Soda ash roasting

Soda. Ash Roasting. Some of the first processes to recover selenium on a commercial basis were based on roasting of copper slimes with soda ash to convert both selenium and tellurium to the +6 oxidation state. Eigure 1 shows flow sheets for two such processes. Slimes are intensively mixed with sodium carbonate, a binder such as bentonite, and water to form a stiff paste. The paste is extmded or peUetized and allowed to dry. Care in the preparation of the extmdates or pellets is required to ensure that they have sufficient porosity to allow adequate access to the air required for oxidation. 

Fig. 1. Recovery of selenium by soda ash roasting of slimes (a) process 1 and (b) process 2.

United States production of, 22 793t, 794 Soda ash roasting, selenium recovery via, 22 79-81... 

Selenium is distributed widely in nature and is found in most rocks and soils at concentrations between 0.1 and 2.0 ppm (Fishbein 1983). However, elemental selenium is seldom found naturally, but it is obtained primarily as a byproduct of copper refining (Fishbein 1983). Selenium is contained in the constituents of the copper anode that are not solubilized during the copper refining process and ultimately accumulate on the bottom of the electrorefining tank. These constituents, usually referred to as slimes, contain roughly 5-25% selenium and 2-10% tellurium. Selenium is commercially produced by either soda ash roasting or sulfuric acid roasting of the copper slimes. 

All commercial processes for the production of selenium involve modifications or combinations of four fundamental methods, for example, smelting with soda ash, roasting with soda ash, direct oxidation, and roasting with sulfuric acid. Current world production of selenium is of the order of 2000 tons annually (Brown 1998),... 

Pure chromium metal is obtained commercially from the aluminothermic or silicother-mic reduction of chromium sesquioxide, or chromia (Cr Oj), and to a lesser extent by electrowinning. In the aluminothermic process, first the chromia is prepared by the soda-ash roasting of chromite ore. After mixing the raw chromite ore with sodium carbonate or soda... 

Historically, soda ash was produced by extracting the ashes of certain plants, such as Spanish barilla, and evaporating the resultant Hquor. The first large scale, commercial synthetic plant employed the LeBlanc (Nicolas LeBlanc (1742—1806)) process (5). In this process, salt (NaCl) reacts with sulfuric acid to produce sodium sulfate and hydrochloric acid. The sodium sulfate is then roasted with limestone and coal and the resulting sodium carbonate—calcium sulfide mixture (black ash) is leached with water to extract the sodium carbonate. The LeBlanc process was last used in 1916—1917 it was expensive and caused significant pollution. 

Recovery of Bismuth from Tin Concentrates. Bismuth is leached from roasted tin concentrates and other bismuth-beating materials by means of hydrochloric acid. The acid leach Hquor is clarified by settling or filtration, and the bismuth is precipitated as bismuth oxychloride [7787-59-9] BiOCl, when the Hquors are diluted usiag large volumes of water. The impure bismuth oxychloride is usually redissolved ia hydrochloric acid and reprecipitated by diluting several times. It is then dried, mixed with soda ash and carbon, and reduced to metal. The wet bismuth oxychloride may also be reduced to metal by means of iron or 2iac ia the presence of hydrochloric acid. The metallic bismuth produced by the oxychloride method requites additional refining. 

The chemical-grade ore, containing about 30% chromium, is dried, cmshed, and ground in ball mills until at least 90% of its particles are less than 75 ]lni. It is then mixed with an excess of soda ash and, optionally, with lime and leached residue from a previous roasting operation. In American and European practice, a variety of kiln mixes have been used. Some older mixes contain up to 57 parts of lime per 100 parts of ore. However, in the 1990s manufacturers use no more than 10 parts of lime per 100 parts of the ore, and some use no lime at all (77). The roasting may be performed in one, two, or three stages, and there maybe as much as three parts of leached residue per part of ore. These adaptations are responses to the variations in kiln roast and the capabihties of the furnaces used. 

