The materials used in a successful lithopone plant comprise barytes (crude), coal, zinc spelter, oil of vitriol, common salt, sal ammoniac, sodium phosphate, chlorate of potash, calcium chloride and caustic soda. Taking the average figures for several years, the percentages by weight of the consumption of each of these materials are as follows:-Crude barytes, 37.30 per cent; bituminous coal, 26.59 per cent; zinc spelter or scrap zinc, 9.63 per cent.; oil of vitriol, 22.05 per cent.; rock salt, 3.39 per cent.; sal ammoniac, .44 per cent.; sodium phosphate, .14 per cent.; chlorate of potash, .31 per cent.; calcium chloride .13 per cent.; caustic soda, .2 per cent. The average yield of lithopone from this material was 41 per cent of a normal grade, averaging 29.85 per cent zinc sulphide, 1.63 per cent zinc oxide and 68.52 per cent barium sulphate.
The cost of manufacture naturally depends, in the first place, upon the location of the plant, a plentiful supply of clear water, as enormous quantities are required for the washing of the product; facilities for discharging the materials from railroad cars or boats, economical methods of handling the same, up-to-date crushers, mills and furnaces. Neither must the placing of levigating tanks, filter presses and drying ovens or kilns be lost sight of and it will be noted that it requires quite a plant full of apparatus to manufacture a paying quantity of lithopone. On the other hand, to run it most economically, is to have it going day and night, with double shifts of men and a shut down only when it becomes necessary to clean out and repair furnaces, etc., because every shutdown means quite an item of expense and a liability to produce inferior products until the plant is again in normal working order. Without going into minute details we may say, that for a plant with a capacity of ten tons every twenty-four hours a building, at a cost of about $25,000, fitted out with apparatus, costing as much more is required. The apparatus, including engine for running two reverberatory furnaces, a crusher for breaking the crude barytes, at least four large buhrstone mills, disintegrator, elevator, etc., and to furnish steam heat for drying rooms, consists of one or two reverberatory furnaces, a series of muffle furnaces, lined tanks for the zinc sulphate and barium sulphide solutions, thirty vats of about 1,500 gallons each capacity for washing the product, filtering presses, trays for handling the filtrate and drying chambers for same, aside from numerous other tools and appliances. Where perfectly clear water cannot be had large tankage for filtering the water for levigating the product is absolutely needed.
The process of manufacture itself begins with the preparation of barium sulphide, usually termed black ash, which consists of breaking up the crude baryta rock in a crusher and running it through a buhr stone mill to a certain size, about like a small pea, screening it to avoid the dust, which is liable to clog the furnace, adding to every 100 pounds of this broken rock about twenty pounds of bituminous coal of small pea size, and about ten pounds of common salt to assist fusion. This mixture is placed in a reverberatory furnace, where it is heated to dull redness without admission of air. When the reaction has taken place the mass is placed in vats, digested and filtered. When the crude baryta rock contains, as is often the case, much clay, iron, etc., the pure barium sulphide must be leeched out and the foreign residue removed. When this black ash and its solution is being prepared, zinc spelter, zinc dross or scrap zinc or any available zinc salt is being dissolved in separate vats or tanks in sulphuric acid, forming a solution of zinc sulphate. The barium sulphide solution and the zinc sulphate solution should be of a concentration of 60 per cent and at a temperature of 140 to 150 degrees Farenheit, when they are poured together, and the zinc solution should be poured at double the rate of the barium solution, in which case a precipitate is obtained that must be filtered and dried. It is self-evident that when zinc dross is used and in any case unless refined spelter is employed, the zinc solution must be treated with a compound that will eliminate all traces of iron and other foreign materials. For this purpose chlorate of potash is added. The precipitate, after being filtered and dried, is now placed in muffle furnaces and heated at as high a temperature as 900 degrees Fahrenheit, when it is suddenly plunged into cold water. In calcining in the muffle furnace salammoniac to about 1 per cent of the dry weight of the pigment is added to assist fusion. When the material is thus prepared it is carried to the buhr stone mills by means of centrifugal pumps and pipes or by other mechanical appliance and there ground in the pulp state to the standard degree of fineness, and then conveyed to the washing tanks, where it is washed over as often as it is necessary to eliminate all traces of impurities, free sulphur, iron, etc. This often requires as many as twelve to fifteen washings and sodium phosphate and caustic soda are added in very small portions to aid in the process. Calcium chloride is also added as a bleaching agent when the whiteness is defective. A minute quantity of ultramarine blue is also one of the ingredients to assist in eliminating too creamy a cast. When the test shows the pigment to be thoroughly washed, it is filter pressed, the cakes so formed are placed on wooden trays, the trays stacked up on suitable trucks and these put into drying rooms usually built of metal and heated by exhaust steam, which is augmented by jets of live steam. When the cakes of lithopone have dried, they are fed into the hopper of a disintegrator, which pulverizes the lumps into a powder of uniform fineness, and when this powder has been put up in barrels or casks of suitable size, the product is ready for the market.
In our percentages of materials required to produce lithopone of standard quality, we have included in the item of bituminous coal, not only the percentage for use in the black ash, but also the coal required as fuel for the engine and the furnaces. It must be noted that of late crude petroleum has become quite a favorite for firing boilers, furnaces, etc., on account of the greater heat produced, but it is an open question whether or not its use is not detrimental to the life of a furnace and whether it is really more economical. The one great feature about the manufacture of lithopone, as compared with that of white lead is in its quick manufacture, for a working batch of lithopone can be turned out in three or four days, whereas even quick process white lead will require at least one month from the time the pig lead is unloaded, with one single exception, that of the so-called mild process of white lead. It stands to reason that the capital tied up in a lithopone plant is very much smaller than in the case of white lead manufacture. Another advantage of the process as against that of white lead making is that the health of the workmen is not as much in danger, though by no means pleasant, on account of the vapors and the heat. The cost of manufacture, exclusive of the capital invested, but including repairs to apparatus, superintendence and labor should not exceed $10 per ton (2,000 pounds), while the cost of material, packages, etc., necessarily varies with the condition of the market.