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:
1. It can be concentrated and used in preparing a new charge, the
small quantity of calcium chromate present being an assistance, or:
2. It can be used for making chromate of lead (chrome yellow), by
precipitating the calcium chromate with a lead salt; this being a very
economical process for the manufacture of this color.
The mass after lixiviation, being treated with a solution of sulphate
or carbonate of potash or soda, will yield chromate of potash or soda,
and by the employment of sulphuric acid, the corresponding
bichromates. The solutions are then filtered, to get rid of the
insoluble deposits, concentrated, and crystallized.
If, instead of chromate or bichromate of potash or soda, chromic acid
is sought, the mass after lixiviation is treated with sulphuric acid,
and the chromic acid is obtained directly without any intermediate
steps.
This process has the following advantages:
1. The oxidation can be effected at the ordinary temperature, thus
saving expense in fuel.