Conversion of carbonate ions in vacuum salt production and scale inhibition and prevention

In view of CO32- concentration and enrichment in the co-production process of salt and nitrate, which leads to scaling problems in the evaporation system, the study used multi-stage hydrochloric acid to adjust the brine pH to convert CO32- to HCO3-, alleviating system scaling and extending the production cycle effectively. The conversion behavior between CO32- and HCO3- during the acid addition process was investigated by thermodynamic calculation, XRD and SEM-EDS. The results showed that the scale of the salt and nitrate evaporation system was mainly composed of CaCO3 scale and Mg(OH)2 scale, but the scaling mechanism was different. The cause of scale formation in the salt system was that with the increase of temperature, the Ca2+ enriched would form CaCO3 with low solubility. When CaCO3 reached supersaturated state, it would be deposited and precipitated out to form scale layer. While the main reason of scale formation in nitrate system was that the multiple enrichment of CO32- and OH- in brine. The theoretical calculation and experimental results indicated that adjusting the pH of mother liquor to 9.0 could convert local CO32- to HCO3-. SEM-EDS and composition analysis of scale and sludge revealed that adjusting the pH of brine to 8.59 could effectively dissolve Mg2+ in scale and sludge, and part of Ca2+. The undissolved Ca2+ formed a thin scale layer between the brine and the heat exchanger, avoiding the brine with a neutral pH to be in direct contact with the heat exchanger and realizing the conversion of CO32- to HCO3-. At the same time, the corrosion process of heat exchange tube was restrained to some extent. © 2020, Editorial Office of FINE CHEMICALS. All right reserved.