Development of a method to separate Mg(OH)2 from gypsum precipitates generated during the lime treatment of metallurgical processing effluents (Lhoist) (Université de Liège)

Abstract

Magnesium has been identified a raw critical material by the European Union. It is mainly used in the pharmaceutical, refractory, water treatment and desulphurization of fuel gases industries. Brines and some minerals are the conventional sources for the production of Mg(OH)2. Over the last 20 years, China has been the main magnesium producer. However, the Chinese exports are reducing due to the country policy. Therefore, the interest of the scientific community on the possibility to recover magnesium compounds from waste streams is growing. The two main purposes are: to recycle the process water and to secure the supply of magnesium by extracting it from unconventional sources. In this context, this project studied and evaluated the possibility to recover magnesium from hypersaline and process water by precipitation using milk of lime. Magnesium ions precipitate as hydroxide with the addition of hydroxide ion. The source of OH- ions can be any alkaline solution such as NaOH, NH4OH and Ca(OH)2. Caustic soda is usually used in order to precipitate magnesium. However, the economic gain is low as the OPEX is high. As more profitable alternative, milk of lime has been proposed instead of caustic soda. The main challenges arising from the use of milk of lime are its low solubility in water and the presence of insoluble impurities. Additionally, the use of lime as an alkaline reagent for the precipitation of magnesium hydroxide may result in the formation of calcium sulphate in presence of dissolved sulphate. In this study, magnesium hydroxide was precipitated from a solution containing magnesium sulphate. Solubilized magnesium ion reacted with OH- from the MoL to form Mg(OH)2, a compound that has a very low solubility. However, the formation gypsum was also observed during this process. In order to separate Mg(OH)2from gypsum, different methods such as physical separation, flotation and desulfation were considered and studied. The characterization of the precipitate showed that gypsum can grow as pure crystals while magnesium hydroxide frequently precipitated as much smaller particles with gypsum in its structure. The small particle size was one of the main reasons for the poor performances of centrifugation and flotation separation methods. Flotation of Mg(OH)2 from the precipitate was carried out in a water-bearing pulp at pH levels between 9.5 and 10. Collectors were sodium dodecyl sulphate (SDS) and sulfonate. Modifiers were sodium silicates (glass water), CMC and sulphuric acid. Stable froth for flotation was provided by the collectors themselves. Flotation gave poor results as poor selectivity between the two products was observed. Physical separation by centrifugation gave better results. However, the performance of the process cannot yet be predicted as the size-morphology analysis is required. The best result obtained by centrifugation was a concentrate with 66% of magnesium hydroxide. The results also suggested that the use of glass water (Na2SiO3) as a dispersant can increase the performance of the centrifugation. But the intensity of the improvement is particle size-dependent. Finally, to prevent the formation of the gypsum contaminated magnesium hydroxide, a two-stage precipitation procedure was investigated: the desulfation of the hypersaline water on reaction with a solution of CaCl2 followed by the precipitation of Mg(OH)2 using MoL. A scoping exercise was designed in order to get a better understanding of the different processes. The laboratory tests showed that the best purification was 95%, resulting Mg(OH)2 containing 79-80 wt% of the main compound.