Potassium hydroxide is prepared by chemical process. At present, most manufacturers adopt ion-exchange membrane method. The core process of the process flow is primary refining of brine, and the key equipment is electrolytic cell and ion-exchange membrane. The main processes are as follows:
1. Primary refining of brine
The task of primary brine process is to treat potassium chloride, recovered brine and light brine dechlorinated by ion-exchange membrane electrolyzer into qualified filtered brine. Because these brine contains impurities (organic matter, bacteria and algae, SO42 -, Ca2 +, Mg2 +, NH3, heavy metals, SS, etc.) that cannot be allowed by the diaphragm, especially SO42 -, Ca2 +, Mg2 + will block the diaphragm or ion membrane in the electrolytic cell, resulting in the rise of brine level in the electrolytic cell, the decrease of current efficiency, the increase of cell voltage, the shortening of cell life, and even shutdown. Add refining agent barium chloride (or membrane filtration, but many small and medium-sized devices still use barium method), potassium hydroxide, potassium hypochlorite, potassium carbonate, flocculant and potassium sulfite to remove SO42 - and Mg2 + impurities in the brine respectively, and then remove SS, CaCO3 and other impurities through the filter to meet the process requirements.
2. Secondary refining of brine
Ion membrane electrolysis has high requirements for the quality of brine. The filtered refined brine from the primary brine process needs to undergo ion exchange to further reduce Ca and Mg ions. Ion exchange resin tower can be divided into two tower processes and three tower processes. Three tower process is adopted. Under normal conditions, two towers operate in series and one tower regenerates.
After secondary refining, that is, the brine enters the chelating resin tower to remove calcium and magnesium, which is pumped into the high-level tank and then flows into the anode chamber of the ion-exchange membrane electrolyzer. After DC power is applied, chlorine gas and outgoing light brine are generated in the anode chamber and separated by the separator. The chlorine gas is transmitted to the chlorine main pipe. The light brine generally contains 200-220g / L potassium chloride and passes through the dechlorination gas desaturation tank. Hydrogen and 30-32% liquid alkali generated in the cathode chamber of the electrolytic cell also pass through the separator, and the hydrogen is transmitted to the hydrogen main pipe. 30-32% of liquid alkali can be sold as a commodity, or it can be sent to the evaporation unit to evaporate to 48% of liquid alkali.
4. Steam concentration and treatment of chlorine, hydrogen and waste gas