837
The two last-mentioned mixtures, however, are quiteexceptional, and ought not to be used, but ratherthe first compounded mixture.
There is no difference whatever in the lixiviationfrom that practised for soda ash ; but of course thecarbonating process is never performed, instead ofwhich sometimes the oxidising by air, mostly byHargreaves’ blower, is carried out. This is, how-ever, quite unnecessary as a special process, since itcan be combined with the causticizing, as will beshown presently. The tank liquor is, therefore, nearlyalways pumped or run direct from the liquor settlersinto the causticizing vessels (operation pans). Theseare always of iron (cast, or mostly wrought), but ofno definite shape; frequently old boilers cut in halvesare employed for this purpose. When the agitationtakes place by a mechanical stirrer, circular vesselsare often preferred, but not always even then. Inthis case a small bracket engine is bolted to the pan,which is, moreover, provided with one or moresteam-pipes for heating the charge, a sludge valvefor the mud, and a drop siphon for removing theclear liquor, except when the whole of the charge isrun out at once. Small causticizers can be workedwithout an engine, the agitation produced by theblowing in of steam being sufficient; but the bestsystem of all is to produce the agitation of the mudand liquor by means of a “blower” worked by asteam jet, of which that constructed by KoertingBrothers is the most economical in working. Thislast method possesses the very great advantage thatthe mixture of air and steam, which serves for agi-tation, at the same time heats the liquor, and lastly,oxidises the sulphides, so that the preliminary oxida-tion can be dispensed with, as above mentioned.
The tank liquor (which ought to be well settledfor making white caustic) is always diluted with hotwater, but not always to the same degree. Theminimum strength found in practice is 15° Tw.,the maximum 20° Tw., but most factories work at16° to 18° Tw. Davies has proved that a liquorof 20° Tw. can be quite as thoroughly causticizedas one of lower strength ; and although it takesnaturally a little more time, there is not only morework done, but there is much less water to evapo-rate. When the total has been brought to the boilby injection of steam, quicklime is put into the cage,and the steaming and agitation is carried on untilthe contents of the pan are thoroughly causticized.An experienced workman can tell pretty nearly, bythe kind of boiling, the colour, and other indications,when the liquor is completely causticized; but atest is always made by taking out a sample, settlingor filtering it, and trying the clear liquor for effer-vescence by addition of a little dilute sulphuric orhydrochloric acid. After this the whole is some-times run off to a separate settling pan ; it is, how-ever, more usual to allow it to stand for half anhour, remove the clear liquor by means of a swivelor drop siphon, as nearly as possible into the causticliquor settlers, and to run a fresh quantity of tankliquor upon the lime mud, which always containssome free lime, so that the second charge docs not
require quite so much lime as the first. When thissecond charge in its turn has been causticized, set-tled, and the clear liquor run off, the agitator isstarted again, fresh water is run on to the lime mudin the pan and steamed up with it; it is allowed tosettle again, and the clear liquor is used for dilutingthe tank liquor of a further batch. The mud nowremaining is stirred up with a little water and runon to the filters, where it is allowed to drain, eitherby itself, or by preference with the aid of a vacuumpump. The filter is also again a half-boiler, butwhen made on purpose, is something like 20 feet long,10 feet wide, and 4 feet deep. On the bottom areplaced bricks on edge 2 inches apart from each other,and leaving a clear channel in the centre, coveredover with bricks or slabs. The whole arrangementis covered with 8 or 9 inches of limestone, or better,of coke, in pretty large pieces; over these anotherlayer of smaller, and over this a third layer of stillsmaller pieces. Lastly, a layer of clean coarse sandor small coals, on which cast or wrought iron gridsare laid to afford a smooth shovelling bottom. A fewclean riddled cinders from one of the furnaces rendersthe filter, of which there still remains 18 to 24 inchesof clear space, available for use; but communicatingwith the bottom channel above described there is aniron pipe (2 or 3 inches wide) connected with a smallboiler or air-tight tank of about 150 to 200 cubicfeet capacity, erected above the level of the causti-cizing pan, and connected in its turn with a vacuumpump. The top of this boiler is provided with anair-cock, and its bottom with a running-off cock forthe collected water; it has also a water-gauge. Assoon as this filter has received a batch of mud fromthe causticizing pans the vacuum pump is set inmotion, and first the air and then, as rapidly as itcollects, the filtrate is withdrawn from underneaththe bed. Very soon cracks appear on the surface ofthe mud. These are immediately obliterated with awooden rake, whilst a supply of water is run on andworked well into the mud to remove as much sodaas possible. After three or four hours the magmahas become very firm and all available liquor is gotfrom it; the vacuum pump is now stopped, theliquor which is now in the receiving-vessel or boileris run into the causticizing pan to assist in diluting afresh portion of tank liquor, and the mud is shovelledout of the filter and wheeled to the ball-furnaces,where it is used as above mentioned (Mokison).Its composition as it goes into the ball-mixture isstated by Davies as follows:
A.
B.
f Calcium hydrate,...
2-825
2-652
Sodium hydrate,.
1-906
1-663
(Sodium chloride and sulphate,....
0-244
0-206
Calcium carbonate,.
40-170
42-031
Calcium hydrate,.
3-759
3-072
Silica,.
0-673
0-774
Alumina,.
0-255
0-324
Ferric oxide,.
0-048
0-708
Magnesia,.
0-086
0-095
Soda,.
1-832
1-600
Water,..
47-986
46-577
Total,.
99-781
99-702