| Per cent. | |
| Tin dioxide | 0.0158 |
| Grains per pound | 1.11 |
| Equivalent to stannous chloride | 1.40 |
| Minimum doses | 22.40 |
| Maximum doses | 2.80 |
Can eroded.
—Jour. Amer. Chem. Soc.
NEW PROCESS FOR THE MANUFACTURE OF CHROMATES.
By J. MASSIGNON and E. VATEL.
The ordinary method of manufacturing the bichromates consists in making an intimate mixture of finely pulverized chrome ore, lime in large excess, potash or soda, or corresponding salts of these two bases. This mixture is placed in a reverberatory furnace, and subjected to a high temperature, while plenty of air is supplied. During the operation the mass is constantly puddled to bring all the particles into contact with the hot air, so that all the sesquioxide of chromium of the ore will be oxidized. After the oxidation is finished, the mass is taken from the furnace and cooled; the bichromate is obtained by lixiviation, treated with sulphuric acid and crystallized. This method of manufacture has several serious objections.
The authors, after research and experiment, have devised a new process, following an idea suggested by Pelouze.
The ore very finely pulverized is mixed with chloride of calcium or lime, or carbonate of calcium, in such proportions that all the base, proceeding from the caustic lime or the carbonate of calcium put in the mixture, shall be in slightly greater quantity than is necessary to transform into chromate of calcium all the sesquioxide of chromium of the ore, when this sesquioxide will be by oxidation changed into the chromic acid state. The chloride of calcium employed in proportion of one equivalent for three of the total calcium is most convenient for the formation of oxychloride of calcium. If the mixture is made with carbonate of lime (pulverized chalk), it will not stiffen in the air; but if lime and carbonate of calcium are employed at the same time, the mass stiffens like cement, and can be moulded into bricks or plates. The best way to operate is to mix first a part of the ore and well pulverized chalk, and slake it with the necessary concentrated chloride of calcium solution; then to make up a lime dough, and mix the two, moulding quickly. The loaves or moulds thus formed are partially dried in the air, then completely dried in a furnace at a moderate temperature, and finally baked, to effect the reduction of the carbonate of calcium into caustic lime. It is only necessary then to expose the loaves to the air at the ordinary temperature, for the oxidation of the sesquioxide of chromium will go on by degrees without any manipulation, by the action of the atmospheric air, the matter thus prepared having a sufficient porosity to allow the air free access to the interior of the mass. Under ordinary conditions the oxidation will be completed in a month. The division of this work—mixing, slaking or thinning, roasting or baking, and subjection to the air—is analogous to the work of a tile or brick works. The advance of the oxidation can be followed by the appearance of the matter, which after baking presents a deep green color, which passes from olive green into yellow, according to the progress of calcium chromate formation. When the oxidation is completed, the mass contains: Chromate of calcium, chloride of calcium, carbonate of lime and caustic lime in excess, sesquioxide of iron and the gangue, part of which is united with the lime. This mass is washed with water by the ordinary method of lixiviation, and there is obtained a concentrated solution containing all the chloride of calcium, and a small quantity only of chromate of calcium, the latter being about 100 times less soluble in water.
This solution can be used in the following ways: