WASHING APPARATUS.
In all colour works operating on a large scale, special apparatus is
used, in which are carried out the washing, pressing and drying of the
pigments obtained as precipitates. Only pigments prepared in small
quantities are filtered through filter paper. The treatment of enamel
white and white lead requires the use of apparatus for this purpose in
a special degree. We insert a short description here.
The preparation of enamel white takes place most conveniently in tubs
provided with a stirring apparatus. When the precipitate of barium
sulphate has once settled to the bottom, it is very difficult to
again mix it up with water by means of a hand stirrer, an operation
which must be often performed in washing. If vessels be used provided
with a suitable mechanical stirrer, the precipitates are rapidly and
thoroughly washed. We have already stated that vessels with a stirrer
capable of being raised out of the liquid were specially suitable for
washing white lead. Such an arrangement can with advantage be used in
washing all precipitates. Many of the mineral pigments have to be freed
from admixed salts by washing. A description follows of an effective
washing apparatus with movable stirrer. In the cylindrical vessel (Fig.
21), which may be of any size, is a vertical iron shaft rotating upon
a pin in the bottom of the vessel. On this axis is a horizontal wooden
crosspiece, the under surface of which is studded with brushes; the
disc, which this crosspiece carries, is united by means of two bars
with a second, in which is cut a screw moving on the screwed shaft. The
shaft is rotated by means of the cog-wheels shown in the illustration.
The handle fastened to the upper disc enables the crosspiece carrying
the brushes to be raised or lowered. When the handle is held fast, this
crosspiece rises or falls according to the direction of rotation of the
axis. The pipe shown at the side supplies the water for washing the
precipitates.
[Illustration: FIG. 21.]
In the preparation of colours liquids have often to be brought into
the precipitating vessel which would attack the iron of the stirring
apparatus, so that it is advisable to make the connecting rods between
the two discs of such a length that the screws may be above the vessel.
All iron parts of the apparatus dipping into the liquid should be
protected by asphaltum varnish.
When a specially heavy precipitate is to be washed, such as enamel
white, chrome yellow or white lead, the stirrer is raised as high
as possible before the commencement of the operation. When the
precipitate has formed and settled, the liquid is run off, the water
tap opened, and the stirrer slowly brought down to the precipitate;
the brushes fastened to the crosspiece stir up first the top of the
precipitate, then the next portions, and so on until the whole of the
precipitate has been stirred up into the liquid. When this has been
accomplished, the stirrer is kept going for some time, so that the
water may take up as much of the soluble materials as possible; it is
then raised out of the vessel, in which the precipitate again settles.
As a rule, two or three washings of permanent white, in an apparatus of
the construction described, are sufficient to render it quite free from
acid. Washing must be continued until the wash water leaves blue litmus
paper quite unchanged. When dry, precipitated barium sulphate is a very
soft powder of great whiteness, which, on account of the fineness of
its particles, can be readily ground with binding materials.
Enamel white loses, in a remarkable manner when completely dry, a great
portion of its covering power and of its valuable property of being
easily ground with oil or size to a homogeneous paste. It is not known
whether this alteration is caused by a molecular change of the barium
sulphate, as is not altogether improbable. In order to preserve the
valuable properties of enamel white it is not, as a rule, completely
dried, but is brought into the market in the form of pulp, which is
obtained by bringing the washed precipitate into strong linen bags
and allowing the water to drop through. This object is more quickly
accomplished when the last wash water is drawn off only to such a point
that, when the precipitate is again mixed up by means of the stirrer,
a thin paste is formed. This paste is run into a centrifugal machine,
of which Fig. 22 is an illustration. In the drum, B, provided with an
outflow, R, the smaller removable drum, C, with perforated walls, is
caused to rotate by gearing, G, to which it is attached by the screw,
V. The drum, C, is lined by a tight linen bag.
[Illustration: FIG. 22.]
When the drum is in rapid motion, the thick liquid in the washing
vessel is run in. In consequence of the centrifugal force due to the
rapid rotation, the whole mass is at once thrown on the sides of
the drum, the liquid penetrates the fine openings, is caught in the
outer vessel, and runs away to a receiver, in which it is kept until
the finest particles of the precipitate, which will penetrate even
the closest fabric, have settled. The operation is continued until
the excess of water has been separated, when the bag containing the
precipitate is lifted out of the drum. Centrifugalised enamel white is
a fairly stiff, white paste, which should be packed in sacks lined with
oiled paper to prevent drying.
[Illustration: FIG. 23.]
FILTER PRESSES.
In recent years the use of filter presses for separating liquids from
precipitates and for washing the latter has become general. A filter
press consists, as shown in Fig. 23, of a number of frames, between
which are perforated plates and sheets of filter cloth, and which can
be pressed tight together by a screw. A powerful pump forces the liquid
containing the precipitate into the hollow spaces of the frames (known
as chambers), in which the solid body remains, whilst the liquid goes
through the filter cloth. By afterwards pumping fresh water through,
the solid remaining in the chambers is soon completely washed.