some of the Flemish limit values for heavy metals (with Cu being the most critical), treatment is required.Natural weathering and accelerated carbonation resulted in a significant decrease of the Cu leaching. Natural weathering during 3 months caused a decrease of Cu leaching to <50% of its original value, whereas accelerated carbonation resulted in an even larger decrease (to ca. 13% of its initial value) after 2 weeks, with the main decrease taking place within the first 48 h.Total organic carbon decreased to ca. 70% and 55% of the initial concentration in the solid phase, and to 40% and 25% in the leachate after natural weathering and after accelerated carbonation, respectively. In the solid material the decrease of the Hy fraction was the largest, the FA concentration remained essentially constant. The decrease of FA in the leachate can be attributed partly to an enhanced adsorption of FA to Fe/Al (hydr)oxides, due to the combined effect of a pH decrease and the neoformation of Al (hydr)oxides (both due to carbonation). A detailed study of adsorption of FA to Fe/Al (hydr)oxides showed that significant adsorption of FA occurs, that it increases with decreasing pH and started above pH 12 for Fe (hydr)oxides and around 10 for Al (hydr)oxides. Depending whether FA or Hy are considered the controlling factor in enhanced Cu leaching, the decreasing FA or Hy in the leachate explains the decrease in the Cu leaching during carbonation.     

Full-scale blending treatment of fresh MSWI leachate with municipal wastewater in a wastewater treatment plant

Fresh leachate, generated in municipal solid waste incineration (MSWI) plants, contains various pollutants with extremely high strength organics, which usually requires expensive and complex treatment processes. This study investigated the feasibility of blending treatment of MSWI leachate with municipal wastewater. Fresh MSWI leachate was pretreated by coagulation–flocculation with FeCl₃ 2 g/L and CaO 25 g/L, plate-and-frame filter press, followed by ammonia stripping at pH above 12. After that, blending treatment was carried out in a full-scale municipal wastewater treatment plant (WWTP) for approximately 3 months. Different operational modes consisting of different pretreated leachate and methanol addition levels were tested, and their performances were evaluated. Results showed that throughout the experimental period, monitored parameters in the WWTP effluent, including COD (<60 mg/L), BOD₅ (<20 mg/L), ammonium (<8 mg/L), phosphorus (<1.5 mg/L) and heavy metals, generally complied with the Chinese sewage discharged standard. Under the experimental conditions, a certain amount of methanol was needed to fulfill TN removal. An estimation of the operation cost revealed that the expenditure of blending treatment was much lower than the total costs of respective treatment of MSWI leachate and municipal wastewater. The outcomes indicated that blending treatment could not only improve the treatability of the MSWI leachate, but also reduce the treatment cost of the two different wastewaters.     

The influence of pH on the leaching behaviour of inorganic components from municipal solid waste APC residues

The influence of pH on the leaching behaviour of air pollution control (APC) residues produced in municipal solid waste incineration (MSWI) is addressed in this study. The residue is considered hazardous waste, and in accordance with their chemical properties, the leaching of contaminants into the environment is the main concern. Several leaching tests can be used for research studies or regulatory purposes, where a wide variety of conditions may be tested. Our work deals mainly with the leaching behaviour of toxic heavy metals (Pb, Cd, Zn, Cr, Ni, Cu) and inorganics associated with soluble salts (Na, K, Ca, Cl). The main goal is to obtain an overview of the leachability of APC residues produced in a Portuguese MSWI process. Among the different variables that may have influence on the leaching behaviour, pH of the leachant solution is the most important one, and was evaluated through pH static tests. The acid neutralization capacity (ANC) of the residue was also determined, which is in the range of 6.2–6.8 meq g^?1 (for pH = 7) and 10.1–11.6 meq g^?1 (for pH = 4). The analysis of the leaching behaviour is particularly important when the leaching is solubility controlled. The amphoteric behaviour of some elements was observed, namely for Pb and Zn, which is characterized through high solubilization at low and high pH and moderate or low solubility at neutral or moderate high pH. The solubility curves for Pb, Cd, Zn, Cr, Ni and Cu as a function of pH were obtained, which are very useful for predicting the leaching behaviour in different scenarios. The solubility of K and Na reveals to be nearly independent of the solution pH and the released amount is mainly availability-controlled. Moreover, the pH static test showed that Cl^? is the most pH-independent species. The APC residue turns out to be a hazardous waste because of the high leaching of lead and chloride. On the other hand, leaching of elements like cadmium, nickel and copper is limited by the high pH of the residue, and as long as the waste keeps its ANC, the risk of mobilization of these elements is low.     

