Direct and Reverse Flotation Processes for Phosphogypsum

Flotation is a key method for removing impurities from phosphogypsum, improving its quality and facilitating its resource recovery. It is primarily divided into direct flotation and reverse flotation, whilst industrial production often employs a combination of multiple processes.

Orthoflotation aims to concentrate calcium sulphate. It relies on cationic collectors to increase the hydrophobicity of the gypsum surface, allowing calcium sulphate to adhere to bubbles and float to the surface, thereby separating it from impurities such as silica and organic matter. The Ca-O bond structure within gypsum is stable, resulting in minimal calcium ion leaching; the mineral surface exhibits electronegativity. When combined with dodecylamine-based cationic reagents, the flotation results are outstanding, effectively enhancing the grade of the gypsum concentrate. The purified phosphogypsum can be processed into building materials such as plasterboard and gypsum blocks, which offer excellent fire resistance, thermal insulation and soundproofing properties. However, conventional flotation has significant drawbacks: its ability to remove fine and micro-impurities is relatively weak, reagent consumption is high, the process flow is complex, and production control is difficult, all of which drive up overall processing costs.

The reverse flotation approach is the opposite of conventional flotation: it uses reagents to modify the surface of impurities, causing organic matter, quartz, fine silt and other impurities to float to the surface for separation, whilst the gypsum remains in the slurry. This process offers good adaptability to low temperatures, reduced reagent consumption and a simplified workflow. It efficiently removes organic impurities and silica, whilst allowing for flexible reagent combinations to suit different impurities. However, reverse flotation struggles to remove eutectic phosphorus from the raw material; some reagents have poor dispersibility and are prone to agglomeration and failure; flotation performance is highly influenced by the pH of the pulp; and production requires precise control of environmental parameters.

As individual flotation methods each have their limitations, combined forward and reverse flotation has gained widespread adoption. This approach not only removes impurities comprehensively but also optimises the particle morphology of phosphogypsum, thereby enhancing the commercial value of the product. Furthermore, the industry commonly integrates flotation with pre-treatment processes such as washing, calcination and ball milling. This multi-stage, synergistic approach to impurity removal significantly improves the whiteness and structural strength of phosphogypsum, promoting the efficient resource utilisation of this industrial by-product.