International Dyestuff Industry - ColorantsHistory.Org

Figure 6. Apparatus for Separation of Product. Edelstein
Collection, Hebrew University.
The crude product of the sulfonation was diluted in large wooden tanks, a, the solution was
brought to boiling by blowing steam into it, and wet slaked lime was added to bring about
neutralization. (This operation was later carried out in wooden tubs fitted with mechanical
stirrers.) The neutral mixture was then run into the vacuum filters, b, connected to iron
reservoirs, dd, that were evacuated by means of an air pump. The mixture was filtered
through successive layers of sand, pebbles, and bricks, covered by a coarse canvas
stretched out on a wooden frame, such as c. One reservoir was connected so as to receive
the concentrated filtrate and first washings; further washings with boiling water were drawn
into a second reservoir. Some product remained mixed with the calcium sulfate in the filters
and was extracted by removing the solid, boiling it with water, and returning the mixture to
the filters.
The salt solution in the first reservoir was concentrated “in iron pans heated either over a
fire or by steam, until they contain about 15 per cent of lime salts. Soda crystals (sodic
[sodium] carbonate) were then added in sufficient quantity to precipitate all the lime as
carbonate, and thus to produce sodic salts of the sulpho acids. These were siphoned off
from the carbonate of lime, and concentrated until they contained about 30 per cent of
sodic salts.”
These “sodic salts” were the sodium salts of the anthraquinonesulfonic acids, generally
referred to as “soda salt,” or “silver salts.” At this stage in the alternative anthraquinone
process, soda salts were prepared in a similar manner by the lime treatment. The salts
were later found to be a mixture of two disulfonic acids and a monosulfonic acid, the
proportions depending on which of the two methods was employed, and the amount of
sulfuric acid added. Later, it was found possible to separate these salts from one another at
this stage, “so that the different coloring matters they produce may be obtained in a more
or less pure state.” However, it is unlikely that Perkin ever carried out such separations in
his factory.
The alkaline fusion was found troublesome to achieve on a large scale by both Perkin and
Caro. The heating range had to be carefully maintained to prevent destruction of the
product at high temperature. Laboratory experiments “were performed in an air bath, the
products being heated in metallic dishes, a mixture of ‘soda salt’ [silver salt, the sodium
salts of anthraquinonesulfonic acids] in a dry state with a saturated solution of the alkali
being taken... and it was considered that caustic potash gave, on the small scale, the best