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Nepheline syenite is primarily an alumino-silicate rock which contains a major amount of potassium and aluminum. After potassium recovery through chloridizing roasting followed by water leaching method, the roast leach residue of nepheline syenite was considered for the aluminum extraction process due to its ample aluminum content. Sulfuric acid is chosen over other acids as it prevents the leaching of calcium. A 2<^>3 full factorial design was employed to study the effects of acid concentration, time, and temperature. Furthermore, to enhance the recovery and to forbid the leaching of silicon, autoclave leaching was preferred.Iron is generally leached with aluminum in the acid leaching process which could be removed effectively by pH adjustment. The leached liquor obtained after purification is evaporated by using a hot water bath. The liquor concentrated on slow evaporation to a point where aluminum sulfate started precipitating. Then, the concentrated liquor was kept overnight at 8 degrees C in a cooling incubator. Aluminum sulfate was produced with 99.80% purity.

The third crucial macronutrient required for the development and growth of plants in addition to nitrogen and phosphorous is potassium. The quantity of soluble potassium in soil that could be directly taken up by plants is less and present in the form of evaporite deposits situated in countries lying in the Northern hemisphere such as Canada, Belarus, Israel, USA and Russia. All other countries are dependent on imports from these countries to fulfil their potash requirement. But a major part of potassium exists in insoluble form as silicate minerals abundantly in these countries which can compensate for the potassium deficit. Mostly feldspar and feldspathoid from the tectosilicate group and micaceous minerals from the phyllosilicate group are the common potassium bearing silicates containing 5–15% K2O. Tectosilicates are framework silicates in which potassium is present in the three-dimensional silicate tetrahedra. Phyllosilicates are sheet silicates having a two-dimensional parallel sheet structure and potassium ions are located in the interlayers of the silicate structure. Various processes have been adopted to release the potassium from these silicate resources such as bio-leaching using various algal, bacterial and yeast strains, acid leaching with different acids of varying concentrations, base leaching with diverse strategies and roast-leach studies with alkali and alkaline salts to optimise the recovery. This review summarizes versatile methodologies that have been approached and the scope of various works that could be further accomplished.

La, Ce and Tb were leached from a residue generated by lixiviating phosphor with 3.3 N H2SO4. Model equations to predict leaching extent of La, Ce and Tb were generated by factorial designing. At the same strength, HCl could leach more La, Ce and Tb than HNO3. Parameters considered for two-level full factorial design were HCl strength, temperature and pulp concentration. Dissolutions achieved at design conditions were entered into Minitab to formulate equation for leaching period of 0.5, 1, 1.5 and 2 h. Leaching achieved at random experimental conditions were within ± 5% of predicted values which established the validity of model equations. Conditions obtained by solving these equations for 99.9–100% leaching were provided in the form of surface plots. Ranges of HCl strength, temperature and pulp concentration for leaching more than 99% of La, Ce and Tb in 2 h were 46–63% v/v, 80–95 °C and 5–20% w/v, respectively. The equations were also utilized for generating data needed for kinetic studies which suggested that leaching was chemical reaction controlled.

To recover potassium from feldspar, a biowaste, i.e., eggshell, was used. The chief composition of eggshells is calcite. As it is a rich source of Ca, hence it is used with HCl to produce calcium chloride. Feldspar is an aluminosilicate mineral that bears potassium in the interstitial sites. To unlock the potassium from the interstitial sites, it was roasted with calcium chloride prepared by mixing eggshell and hydrochloric acid. At the roasting temperature, CaCl2 melts and penetrates into the aluminosilicate matrix to replace K with Ca. Potassium ion released from the silicate matrix combines with chloride ions to form potassium chloride, which solubilized in water during the leaching process of the roasted feldspar. For elucidation of the mechanism of the roasting process, the shrinking core model was applied to the roast-leach data, and diffusion through the product layer was inferred as the rate-determining step. The order of the roasting process was found to be 2.158 and activation energy calculated to be 155.3 kJ/mol. Apart from potassium, sodium and excess calcium also got co-leached. To recover potassium from the leach liquor selectively, sodium perchlorate was added to precipitate potassium as KClO4. Further, potassium perchlorate was thermally decomposed to give fertilizer grade potassium chloride (purity: 99.81%).

An integrated approach has been made to extract potassium and aluminium from a silicate mineral, i.e. nepheline syenite with 5.4% K2O, 19.9% Al2O3, 55.5% SiO2. Chloridising-roasting experiments were carried using CaCl2 at 900 degrees C for 1 h and water leaching at ambient temperature for 1 h. The leach liquor contains impurities like Ca and Na; hence, selective extraction of potassium was carried out through precipitation using perchloric acid at freezing temperatures. XRD data of the final KCl product obtained after decomposition of KClO4 indicated cubical crystal system and was found to be 99.5% pure. In the second stage, leaching of the residue was carried out with HCl, H2SO4 and H2SiF6. For selective aluminium recovery, sulphuric acid was found to be the better leachant. A 2(3) full factorial design with time, temperature and acid concentration was used. Finally, aluminium sulphate crystals were produced from the leach liquor after removing iron through pH adjustment.

This work focuses on extraction of potassium from feldspar through pyro-hydrometallurgical route. Calcium chloride was used as roasting agent for feldspar. Effect of various operating variables, viz. time, roasting temperature and CaCl 2 dose on extent of roasting, were thoroughly studied. Roasting 10 g of feldspar with 17 g of CaCl 2 at 900 °C for 1 h resulted in 99.9% conversion of K 2 O to KCl. Order of feldspar roasting was found to be around 2, and activation energy requirement was 136.6 kJ/mol. Calcium and sodium were leached along with potassium from chloride-roasted K-Feldspar with water at room temperature. To generate a pure solution of potassium from the leach liquor containing K, Ca and Na, it was selectively extracted from leach liquor by using dibenzo-18-crown-6 ether as extractant and m -cresol as diluent. Loading of potassium was carried out at an A / O ratio of 2:1, while stripping was carried out in water at O / A ratio of 2:1. 99.5% pure crystals of KCl were obtained on evaporation of the strip solution. A schematic flowsheet has been provided for the preparation of pure KCl from feldspar.