Application significance of bismuth oxide (bismuth trioxide) in water treatment and zinc hydrometallurgy

Application significance of bismuth oxide (bismuth trioxide) in water treatment and zinc hydrometallurgy
The hazards of chloride ions in water mainly include the following four aspects:
1. Impact on vegetation and crop growth: When the mass concentration of chloride ion in irrigation water reaches 142-355mg/L, some crops cannot synthesize protein, which endangers the normal growth of vegetation and crops. When the mass concentration of chloride ion is greater than 355mg/L, most crops and vegetation will be poisoned to death.
2. Corrosion: chloride ions in the solution can damage the passivation film on metal and alloy surface to varying degrees, causing intergranular corrosion, crevice corrosion, pitting corrosion, etc., affecting the normal operation of industrial equipment and causing potential safety hazards.
3. Toxicity: When the concentration of chloride in water is higher than 100mg/L, people can be poisoned to varying degrees after eating, affecting normal metabolism. When the chloride content is above 8g/kg, the biological function, diversity and microbial community structure in the soil will change significantly. When the chloride ion in the water exceeds 500mg/L, a large number of fish will die.
4. Impact on the normal life of the building: when the chloride ion content in the concrete is large, the reinforcement will be corroded, which will cause the concrete to expand and loosen, reduce its chemical corrosion resistance, wear resistance and strength, and damage the building structure.
The hazards of chloride ion in zinc smelting mainly include the following aspects:
1. The existence of chloride ion affects the normal process of zinc electrodeposition, which not only aggravates the corrosion of lead anode, but also makes it difficult to strip zinc during electrodeposition;
2. The increase of lead anode power consumption also leads to the increase of lead content in cathode zinc; The increase of chlorine above the electrode tank will worsen the operating conditions and seriously affect the health of workers. According to the process requirements, the chlorine ion content in the zinc solution during electrolysis should be controlled below 200mg/l to ensure the smooth progress of production. Otherwise, it will bring a lot of inconvenience to the zinc electrodeposition, which will seriously affect the efficiency of zinc electrodeposition and the quality of zinc products.
Introduction to the current process of removing chlorine from wastewater by bismuth oxide
1. The bismuth oxide method is to add bismuth oxide reagent to the original solution, and the bismuth ion formed under acidic conditions will hydrolyze bismuth ion and chloride ion to generate bismuth oxychloride precipitation that is difficult to dissolve in water within a certain PH range, so as to remove chloride ions from the original solution.
2. With this chlorine removal process, bismuth oxide can be repeatedly used for purification, saving production cost
So how to use bismuth oxide to remove chlorine in zinc hydrometallurgy? Now, we will introduce the methods of removing chlorine in zinc hydrometallurgy at this stage, including alkali washing, copper slag and ion exchange. The material used in the production system is zinc oxide dust produced by the lead smelting top blowing furnace. The lead content of the material is relatively high, reaching about 40%. A part of fluorine and chlorine in the dust exists in the form of PbF2, PbCl2 and other insoluble substances. When sodium carbonate (or sodium hydroxide) is used for alkaline washing, the chlorine removal rate can only reach about 30%, which fails to achieve the desired effect; When copper slag is used for chlorine removal, due to the material characteristics, zinc oxide dust basically does not contain copper, so a large amount of copper sulfate and zinc powder need to be added to create the conditions for chlorine removal by copper slag, resulting in high chlorine removal costs. In addition, when copper slag is returned for use, due to factors such as long-term storage and oxidation of copper slag, the effect of chlorine removal by copper slag return is unstable; When the ion exchange method is used to remove chlorine, only 50% of chlorine can be removed. Because the chlorine content of this material is high, the ion exchange method can not meet the requirements of electrolytic zinc for chlorine ion removal. At the same time, the regeneration of the resin consumes a lot of water and generates a lot of wastewater.
The following characteristics can be achieved by using bismuth oxide to remove chlorine
1. The effect of chlorine removal is stable, basically maintained at about 80%.
2. Bismuth oxide can remove 30% - 40% fluorine while removing chlorine, which provides favorable conditions for normal operation of electrolysis.
3. Main reagent consumption From the perspective of industrial application, in the process of using bismuth oxide to remove chlorine, the unit consumption of per ton zinc of caustic soda is 66kg/t, and the unit consumption of per ton zinc of basic zinc carbonate is 60kg/t, and the unit consumption of water used to wash oxidation secrets is 2m3/t. The reagent consumption is small, the amount of wastewater generated is small, and there is basically no zinc loss. Bismuth oxide is a one-time input and can be used for a long time. After a long period of operation, the chlorine removal effect has declined, because other impurities exceed the standard. After the impurity removal process, it can be recycled and put into the system again, and the effect is still good.