/ 
28 Indian Forest Records. [Vol. Ill 
ing digesters, and it cannot therefore be recommended for practice in the 
neighbourhood of towns. But as pulp factories in India must necessarily 
be situated close to the areas from which the supply of raw material is 
drawn, that is to say, in sparsely populated localities, the objection will 
not have, as a rule, much force. The odour is : ot dangerous to health, 
merely disagreeable until one gets used to it. 
33. As Na 2 S has but small digestive effect, it can only to a small ex¬ 
tent be substituted for the NaOH necessary for bare digestion as per 
Table VI, but it may usefully be used instead of the excess NaOH 
added for bleaching effect in Table VII. Its bleaching effect is so much 
greater than that of excess NaOH that it is desirable to use as much of 
it as circumstances will permit, provided the proportion of NaOH is not 
thereby reduced below the necessary amount, and the factor which chiefly 
controls the respective proportions is the efficiency of the soda recovery 
plant. Upon this depends the amount of sulphate added to make up for 
loss, and upon this again depends the respective proportions of hydroxide 
and sulphide in the final liquor. With a recovery of 80 to 85 per cent., 
these will usually average a ratio of 3 : 1, and our experiments have 
accordingly been made with liquor in this proportion. As the combining 
weights of NaOH and Na 2 S are almost alike (40 and 39) the small error in 
calculating the whole in terms of NaOH may be neglected, and compari¬ 
son with soda process results thus made easier. The following 
are the average results. 
TABLE VIII. 
Digestion results with sulphate liquor in which one-fourth of the NaOH is 
replaced by Na 2 S ■ Crushed , starch-free bamboo. 
Temperature, pressure and time as per Table VI. 
— 
NaOH and 
Na 2 S calcula¬ 
ted as NaOH. 
Un¬ 
bleached 
yield. 
B. 
(a) 
Per 
cent. 
p. 
(&) 
Per 
cent. 
1 
Bleached 
yield. 
Unit 
cost. 
(c) 
Per 
cent. 
Tw. 
B. Tulda 
201 
12 
45 
16 
7-20 
43 
5-86 
B. arundinacea 
22 
12f 
44 
17 
7-48 
42 
6-22 
B. polymorpha 
201 
12 
46 
17 
7-82 
44 
5-85 
C. pergracile . 
21 
45 
15i 
6-97 
43 
5*87 
Melocanna b. . 
221 
44 
| 
18 
7-92 
42 
6-36 
{a) On unbleached pulp. ( b ) On raw material, (c) Unit cost of NaOH=4-5, 
[ 208 ] 