Sodium chromate can be converted to the dichromate by a continuous process treating with sulfuric acid, carbon dioxide, or a combination of these two (Fig. 2). Evaporation of the sodium dichromate Hquor causes the precipitation of sodium sulfate and/or sodium bicarbonate, and these compounds are removed before the final sodium dichromate crystallization. The recovered sodium sulfate may be used for other purposes, and the sodium bicarbonate can replace some of the soda ash used for the roasting operation (76). The dichromate mother Hquor may be returned to the evaporators, used to adjust the pH of the leach, or marketed, usually as 69% sodium dichromate solution. 

When a potential is appHed across the ceU, the sodum and other cations are transported across the membrane to the catholyte compartment. Sodium hydroxide is formed in the catholyte compartment, because of the rise in pH caused by the reduction of water. Any polyvalent cations are precipitated and removed. The purified NaOH may be combined with the sodium bicarbonate from the sodium dichromate process to produce soda ash for the roasting operation. In the anolyte compartment, the pH falls because of the oxidation of water. The increase in acidity results in the formation of chromic acid. When an appropriate concentration of the acid is obtained, the Hquid from the anolyte is sent to the crystallizer, the crystals are removed, and the mother Hquor is recycled to the anolyte compartment of the ceU. The electrolysis is not allowed to completely convert sodium dichromate to chromic acid (76). Patents have been granted for more electrolytic membrane processes for chromic acid and dichromates manufacture (86). 

In the absence of soda ash, Se02 can be volatilized directly from the roast ... 

Vanadium usually is recovered from its ores by one of two processes, (1) leaching raw mineral with hot dilute sulfuric acid, and (2) roasting ore with common salt to convert vanadium into water soluble sodium vanadates. In the sulfuric acid leaching process, vanadium is extracted from acid leach liquors by solvent extraction with an aliphatic amine or an alkyl phosphoric acid in kerosene. The organic solvent extract then is treated with an aqueous solution of ammonia in the presence of ammonium chloride to convert vanadium into ammonium metavanadate. Alternatively, the organic extract is treated with dilute sulfuric acid or an aqueous solution of soda ash under controlled conditions of pH. Vanadium is precipitated from this solution as a red cake of sodium polyvanadate. 

In the process (Fig. 1), the ore is ground and mixed with ground limestone and soda ash, and roasted at approximately 1200°C in an oxidizing atmosphere. The sintered mass is crushed and leached with hot water to separate the soluble sodium chromate. The solution is treated with sulfuric acid to convert the sodium chromate to sodium dichromate plus sodium sulfate. Some of the sodium sulfate crystallizes in the anhydrous state from the hot solution during acidification as well as in the evaporators during concentration of the dichromate solution. From the evaporator, the hot, sat-... 

Chromium metal is commercially produced in the United States by the reduction of chromite ore with carbon, aluminum, or silicon, and subsequent purification. Sodium chromate and dichromate are produced by roasting chromite ore with soda ash. Most other chromium compounds are produced from sodium chromate and dichromate (Hartford 1979 Westbrook 1979). For example, basic chromic sulfate (Cr(0H)S04), commonly used in tanning, is commercially produced by the reduction of sodium dichromate with organic compounds (e.g., molasses) in the presence of sulfuric acid or by the reduction of dichromate with sulfur dioxide. Lead chromate, commonly used as a pigment, is produced by the reaction of sodium chromate with lead nitrate or by reaction of lead monoxide with chromic acid solution (IARC 1990). 

Derivation (1) Finely ground ore is roasted with sulfuric acid at 250C, lithium sulfate is leached from the mass and converted to the carbonate by precipitation with soda ash. (2) Reaction of lithium oxide with carbon dioxide or ammonium carbonate solution. 

Ultramarine A pigment obtained by powdering the product from roasting a mixture of kaolin, soda ash, sulphur and charcoal. 

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