Experimental Study On Extraction Of Chromium From Chromium Residue

As early as 2000, the chromium salt production of China is the world’s largest and currently occupies half of the global market. Chromium salt industry produced a mount of solid waste each year and posed a serious challenge to the society and the environment. According to the chromic salt production of our country which is about 400,000 tons each year, the annual emissions of chromium slag is about 800,000 tons, chromium slag of containing hexavalent chromium compounds is a kind of carcinogen that can harm the DNA of humans and animals.At present, the domestic processing technology is mainly divided into detoxification of chromium slag and chromium slag comprehensive utilization of resources. The detoxification of chromium slag include stockpiled technical method, curing method and reduction method and so on. The resources comprehensive utilization technology include sintered iron, preparation of microcrystalline glass, etc. China and abroad has began to research about how to extract chromium from chromium slag, the research of how to systematically and efficiently use chromium slag is little. This study analyzed a Yunnan company’s plating chromium slag and discovered the copper and nickel content are higher. These elements are valuable. Therefore, this paper study extraction technology of chromium from chromium slag of a Yunnan company. Extraction study of this project provides an effective way for comprehensive utilization of chromium residue. It achieves the detoxification of chromium slag and resource utilization。The main topic of this paper is chromium leaching and the preparation of chromate product. We study the optimum conditions of chromium slag basic roasting, leaching chromium with an aqueous solution of and preparing lead chromates from leaching solution.First of all, electroplating chromium slag of a Yunnan company, oxidizing agent sodium nitrate, additive sodium hydroxide and sodium carbonate were mixed, then roasted at a high temperature, which make the insoluble chromium trioxide become water soluble chromium so as to easily extracting chromium. The roasting temperature, roasting time, the amount of adding sodium hydroxide, the amount of adding sodium carbonate, the amount of adding sodium nitrate and other affective factors were studied and obtained the optimum conditions of basic roasting stage were roasting temperature at 600 ℃, roasting time 2.5h, the amount of adding sodium hydroxide is Og, m (sodium carbonate):m (chromium)= 2:5, m (sodium nitrate):m (chromium)= 3:5. At these process conditions, the leaching rate can reach 91.38%.Secondly, the alkaline roasting slag was leached with sodium carbonate solution. Sodium carbonate solution concentration, solid-liquid ratio, temperature, time, lotion volume, stirrer speed and other factors was researched and obtained the optimum conditions chromium leaching stage were sodium carbonate leaching solution concentration of 0g/L, solid-liquid ratio of 1:6, leaching temperature 80 ℃, leaching time 2h, wash volume of 30mL/times (3 times), stirrer speed 300r/min. Under these conditions, the chromium leaching rate was 93.94%. By characterizing of the leaching residue, results have shown that when the leaching of chrome was completed, the copper and nickel in slag was enriched at the same time.Then, silicon and other impurities were removed from leaching solution by adjusting PH with nitric acid. Lead chromate was prepared by adding the lead nitrate solution, and conducted the experiments of the influences of different PH conditions on lead chromate products. When PH= 7, the lead chromate prepared is yellow-orange, which the purity is higher, and the product yield is 92.99%. At higher PH, there will be aluminum silicate and oxide of lead impurity. The lower PH will form lead oxide impurity.Finally, the experiments of preparing lead chromate was studied by adding lead nitrate to leach liquor at PH= 7, and temperature, time, rotating speed, material ratio and other factors were studied. The optimal process conditions of the largest yield as follows:reaction temperature of 60 ℃, the reaction time of 1 h, the rotor speed of 400 r/min, mol (lead nitrate):mol (leach liquor containing chromium)= 1:1. Under the process conditions, the yield of lead chromate is 94.90%. XRD pattern shows that when the lead nitrate is excessive, chromium oxide will be produced.