Chrome iron ore (FeO · Cr 2 O 3) is a metallurgical raw material in inorganic salt industry, chemical methods may be processed into a series of chromite chromium compound. There are more than 30 kinds of chrome ore, among which the industrial value is mainly chromite. In 1981, the world's chrome ore mining volume was about 9Mt, of which the consumption in the chemical industry accounted for about 20% of the total consumption. South Africa is the largest mining area, with a production of 3.09 Mt in 1981.
In 1850, Russia set up a large-scale (1000-1500t per year) dichromate plant. In 1885, the Baltimore chromium plant in the United States began producing sodium dichromate, and in some countries in Europe, chromium compounds were also produced.
Processing method: Chromium is an amphoteric substance, which exists in a multivalent state. The chemical processing methods of chromite are alkaline oxidation method and reduced ferrochrome method. The former is a method commonly used in industry. Both methods are prepared by first preparing sodium chromate and further processing into a series of chromium compounds.
Alkaline oxidation method: divided into sodium alkali method and potassium base method, mainly sodium alkali method (mainly using soda ash). In the actual production, in order to reduce the consumption of soda ash and to reduce the sintering phenomenon during baking, calcareous fillers may be calcined. Although the soda ash oxidizing roasting method has a large amount of waste slag and is difficult to handle, it is widely used due to factors such as low alkali consumption, economical rationality, and stable production. This method requires that the content of chromium oxide in chromite should be not less than 35%, and the smaller the content of impurities such as silicon and aluminum , the better, because the presence of these impurities will increase the alkali consumption, lower the melting point of the charge, affect the conversion rate and product quality. The ore powder of about 200 mesh with most impurities removed by beneficiation is mixed with soda ash and calcareous filler in a certain proportion, and then granulated and sent to the rotary kiln (see kiln) at a high temperature of 1100 to 1200 °C. The mixture is calcined in an oxidizing atmosphere for about 1 to 4 hours, cooled, pulverized, and then leached with water, filtered, and purified to obtain a sodium chromate solution. The main reactions during the roasting process are:
4(FeO·Cr 2 O 3 )+8Na 2 CO 3 +7O 2 -→8Na 2 CrO 4 +2Fe 2 O 3 +8CO 2
It is important to control the ratio of various materials and the roasting conditions during the processing. The ratio is determined by the quality of the ore and the content of harmful impurities, and the ratio of the materials also affects the choice of roasting conditions. Oxidation roasting is a key process of the process, directly affecting the conversion rate, yield and economic benefits of chromium.
Reduction of ferrochrome: first reduction of chromite to obtain ferrochrome. It is processed from ferrochrome into sodium chromate. There are two methods of calcination oxidation and electrolytic oxidation: calcination oxidation is to oxidize and calcining ferrochrome and soda ash (or caustic soda) at 800 ° C, oxidizing chromium in ferrochrome to hexavalent, forming sodium chromate, and separating from iron; Electrolytic oxidation uses ferrochrome as an anode and lead as a cathode. It is electrolytically oxidized in an alkali solution, and the anode is continuously consumed to be oxidized to sodium chromate.
Product use: The most industrially valuable among the chromium compounds are the sodium and potassium salts of chromic acid, especially sodium dichromate and chromic anhydride, in addition to potassium dichromate (K 2 Cr 2 O 7 ), chromium sulfate. Potassium, basic chromium sulfate, etc., are widely used.
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