Synthetic gypsum treated with controlled amt of alkali - to neutralise insol. acid fluorine cpds, before mfg, plaster. (BE221076)

摘要

Plaster is manufactures from synthetic gypsum by removing the accompanying acid substances with an alkali cpd. and dewatering the gypsum to form plaster. An aq. suspension of the sypsum is homogenised with an amt. (Q) of alkali which depends on the fluorine content (p) of the gypsum; the suspension is then drawn off, washed, filtered, dried, and roasted to form plaster. The alkali is pref. lime, CaCO3, NaOH, Na2CO3, KOH, K2CO3, MgO or MgCO3. The amt. Q is pref. equal to Qo + Kp, where Q is the number of OH equivalents/kg gypsum; Qo = no. of OH equivalents needed to neutralise the acids soluble in water per kg. gypsum; and K is a coefft. with value 0.2-0.6 Used e.g. in the treatment of gypsum obtd. as a by-product in mfg. phosphoric acid. The acid substances interfere with setting and/or result in plaster with inferior properties. The invention overcomes these problems without leaving excess alkali in the plaster, and the mechanical properties obtd. are similar to those of natural plaster.