I. V. Tananaev Institute of Chemistry and Technology of Rare Elements and Mineral Raw Materials of Kola Scientific Center of Russian Academy of Sciences, Apatity, Russia:

B. I. Gurevich, Leading Researcher
V. V. Tyukavkina, Senior Researcher, e-mail: tukav_vv@chemy.kolasc.net.ru

The paper presents a classification of sulphate wastes and possibilities for the recovery of those formed in the perovskite and sphene concentrate sulphuric acid processes. It is shown that phosphogypsum resulting from the perovskite process is a full-fledged substitute for natural gypsum that can be used to control the setting time during the Portland clinker grinding. The optimal content of titanogypsum in cement, in terms of SO₃, was found to be 2.5–3.5 w/o. Physical-chemical experiments did not reveal any fundamental difference in hardening of cements admixed with titanogypsum and natural gypsum. In sphene processing for titanium dioxide pigment using the sulphuric acid method, two acidic wastes are generated: a silicium-calcium cake and wastewater. After chemical cleaning followed by neutralization, wastewater formed a product (titanophosphogypsum) consisting of biaqueous Gypsum with ТiO₂, Р₂O₅ as impurities. It has been shown that titano-phosphogypsum can be turned into building gypsum conforming to the State Standard 125–79 requirements, with impurities not affecting its quality. The silicium-calcium cake was used as pigment to produce both unfired and fired anhydrite cement. The silicium anhydrite material with hardening activators had low strength (1.0–1.9 MPa) and rapid setting (onset after 5–7 minutes, end — after 10 minutes). The strength of fired anhydrite cement after 28 days was 2.4–26.1 MPa depending on firing temperature and admixtures. It is recommended that the waste recovery should be carried out simultaneously with the principal titanium dioxide process.

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