Unreacted gypsum

Common pollutants in a titanium dioxide plant include heavy metals, titanium dioxide, sulfur trioxide, sulfur dioxide, sodium sulfate, sulfuric acid, and unreacted iron. Most of the metals are removed by alkaline precipitation as metallic hydroxides, carbonates, and sulfides. The resulting solution is subjected to flotation, settling, filtration, and centrifugation to treat the wastewater to acceptable standards. In the sulfate process, the wastewater is sent to the treatment pond, where most of the heavy metals are precipitated. The precipitate is washed and filtered to produce pure gypsum crystals. All other streams of wastewater are treated in similar ponds with calcium sulfate before being neutralized with calcium carbonate in a reactor. The effluent from the reactor is sent to clarifiers and the solid in the underflow is filtered and concentrated. The clarifier overflow is mixed with other process wastewaters and is then neutralized before discharge. 

Fig. 5. Phosphoric acid magma with high SO4 concentration showing poor gypsum crystals and unreacted phosphate rock.

Unless otherwise stated, this chapter relates to ordinary Portland cements hydrated in pastes at 15-25°C and w/c ratios of 0.45-0.65. XRD powder studies on such pastes have been reported by many investigators (e.g. C38,M67). The rates of disappearance of the phases present in the unreacted cement are considered more fully in Section 7.2.1. Gypsum and other calcium sulphate phases are no longer detectable after, at most, 24 h, and tbe clinker phases are consumed at differing rates, alite and aluminate phase reacting more quickly than belite and ferrite. The ratio of belite to alite thus increases steadily, and after about 90 days at most, little or no alite or aluminate phase is normally detectable. 

The suspension of gypsum is removed from the absorber and classified in hydroclones, which remove crystals of the required size. Finer particles are recycled to the scrubber, where they act as seed crystals. Any unreacted slaked lime is recycled with the fines to the scrubber. 

Typical filter cake contains about 3.3% of the input P2O5 in insoluble forms of which 1% may be unreacted rock and 2.3% CaHP04 cocrystallized with gypsum. The aggregate weight ratio of CaO combined with P2O5 is about 1.0. 

The carbonation of this matrix leads to the decomposition of ettringite into gypsum and CaCOa. It was suggested [124] that the ettringite in this system bonds together the other components of the matrix (CSFI and unreacted slag) and therefore this phase, even when present in small quantities, exerts a considerable influence on the mechanical properties. Various methods have been considered to overcome the carbonation problem in the supersulphated cement GRC, including surface compaction and the application of a protective layer [123]. 

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