Research on the transformation and immobilization mechanism of fluorine-containing phase in phosphogypsum low-carbon gel backfilling system

As a bulk by-product of the chemical phosphoric acid industry, phosphogypsum (PG) has not been effectively utilised because of the large number of impurities, and the large amount of stockpiled on the surface of the ground poses a potential risk of environmental pollution due to the leaching of its harmful elements. Therefore, the current consideration is to combine the bulk consumption of PG with the management of the underground mining area by cementing paste backfilling technology (CPB), so as to realise the harmless disposal of PG in the management of the mining area at the same time. However, there are several scientific problems as follows:

• PG has a large amount of phosphoric acid residue in the output process, which makes the pH of phosphogypsum around 2~3, and it is difficult to meet the alkaline environment requirements for cement hydration.
• The poor cementing properties of PG and silicate cement lead to limited development of the strength of the PG backfilling body, and increasing the proportion of cementitious materials is bound to increase the cost of backfilling.
• Although PG cemented backfiller can fix/stabilise most of the harmful elements leaching pollution, according to the preliminary experimental research, the fluorine leaching amount in the backfiller is still far more than the original groundwater quality grade standard (>2.0mg/L).

Therefore, on the basis of CaO neutralisation and modification of PG, we choose diversified active solid wastes such as steel slag and slag as backfilling cementitious materials, and use PAC or biochar as targeted fluorine fixation materials. Afterwards, we can obtain the parameters of PG backfilling ratios in line with the requirements of strength and environmental protection through a large number of experiments. The hydration and fluorine leaching mechanisms were explained by XRD, EPMA-WDS, heat of hydration test, compressive strength test and fluorine leaching test.