Part IV.] Puran Singh: Preparation of Tannin Extracts. 5 
calculated according to the quantity of tan liquors to be cooled from 
100°C to n degrees (n being the temperature of the cold water used in 
the factory). According to the calculations of Messrs. Dumsney and 
Noyer the condensing surface required per ton of liquor at 100°C to be 
cooled from 100° to 20°C is 1’2 to 1*5 sq. metres. 
Filtration .—The artificial cooling of the tan liquors as given above 
has a marked effect on their clarification. Most of the insoluble 
“ reds ” or “ phlobaphene,” pectosic and resinoid bodies are precipitated 
out. The next step, therefore, for completing the clarification is to 
rapidly filter them through filter presses or decanting centrifugals. The 
filter presses best suited for this purpose are those with 13 chambers 
having 12 interposed wooden frames of 13J in. square. One filter press 
of this kind would give 22 gallons of clear filtrate per hour. In this 
process, pasty cakes consisting of wooden dust, pectosic and resinoid 
bodies, etc., are formed which fill the interposed space. The liquors thus 
treated are freed from a large amount of insoluble matter. 
Mechanical Clarification .—Finding that filter presses are too slow 
and too costly in their action, the extract manufacturers prefer to employ 
only decanting vats. These are large vats in which the liquors are 
allowed to stand and deposit their insolubles. The latest improvement, 
however, consists in the use of decanting centrifugals. A form of the 
decanting centrifugal is given in Plate II. In this centrifugal, the forma¬ 
tion of “ cakes ” takes place on the circumference of the copper basket 
of the centrifugal and the clarified liquor is decanted in a continuous jet 
from the upper portion of the basket. The advantage of this method 
can at once be seen from the fact that for clarifying 19,800 gallons in 
24 hours, one requires a decantation battery of 110,000 gallons capacity, 
while four decanting centrifugals of 1*2 metres diameter will do the same 
work. The decanting battery would require 300 sq. metres of surface 
instead of 30 sq. metres required by these centrifugals. The centri¬ 
fugals can be installed at less than half the cost of huge decanting 
batteries. It is, therefore, recommended that in place of filter presses 
and decanting batteries, decanting centrifugals made by Robatel Buff and 
Sf Co., Lyons, France, can be used with great advantage and economy. 
A factory treating 60 tons of wood with liquor amounting to 20,000 
gallons in 24 hours would require only four decanting appliances costing 
£640. 
Evaporation .—The mechanically clarified liquors are then evaporated 
222 
