Chromate of Potassium. (FR., Chromzte c7,e Potasse ; GER., Chromsaures Kali.) Formula, Ii2Cr04. This salt is a source of chromium preparations. It crystallizes in yellow, six-sided pyramids, isomorphous with sulphate of potassium. The solution in water-2 partsis also yellow, with an alkaline reaction and a bitter saline taste. It is efflorescent, and exceedingly poisonous in all forms.
Chromate of potassiuni is prepared direct from native chrome-iron ore by calcination with salt petre, or carbonate of potassiuni, or caustic lime, the ore being powdered aud carefully heated with the alkali in a reverberatory furnace ; or the bichromate may be subjected to a strong heat, and split up into oxide of chromium and chromate.
Chrome-iron ore is a compound of the sesquioxide of chromium and protoxide of iron, with certain admixtures of alumina, magnesia, and silica. It is found in considerable quantities iu Russia, Greece, North America, and Turkey.
Biehromate of Potassium. (FR., Bichromate de Potasse ; GER., Zweifaeh Chromsaures Kali.) Synonym, bichrome. Formula, 11,0, 2Cr03, or 112Cr,0By slow evaporation, this salt crystallizes in fine, red, tabular crystals, derived from an oblique rhombic prism, which are anhydrous, and melt at a low red heat. At an ordinary temperature, it is soluble in about 10 parts of water, the solubility increasing rapidly with an increase of temperature. Thus, 1 part is soluble in 20-14 parts of water at .. .. 0° 11-81 .. 10 7-65 .. 20 1-98 .. 60 1- 37 a ' . 8° 98 .. 100 At a high temperature, it is split up into the neutral chromate, oxygen, and oxide of Chromium.
Biehrome is manufactured from the chromate by adding sulphuric acid, which unites with one half of the base to form sulphate of potassiuni. The process is usually carried out direct from the chrome ore, and is as follows :The ore is carefully ground and sieved through a very fine mesh. It is then mixed with potash lime, prepared from the purest obtainable limestone and a solution of carbonate of potassium, as free from chloride as possible. The proportions are 7 cwt. of lime to 2i of carbonate. After being thoroughly mixed by any convenient apparatus, the whole mass is thrown into a reverberatory furnace constructed in similar fashion to a double-bedded sulpha of soda furnace, with a bed about 10 ft. long by 7 ft. wide, a fireplace 2 ft. 6 in. wide, and a crop of
areh 2 ft. 6 in., narrowing down to 18 in. at the end farthest from the fire. The charge is spread over one of the beds and paddled carefully, under a bright flame. After about two hours, it is transferred to the bed nearest the fire, and a fresh charge introduced upon the back bed. Each charge is worked for about four hours, by which time nearly the whole of the oxide of chromium is completely oxidized. It is then withdrawn, aud should have a greenish-yellow appearance, with bard lumps dispersed through the whole mass. With the chromate of potassium are' now mixed chromate of calcium, free lime, silicate of potassium, and oxide of iron. The lumps are roughly broken up, and the whole is th own into lixiviating tanks, and digested with a hot saturated solution of sulphate of potassium. The chromate of calcium is thereby converted into chromato of potassium, with the formation of sulphate of lime. The tanks are arranged iti fashion somewhat resembling black ash vats, the liquors running from tank to tank until they are sufficiently strong. They are then drawn off and allowed to settle. Sulphate of lime is deposited, and the clear chromate of potassium run into cisterns lined with lead. Here it is trtated with sulphuric acid, which abstracts a portion of the base, converting the chromate into bichromate. And inasmuch as the latter salt is not nearly as soluble in water as the chromate, a precipitation of the greater part of the bichrome takes place, leaving sulphate of potassium and a portion of bichromo in the mother liquors. This precipitate is removed, redissolved, and recrystallized in iron cones. The mother liquors are returned to the lixiviating tanks.
Bichrome is also occasionally made by heating the chrome ore with saltpetre in the manner described when treating of the chromate.
The manufacture of bichrome is a rapidly increasing one; as much as 11,000 tons per annum being now turned out in this country. The chief seats of the industry are in Lancashire and Glasgow.