The aluminothermic production includes two processes: preparing chromium oxide from chromium ore and preparing metallic chromium by reducing chromium oxide with aluminum. The main process flow is shown in figure 1.
The chromite prepared by chromium oxide is ground to 160 ~ 200 mesh, mixed with soda ash and dolomite, oxidized and roasted at1050 ~150℃, then leached with water (see leaching) and filtered to obtain a solution containing Na2CrO4 more than 200 g/L. Adjust the pH of the solution to 4 0.2 with sulfuric acid, and then filter out Na2SO4 crystals to obtain sodium dichromate (Na2Cr2O7) solution. Concentrate the solution until it contains about Na2Cr2O7 1 100g/L, cool and filter out Na2SO4 crystals, then concentrate the solution until it contains na2cr2o71500 ~1550g/l, and keep the temperature at 90 ~1l. Sodium chromate can also be converted into sodium dichromate by carbonic acid method, that is, gas containing 50%CO2 is introduced at 15 ~ 16 atmospheric pressure, and the precipitated precipitate is sodium bicarbonate: 2na2cro4+2co2+H2O → Na2Cr2o7+2nahco3 sodium bicarbonate can be recycled. This method can recover half of the soda ash added during roasting, which is more conducive to obtaining sodium sulfate than sulfuric acid method, but sodium chromate can not be completely converted into sodium dichromate.
Chromium trioxide can be prepared by: ① ammonium chloride reduction method. That is to say, a certain amount of ammonium chloride is added into sodium dichromate crystals, mixed uniformly and reduced in a reduction furnace at 700-800℃, then NaCl is washed off, filtered to obtain a filter cake of chromium trioxide, dried, crushed and calcined in a rotary kiln at1150-1200℃. The chromium trioxide obtained by this method is dark green with large particles and high purity, but there are many production processes and harmful gas HCl is produced. ② Calcined chromic anhydride method. Namely, adding sodium dichromate into a reaction tank and injecting concentrated sulfuric acid, and reacting the sodium dichromate with sulfuric acid at 200 DEG C to generate chromic anhydride;
After na2cr2o7+2h2so4 ─→ 2cro3+2nahso4+H2O stood, chromic anhydride and sodium bisulfate precipitated into two layers. Scoop out the sodium bisulfate, reheat the chromic anhydride left in the pot, wash off the residual sodium sulfate with water, and release chromic anhydride from the bottom. Chromic anhydride is calcined and decomposed at 800 ~ 950℃, and the undecomposed chromic anhydride is washed with water and filtered to obtain chromic trioxide. The method has few working procedures, but the product has high impurity content. ③ Calcined chromium hydroxide method. That is, the solution containing more than 200 g/L Na2CrO4 is heated to more than 95℃, pure sodium sulfide solution is added, and large particles of chromium hydroxide Cr(OH)3 are precipitated after stirring. Chromium hydroxide is calcined in a rotary kiln at 1300℃ and decomposed into chromium trioxide Cr2O3. This method has less working procedures, low product cost and high purity, but the particles are fine and easy to lose.
The thermite reduction requires that the content of Cr2O3 in raw materials is more than 99%, the content of sulfur is less than 0.02%, and the contents of lead, arsenic, tin and antimony are less than 0.00 1% respectively. The particle size of aluminum particles should be less than 0.5 mm, and the amount of aluminum should not exceed 98% of the theoretical amount. Nitrite, magnesium chips and aluminum particles are used as kindling materials.
The reaction enthalpy Δ δH =-65.0 kcal/mol (aluminum). In order to keep the autothermal reaction process, metal particles can be successfully separated from the slag, and δH-δ should be at least -72 kcal/mol (aluminum), and oxygen supply agents such as sodium nitrate, potassium chlorate, chromic anhydride or alkali dichromate can be added to supplement the heat. The mixture can also be preheated to 350 ~ 400℃ before entering the furnace. The reduction reaction is carried out in a conical furnace barrel with built-in magnesia bricks. Firstly, part of the mixed burden is added into the furnace, and an igniter is added in the center of the burden surface. After ignition, when the burden begins to react, the remaining burden is continuously sent to the chute. Terminate the reaction, cool to room temperature, disassemble the furnace tube, take out the metal ingot, and remove the slag inclusion and oxide film on the surface by sandblasting. The production of large metal ingots can improve the recovery rate of chromium and the fluidity of slag. The purity of chromium metal can be more than 98.5% by aluminothermic method, in which the aluminum content is not more than 0.5%. The slag contains up to 90% Al2O3, which can be used as grinding material.
Electrolytic production generally takes ferrochrome as raw material and adopts chromite ammonium alum electrolysis process (Figure 2). Crushing ferrochrome, dissolving it in the mixed solution of electrolytic anode return liquid, crystallization mother liquid and sulfuric acid, filtering to remove silicate and other residues, and treating the filtrate with ammonium sulfate to remove iron. After aging (65438 05 days at 30 ~ 35℃), pure chromite Cr2 (SO4) 3 (NH4) 2SO4 2H2O crystallized. Pure chrome ammonium alum is dissolved in hot water and sent to a diaphragm electrolyzer for electrolysis. Stainless steel as cathode, lead-silver alloy (1%Ag) as anode, current density 753 A/m2, cell voltage 4.2 V, electrolyte temperature 52 ~ 54℃. The flow of electrolyte into the anode chamber should be controlled, and the pH value of the solution should be accurately controlled at 2. 1 ~ 2.4. The average current efficiency is 45%, and the power consumption is about18.5 kw h/kg. The purity of the product is 99.2 ~ 99.4%, the oxygen content is 0.3 ~ 0.5%, and it is fragile. In order to improve the purity of chromium, the oxygen content can be reduced by vacuum treatment or hydrogen reduction. High purity metallic chromium (containing 0.0 1 ~ 0.02% oxygen) can be obtained by electrolysis with +6 valence chromium solution (current density 9500 A/m2, temperature 84 ~ 87℃), but the current efficiency is very low (6 ~ 7%).