Gangue leaching

Gangue- ----[ Leach j [Pyro-pr reatmcnt] j Pyro-processing j... 

Leaching Chemistry. The purpose of the leaching operation is to dissolve the desired mineral and separate it from the gangue material. The reaction should be selective and fast, the solvent inexpensive or easHy regenerated. Several leaching agents are commonly used. 

Decomposition with Bases. Alkaline decomposition of poUucite can be carried out by roasting poUucite with either a calcium carbonate—calcium chloride mix at 800—900°C or a sodium carbonate—sodium chloride mix at 600—800°C foUowed by a water leach of the roasted mass, to give an impure cesium chloride solution that is separated from the gangue by filtration (22). The solution can then be converted to cesium alum [7784-17-OJ, CS2SO4 Al2(S0 2 24H20. Extraction of cesium from the poUucite is almost complete. Solvent extraction of cesium carbonate from the cesium chloride solution using a phenol in kerosene has also been developed (23). 

The reaction mixture is filtered. The soHds containing K MnO are leached, filtered, and the filtrate composition adjusted for electrolysis. The soHds are gangue. The Cams Chemical Co. electrolyzes a solution containing 120—150 g/L KOH and 50—60 g/L K MnO. The cells are bipolar (68). The anode side is monel and the cathode mild steel. The cathode consists of small protmsions from the bipolar unit. The base of the cathode is coated with a corrosion-resistant plastic such that the ratio of active cathode area to anode area is about 1 to 140. Cells operate at 1.2—1.4 kA. Anode and cathode current densities are about 85—100 A/m and 13—15 kA/m, respectively. The small cathode areas and large anode areas are used to minimize the reduction of permanganate at the cathode (69). Potassium permanganate is continuously crystallized from cell Hquors. The caustic mother Hquors are evaporated and returned to the cell feed preparation system. 

The separation of solids from liquids forms an important part of almost all front-end and back-end operations in hydrometallurgy. This is due to several reasons, including removal of the gangue or unleached fraction from the leached liquor the need for clarified liquors for ion exchange, solvent extraction, precipitation or other appropriate processing and the post-precipitation or post-crystallization recovery of valuable solids. Solid-liquid separation is influenced by many factors such as the concentration of the suspended solids the particle size distribution the composition the strength and clarity of the leach liquor and the methods of precipitation used. Some important points of the common methods of solid-liquid separation have been dealt with in Chapter 2. 

Comminution leads to an increase in surface area and exposes the mineral grains to attack by the solvent. The mineral grains may not, however, be liberated completely from the associated gangue material by this process. As long as a direct contact between some portions of the mineral grain and the leachant is not inhibited or lost, the mineral leaching process proceeds in an uninterrupted manner. 

The changes that occur in minerals as a result of thermal pretreatments are wide and varied. Some examples are given in Table 5.5. The resources listed, in their untreated conditions, are not amenable to dissolution by common reagents. The treatments mentioned have converted them to altered forms, which dissolve more readily. In these examples only the valuable components of the different resources have been altered. There are however, examples where the gangue is converted into its insoluble or less-soluble forms. In such situations the leaching reagent loss is cut down. As an example, reference may be drawn to... 

It can be seen, therefore, that ferrous iron and chalcopyrite oxidation are acid-consuming reactions, while pyrite oxidation and iron hydrolysis are acid-producing reactions. Thus, whether the overall reaction in a dump is acid producing or acid-consuming depends on the relative proportions of chalcopyrite and pyrite and on the pH conditions. In practice, sulfuric acid additions to the leach solution applied to the dump are usually required to overcome the acid consuming reactions of the gangue minerals and to keep the pH in a suitable range, typically 2 to 2.4, to optimize bacterial activity and minimize iron hydrolysis. 

Micro-XRD and micro-XANES analyses showed that the earliest stage of sulfide alteration is marked by the progressive oxidation of sulfide-S to sulfate-S that is then rapidly leached out from the system. Sulfide oxidation starts from particle rims or from intra-grain microfractures and is accompanied by a progressive loss of sulfur sulfides are then pseudomorphically replaced by goethite and minor bemalite. In addition to sulfides, many gangue silicates are efficiently altered and only quartz is preserved within the altered layers. 

Gangue- -----1 Leach jPyro-pret reatment Pyro-processing ... 

The selective leaching of the nickel and cobalt from the gangue material is carried out in Pachuca-type leaching vessels with 98% sulfuric acid at temperatures between 450 and 500°F and with equivalent pressures between 400 and 600 psi. A typical leach liquor contains 6 gm/liter Ni, 0.6 gm/liter Co, 0.6 gm/liter Fe, 2.3 gm/liter Al, and 67 gm/liter SO4. Each batch is leached for about 90 min to extract 95% of the metal values. Preheating and heating of the ore slurry is by direct steam absorption in the reaction vessel. The barren residue is separated from the leach liquor by countercurrent decantation to a density of 55-60% solids. The solids are washed with very little dilution and about 99.5% of the soluble value are recovered. 

In contrast to the extensive data collected on trace-mineralized rocks between ore districts, less information is available on the arsenic contents of the ore deposits. Because iron sulfide minerals are typically gangue phases, they are commonly ignored in trace element studies of ore deposits, which tend to focus on ore minerals such as sphalerite and galena. Therefore, there have not been extensive analyses for arsenic in iron sulfides in many of the MVT ore districts or trace-mineralized areas. However, a few studies on trace element contents in iron sulfides from the Ozark region have been performed. Wu et al. (1996) analyzed 80 pyrite and marcasite samples from the Viburnum Trend of the Southeast Missouri Lead District and found arsenic in concentrations of 2 to 900 ppm. Bhati and Hagni (1980) also analyzed iron sulfide minerals from this area, but did not publish results for arsenic. Hagni (1993) described the relatively rare occurrence of nickel-arsenic-sulfide ores from the Magmont-West ore deposit of the Viburnum Trend. Leach et al. (1995) list arsenic as a trace constituent in ores from the Northern Arkansas and Southeast Missouri MVT ore districts, but without abundances specified. 

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