Chemical stabilization of medical waste fly ash using chelating agent and phosphates: Heavy metals and ecotoxicity evaluation

Chemical stabilization of heavy metals in medical waste fly ash has been carried out using the following compounds: a chelating agent (Ashnite S803), a commercial acidic phosphoric acid solution (Ashnite R303) as well as basic one (Ashnite R201). In order to predict the leachability of heavy metals, Japanese Leaching Test (JLT-13) procedure was applied to the stabilized fly ash products. An ecotoxicity assessment of the stabilized fly ash products leachate and the unstabilized fly ash leachate was conducted using a battery of bioassays based on lettuce root elongation inhibition, Daphnia magna mortality and Vibrio fischeri photoinhibition. The results showed that the three stabilizing agents were able to significantly decrease (ANOVA, P < 0.05) the concentration of heavy metals in the leachates. Although the leachate from both stabilized and unstabilized fly ash were very toxic to lettuce and daphnids, the incorporation of these stabilizing agents diminished significantly (ANOVA, P < 0.05) the toxicity of the leachates towards the three tested organisms. Pearson correlation analysis was used to analyze the strength of the relationship between chemical elements concentration in the leachate and bioassays results. Most of the heavy metals in the leachate were significantly correlated (ANOVA, P < 0.05) with the toxicity values of the bioassays. However, the correlation was not found between the concentration of dissolved organic carbon (DOC) and the toxicity effect of the leachate to the tested organisms.     

An LCA model for waste incineration enhanced with new technologies for metal recovery and application to the case of Switzerland

A process model of municipal solid waste incinerators (MSWIs) and new technologies for metal recovery from combustion residues was developed. The environmental impact is modeled as a function of waste composition as well as waste treatment and material recovery technologies. The model includes combustion with a grate incinerator, several flue gas treatment technologies, electricity and steam production from waste heat recovery, metal recovery from slag and fly ash, and landfilling of residues and can be tailored to specific plants and sites (software tools can be downloaded free of charge). Application of the model to Switzerland shows that the treatment of one tonne of municipal solid waste results on average in 425 kg CO₂-eq. generated in the incineration process, and 54 kg CO₂-eq. accrue in upstream processes such as waste transport and the production of operating materials. Downstream processes, i.e. residue disposal, generates 5 kg CO₂-eq. Savings from energy recovery are in the range of 67 to 752 kg CO₂-eq. depending on the assumptions regarding the substituted energy production, while the recovery of metals from slag and fly ash currently results in a net saving of approximately 35 kg CO₂-eq. A similar impact pattern is observed when assessing the MSWI model for aggregated environmental impacts (ReCiPe) and for non-renewable resource consumption (cumulative exergy demand), except that direct emissions have less and no relevance, respectively, on the total score. The study illustrates that MSWI plants can be an important element of industrial ecology as they provide waste disposal services and can help to close material and energetic cycles.     

