Calc Sulph: From Gypsum to Pharmaceutical-Grade Calcium Sulphate
At a glance
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Chemical name |
Calcium sulphate dihydrate |
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Chemical formula |
CaSO₄·2H₂O |
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Water solubility |
Slightly soluble |
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Natural mineral |
Gypsum |
Calcium sulphate dihydrate occurs naturally as gypsum, one of the world’s most abundant sulphate minerals. Gypsum forms as mineral-rich waters evaporate and is found in extensive sedimentary deposits. It is widely used in agriculture, construction, food production, and pharmaceutical manufacturing.
The “·2H₂O” in CaSO₄·2H₂O is important. Each calcium sulphate unit is associated with two water molecules that form part of the crystal structure. This is not merely moisture sitting on the surface of the powder; the water is incorporated into the mineral lattice.
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Mineral connection: Why Plaster of Paris hardens Natural gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. Gentle heating removes one and a half molecules of its bound water to produce calcium sulphate hemihydrate, CaSO₄·½H₂O, better known as Plaster of Paris. When water is added again, gypsum reforms as countless interlocking crystals, causing the material to set into a hard solid. The name remains from the historically important, exceptionally pure gypsum deposits near Paris, France. |
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Body connection: Sulphur helps stabilize proteins The sulphur-containing amino acid cysteine, C₃H₇NO₂S, can form disulfide bonds, –S–S–, that link different parts of a protein. These bonds help stabilize keratin in hair, skin, and nails, as well as many enzymes and antibodies. |
Two routes to pharmaceutical-grade calcium sulphate
Calcium sulphate dihydrate may be purified from naturally occurring gypsum or formed chemically from purified calcium and sulphate sources. Both routes can produce CaSO₄·2H₂O that meets a pharmaceutical specification.
Route one: Purification from gypsum
Gypsum-bearing rock is extracted from open-pit or underground deposits. Depending on the source, the ore may also contain limestone, clay, silica, dolomite, iron oxides, other evaporite minerals, moisture, and fine rock particles.
The material is crushed and ground to liberate gypsum from the surrounding rock. Screening, washing, classification, and other separation steps remove unwanted minerals and produce a high-purity calcium sulphate concentrate. The exact process depends on the deposit and the required impurity limits.
The purified material is dried carefully, milled, and classified to obtain the desired particle-size distribution. Throughout this route, it remains calcium sulphate dihydrate, CaSO₄·2H₂O. Processing changes its purity and physical characteristics rather than its chemical identity or hydration state.
Route two: Chemically manufactured calcium sulphate
A purified calcium solution can be reacted with a purified sulphate solution. One simplified example uses calcium chloride and sodium sulphate:
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CaCl₂ + Na₂SO₄ + 2H₂O → CaSO₄·2H₂O↓ + 2NaCl |
The downward arrow indicates precipitation of calcium sulphate dihydrate as a crystalline solid.
1. Preparation and precipitation
The calcium and sulphate compounds are dissolved separately, and their concentrations are adjusted to the correct stoichiometric ratio. The solutions are combined while temperature, pH, mixing speed, addition rate, ion concentration, and reaction time are controlled.
As calcium and sulphate ions meet, CaSO₄·2H₂O crystals form. During crystallization, two water molecules become incorporated into each calcium sulphate unit, creating the characteristic gypsum structure.
2. Separation, washing, and controlled drying
The crystals are separated by filtration, centrifugation, or a similar method and washed repeatedly with purified water. In the example reaction, sodium chloride remains dissolved and is removed with the wash liquid.
Drying conditions remove surface moisture while preserving the two bound water molecules that define gypsum. Excessive heating would remove this structural water and convert the material toward calcium sulphate hemihydrate, CaSO₄·½H₂O, or anhydrous calcium sulphate, CaSO₄.
3. Quality testing
Representative samples are tested for identity, assay, moisture or water content, hydration state, physical appearance, particle characteristics, heavy metals, and other elemental or process-related impurities.