(a) Natural baryta sulphate, barytes, or heavy spar, is employed in the manufacture of a handsome innocuous white colour, fast, and resisting most reagents, but with little body or covering power. This white, fixed with glue size, is largely employed in the manufacture of paper-hangings, and for adulterating white lead and zinc white. In preparing it, the whitest lumps are picked out, coarsely broken, and heated in reverberatory furnaces to disintegrate the substance, and produce a finer degree of pulverization. The grinding is done dry, and the resulting fine powder is thrown into tanks of water, stirred, and let stand a little while, when the heavier and coarser particles fall to the bottom. The milky-looking supernatant water is decanted into settling basins, where the lighter suspended material deposits; after another decantation of the clear liquor, the pasty white is collected, and dried in the air or a stove-room.
(6) Artificial baryta sulphate, found in the market under the name of blanc fixe (fast white), is much better than native sulphate. According to Kuhl-mann, the cost of the acids constitutes the main expense of manufacture. By putting natural baryta carbonate (withe-rite) in contact with the vapours escaping from salt-decomposing furnaces or from lead chambers, he succeeded in saving a large proportion of the un-condensed vapours. In his works, the baryta, dissolved by the condensed acids, is converted into the artificial sulphate by an addition of sulphuric acid. The recovered nitric and hydrochloric acids return to take part in a new operation, and increase the yield. In another process of Kuhlmann's, a mixture of natural baryta sulphate, manganese chloride, and coal, is transformed at a high temperature into insoluble manganese sulphide and barium chloride, which is easily separated by washing. The reaction is represented by the formula -
BaO, S03 + MnCl + 4C = BaCl + MnS + 4CO
The coal intervenes as a deoxidizing agent, and is converted into carbonic oxide. He is thus able to transform native baryta sulphate into barium chloride' without a loss of more than 3 to 4 per' Cent, of baryta sulphate. The transformation is effected in large re-verberatory furnaces, similar -to those employed for decomposing salt in soda-works, with a hearth divided into two compartments by a low wall. When these furnaces have been heated, the portion most remote from the fireplace is charged with a finely-pulverized mixture of native baryta sulphate and bituminous coal; and above it is poured the liquid residue from the manufacture of chlorine, the free acid of which has been previously saturated with chalk or native baryta carbonate. The mixture is well stirred, and thickened by heat. When it has become a thick paste, it is passed over the partition wall, with proper iron tools, into the compartment near the fire. There the mass swells, and soon disengages small jets of carbonic oxide. After an hour's calcination at red heat, the semi-fluid paste, which has little more consistency than that of crude soda, is removed from the furnace, and, when cold, forms a black mass of barium chloride, with manganese and iron sulphides, and a small proportion of baryta hyposulphite.
After several days' exposure to the air, the mass becomes disintegrated, and the hyposulphite passes to the state of sulphate. The substances are then lixiviated with hot water in an apparatus disposed like that for crude soda. The liquors are a clear solution of nearly pure barium chloride. Should there be slight excess of barium sulphide, causing a yellow colour, there is poured in, until complete decoloration, a solution of manganese chloride (residue of the manufacture of chlorine) which has been deprived of iron by digestion with powdered baryta carbonate. Conversely, any excess of manganese chloride is separated with barium sulphide. The solution of barium chloride, obtained from the raw product, marks 24° or 25° B. When purified in the manner indicated, chamber acid (sulphuric), diluted with water to 30° B., is poured in as long as a precipitate forms. The whole is well stirred and let stand. The baryta sulphate is rapidly deposited, and the syphoned liquors constitute a hydrochloric acid marking 6° B. The artificial sulphate thus obtained is washed, to remove the last trace of free acid, drained to the consistency of a firm paste in cloth filters, and the filters are pressed, or subjected to centrifugal action.
When the paste has become thick enough, it is packed in barrels, and contains 30 to 32 per cent, of water. It may be dried and moulded like white lead, but in the majority of cases it is more advantageous. to use it in the pasty state, because, once dried, it does not require the same degree of comminution.