Thermal and hydrometallurgical recovery methods of heavy metals from municipal solid waste fly ash

Heavy metals in fly ash from municipal solid waste incinerators are present in high concentrations. Therefore fly ash must be treated as a hazardous material. On the other hand, it may be a potential source of heavy metals. Zinc, lead, cadmium, and copper can be relatively easily removed during the thermal treatment of fly ash, e.g. in the form of chlorides. In return, wet extraction methods could provide promising results for these elements including chromium and nickel. The aim of this study was to investigate and compare thermal and hydrometallurgical treatment of municipal solid waste fly ash. Thermal treatment of fly ash was performed in a rotary reactor at temperatures between 950 and 1050 °C and in a muffle oven at temperatures from 500 to 1200 °C. The removal more than 90% was reached by easy volatile heavy metals such as cadmium and lead and also by copper, however at higher temperature in the muffle oven. The alkaline (sodium hydroxide) and acid (sulphuric acid) leaching of the fly ash was carried out while the influence of temperature, time, concentration, and liquid/solid ratio were investigated. The combination of alkaline-acidic leaching enhanced the removal of, namely, zinc, chromium and nickel.     

Greenhouse gas emissions from MSW incineration in China: Impacts of waste characteristics and energy recovery

Determination of the amount of greenhouse gas (GHG) emitted during municipal solid waste incineration (MSWI) is complex because both contributions and savings of GHGs exist in the process. To identify the critical factors influencing GHG emissions from MSWI in China, a GHG accounting model was established and applied to six Chinese cities located in different regions. The results showed that MSWI in most of the cities was the source of GHGs, with emissions of 25–207 kg CO₂-eq t^?1 rw. Within all process stages, the emission of fossil CO₂ from the combustion of MSW was the main contributor (111–254 kg CO₂-eq t^?1 rw), while the substitution of electricity reduced the GHG emissions by 150–247 kg CO₂-eq t^?1 rw. By affecting the fossil carbon content and the lower heating value of the waste, the contents of plastic and food waste in the MSW were the critical factors influencing GHG emissions of MSWI. Decreasing food waste content in MSW by half will significantly reduce the GHG emissions from MSWI, and such a reduction will convert MSWI in Urumqi and Tianjin from GHG sources to GHG sinks. Comparison of the GHG emissions in the six Chinese cities with those in European countries revealed that higher energy recovery efficiency in Europe induced much greater reductions in GHG emissions. Recovering the excess heat after generation of electricity would be a good measure to convert MSWI in all the six cities evaluated herein into sinks of GHGs.     

Metal toxicity assessment of mobile phone parts using Milli Q water

Environmentally safe disposal of end-of-life (EoL) or discarded mobile phone is a serious problem on account of their ever increasing number and toxic metals contents. In the present work, metal toxicity of mobile phone plastics, printed wire boards (PWBs) and batteries were assessed through dynamic batch leaching using Milli Q (MQ) water. Phone plastics failed Toxicity Characterization Leaching Procedure (TCLP) and Waste Extraction Test (WET) for Pb as the cumulative amount of Pb leached from plastics (5.33 mg/l) exceeded the regulatory limits (5.0 mg/l) used in characterizing a waste as hazardous. Similarly, the average cumulative amount (21.83 mg/l) of Ni leached from PWBs exceeded the regulatory limit of 20 mg/l and thus PWBs failed WET. Metals leached from batteries in small amounts (Cr: 0.40 mg/l and Ni: 0.15 mg/l). The presence of Fe in the batteries and its precipitation as oxides/hydroxides in the leaching solution hindered the leaching of other metals in MQ water. Both plastics and PWBs should be treated as hazardous waste and should not be disposed in open landfills. Further, MQ water leaching could provide good simulation of metals leaching from the mobile phones disposed at landfill sites.     

Dioxin emissions from municipal solid waste incinerators (MSWIs) in France

The objective of this study was to determine whether the fear of dioxin/furan emissions from waste-to-energy plants was justified by the 2007 status of emissions of French municipal solid waste incinerators (MSWIs). All emissions were examined, plant by plant, but this paper focuses on the incinerator emission that is most frequently mentioned in the French media, toxic dioxins and furans. The study showed that there are 85 large MSWI that generate electricity or heat, i.e., waste-to-energy (WTE) plants, and 39 smaller MSW incinerators. The results showed that all French MSWI are operated well below the EU and French standard of 0.1 ng TEQ Nm^?3 (toxic equivalent nanograms per standard cubic meter) and that their total dioxin/furan emissions decreased from 435 g TEQ in 1997 to only 1.2 g in 2008. All other industrial emissions of dioxins have also decreased and the major source is residential combustion of wood (320 g TEQ). It was extremely difficult to obtain MSWI emission data. This unwarranted lack of transparency has resulted in the public perception that MSWI plants are major contributors to dioxin emissions while in fact they have ceased to be so.     

