Alkali leaching

Alkali leach methods are exemplified by the Bayer process for the preparation of pure a-AhOa for electrolysis (Section 17.5) from the mineral bauxite. Bauxite consists mainly of o -A10(0H) (diaspore) and/or 7-A10(0H) (boehmite), the difference between these being essentially that the oxygen atoms form hep and cep arrays, respectively. The chief contaminants are silica, some clay minerals, and iron(III) oxides/hydroxides, which impart a red-brown color to the mineral. Aluminum(III) is much more soluble than iron (III) or aluminosilicates in alkali, so that it can be leached out with aqueous NaOH (initially 10-15 mol L ) at 165°C under approximately 0.6 MPa pressure, leaving a red mud of iron (and other transition metal) oxides/hydroxides and aluminosilicates  

Formation of mineral deposits occurs through geochemical reactions which often concentrate and precipitate specific compounds quite locally and separately fi m others of sufficiently different chemical properties. In such cases, natural processes do much of the work of concentration and separation for us. If, however, the elements of interest have closely similar chemistries. 

---

W ter ndAlcohols. Silanes do not react with pure water or slightly acidified water under normal conditions. A rapid reaction occurs, however, in basic solution with quantitative evolution of hydrogen (3). Alkali leached from glass is sufficient to lead to the hydrolysis of silanes. 

In the simulation, CO2 in the soil gas reacts with the feldspars, leading to the alkali leaching and separation of silica from alumina observed to result from soil weathering. Near the top of the profile, the reaction produces gibbsite and adds Na+, K+, and Si02(aq) to the migrating pore fluid, according to the reactions,... 

Alumet A process for extracting alumina and potassium sulfate from alunite ore (a basic hydrated potassium aluminum sulfate) involving reductive calcination and alkali leaching. Developed and piloted by the Alunite Metallurgical Company, UT, in the mid 1970s but not commercialized. See also Kalunite. 

Leaching of the alloy is usually done in strong alkali solution. Acid leaching has been examined however it generally results in lower activities compared to alkali leaching [44-46], The general equation for the leaching reaction can be written as... 

Can skeletal catalysts be prepared by acid leaching instead of alkali leaching Why is alkali leaching preferred ... 

Following the development of sponge-metal nickel catalysts by alkali leaching of Ni-Al alloys by Raney, other alloy systems were considered. These include iron [4], cobalt [5], copper [6], platinum [7], ruthenium [8], and palladium [9]. Small amounts of a third metal such as chromium [10], molybdenum [11], or zinc [12] have been added to the binary alloy to promote catalyst activity. The two most common skeletal metal catalysts currently in use are nickel and copper in unpromoted or promoted forms. Skeletal copper is less active and more selective than skeletal nickel in hydrogenation reactions. It also finds use in the selective hydrolysis of nitriles [13]. This chapter is therefore mainly concerned with the preparation, properties and applications of promoted and unpromoted skeletal nickel and skeletal copper catalysts which are produced by the selective leaching of aluminum from binary or ternary alloys. 

The results of leaching Illinois No. 6 coal with different alkaline solutions followed by acid treatment are shown in Table II. In each leaching experiment 15 g. of coal was leached with 120 ml. of alkaline solution at 250°C for 1 hr. In some experiments the alkali-leached coal was washed with hydrochloric acid, and in other experiments the coal was washed with sulfuric acid. When hydrochloric acid was employed, the acid washing step was conducted at the boiling point and the final water washing step at room temperature. However, when sulfuric acid was used, the washing steps were conducted at various temperatures to study the temperature effect. 

The difficulty of thermal treatment of slurry-phase catalysts has resulted in the predominant use of Raney metals and precious metals in the fine-chemical industry. Raney metals are produced from an aluminum alloy of the metal to be used as a catalytically active component. Treatment with alkali leaches aluminum from the alloy and leaves a very finely divided metal [10] as ca 10 to 20 nm metal particles clustered into conglomerates of several microns. Aluminum remaining in the metal after treatment with alkali protects the metal against oxidation. The aluminum reacts very slowly with the water in which the Raney metals are stored and... 

---