DRYING OF CALCIUM SULPHATE SLURRY
TABLE OF CONTENTS
Calcium sulphate hemi hydrate as it is called has a chemical formula (CaS04 ½ H2O). Calcuimsulphate is made from mineral gypsum with chemical formula (CaSO4 2H2O)
The material gypsum is of fairly widely occurrence almost pure and wide various impurities which colour it and modify the properties of the plasters of paris (POP) made from it
DIFFERENCES IN CHEMICAL FORMULA
The slight difference in chemical formula of the mineral gypsum and plaster of paris (P.O.P) is half molecule of water crystallization thus. Crypsum caso42H20 M.W 172. 18 Calcium sulphate 79.1% H2O 20. 19% Plaster of paris (POP) Caso4 ½ H2O M.W 154. 16 CaSoO4 93.8% H2O 6.2% Anhydrous caso4 M.W 136.15 Calcium sulphate (Caso4) 100% Successful production of calcium sulphate from gypsum is complicated by the number of possible dehydration product.
TYPES OF HEMIHYDRATES
Theme are two hemihydrates & and B. the & form markes much stronger and generally more satisfactory plaster and is therefore the desired dehydration product.
TYPES OF ANHYDROUS CALCUIM SULPHATE
There are also four types of anhydrous calcium sulphate obtained by stronger heating of gypsum.
The & hemihydrate forms by recrystallization of gypsum from water above 1150C (239of) it’s formation is therefore favoured by heating gypsum in a sufficiently damp atmosphere for there to be a thin absorbed water layer on the particle B – hemihydrat is formed when gypsum is heated rapidly in dry atmosphere above 1000C (2120f), On heating ground gypsum the temperature rises until 120‑C (2620f) when violent boiling occurs. The temperature does not rise again until this has ceased and the plaster enters the first settle, on further heating a second sharter period of boiting begins at 1630C (32.0f) after which plaster enters the “second settle”.
On continued further heating the hemihydrate begins to decompose, giving off water and being converted into the anhydrous salt at 800 – 10000C (1470-18300f) the dead Bioned gypsum or keene’s cement is attained.
First settle plaster is more plastic and easier to work than second settle material. The later, however gives a denser and stronger set.