Recycling municipal incinerator fly- and scrubber-ash into fused slag for the substantial replacement of cement in cement-mortars

Fly- and scrubber-ash (weight ratio of approximately 1:3) from municipal solid waste incinerators (MSWI) are a major land-fill disposal problem due to their leaching of heavy metals. We uniformly mixed both types of ash with optimal amounts of waste glass frit, which was then melted into a glassy slag. The glassy slag was then pulverized to a particle size smaller than 38 μm for use as a cement substitute (20–40% of total cement) and blended with sand and cement to produce slag-blended cement-mortar (SCM) specimens. The toxicity characteristics of the leaching procedure tests on the pulverized slag samples revealed that the amount of leached heavy metals was far below regulatory thresholds. The compressive strength of the 28-day cured SCM specimens was comparable to that of ordinary Portland cement mortars, while the compressive strength of specimens cured for 60 or 90 days were 3–11% greater. The observed enhanced strength is achieved by Pozzolanic reaction. Preliminary evaluation shows that the combination of MSWI fly- and scrubber-ash with waste glass yields a cost effective and environmentally friendly cement replacement in cement-mortars.     

Hydrothermal treatment of MSWI bottom ash forming acid-resistant material

To recycle municipal solid waste incinerator (MSWI) bottom ash, synthesis of hydrothermal minerals from bottom ash was performed to stabilize heavy metals. MSWI bottom ash was mixed with SiO(2), Al(OH)(3), and Mg(OH)(2) so its chemical composition was similar to that of hydrothermal clay minerals. These solid specimens were mixed with water at a liquid/solid ratio of 5. The reaction temperature was 200 degrees C, and reactions were performed for 24-240h. Generation of kaolinite/smectite mixed-layer clay mineral was found in the samples after the reaction of the mixture of bottom ash, SiO(2), and Mg(OH)(2). Calcium silicate hydrate minerals such as tobermorite and xonotlite were also generated. X-ray powder diffraction suggested the presence of amorphous materials. Leaching tests at various pHs revealed that the concentration of heavy metals in the leachates from MSWI bottom ash hydrothermally treated with SiO(2) and Mg(OH)(2) was lower than that in leachates from non-treated bottom ash, especially under acid conditions. Hydrothermal treatment with modification of chemical composition may have potential for the recycling of MSWI bottom ash.     

Recovery of lithium and cobalt from waste lithium ion batteries of mobile phone

In view of the stringent environmental regulations, availability of limited natural resources and ever increasing need of alternative energy critical elements, an environmental eco-friendly leaching process is reported for the recovery of lithium and cobalt from the cathode active materials of spent lithium-ion batteries of mobile phones. The experiments were carried out to optimize the process parameters for the recovery of lithium and cobalt by varying the concentration of leachant, pulp density, reductant volume and temperature. Leaching with 2 M sulfuric acid with the addition of 5% H₂O₂ (v/v) at a pulp density of 100 g/L and 75 °C resulted in the recovery of 99.1% lithium and 70.0% cobalt in 60 min. H₂O₂ in sulfuric acid solution acts as an effective reducing agent, which enhance the percentage leaching of metals. Leaching kinetics of lithium in sulfuric acid fitted well to the chemical controlled reaction model i.e. 1 ? (1 ? X)^1/3 = k_ct. Leaching kinetics of cobalt fitted well to the model ‘ash diffusion control dense constant sizes spherical particles’ i.e. 1 ? 3(1 ? X)^2/3 + 2(1 ? X) = k_ct. Metals could subsequently be separated selectively from the leach liquor by solvent extraction process to produce their salts by crystallization process from the purified solution.