The Soda industries

THE LEBLANC SODA PROCESS

Nearly ll the soda of trade was forme1'1yobtained from certain natural deposits of the so-called "sesquicarbonate," or from the ashes of sea plants. But towards the end of the last century, the supply from these sources became insufficient to meet the increasing demands. About 1775 the French Academy of Science offered a large prize for a method of making soda from salt. Among other processes submitted was one by Nicolas Leblanc, which seemed promising, and being granted a patent in 1791, he began manufacturing on a commercial scale. But in the French Revolution his factory was seizec1, the patent declared public property, and no indemnity was paid to him. Having lost l his property, he finally committed suicide.
   Leblanc's process was so perfect and complete that very slight changes, and those only in minor details, have been made up to the present. It has been in use now for nearly a century, and although very seriously threatened by newer processes, it still produces about half of the wo1'1u's supply of soda. Owing to the fact that it proves hydrochloric acid and bleaching powder as by-products, it has been able to survive competition, although its condition is becoming more desperate every year. Its chief rival is the ammonia or Solvay Process. Within a few years many electrolytic methods for caustic soda have appeared, and the successful product ion of bleaching material by any of these processes will sweep away about the only source of profit now left to the Leblanc manufacturer'. It is not probable that this change will come immediately, for the electrolytic proc3sses have not yet entirely emerged from the experimental stage; but the decline of the Leblanc process is generally regarded as inevitable, and inventors have, for the most part, abandoned further attempts to improve it.
The reactions of the Leblanc process are generally expressed as follows : -

1) 2 NaCl +H2S04 = Na2S04 +2 HCl.
2) Na2S04 + 2 C = Na2S + 2 CO2•
3) Na2S +CaC03 = Na2COS + CaS.
4) CaC03 + C = CaO + 2 CO.

But these equations do not represent all the reactions which take place during the process, for a number of other substances are formed. The first equation represents the preparation of sodium sulphate and hydrochloric acid (p. 67). The second and third reaction are realized in one operation. The fourth has no direct relation to the process, as the formation of carbonic oxide does no become marked until all the salt-cake has been decomposed. This serves to indicate the end of the process, and aids in the formation of a porous product.
The salt-cake should be friable and porous, containing very little free sulphuric acid, and no undecomposed chloride. The carbon is supplied in the form of powdered coal, which should contain very little ash-forming impurity. A little pyrite does no harm, but the coal should be as free as possible from nitrogen, in order to prevent the formation of cyanides and cyanates. Calcium carbonate in the form of pure limestone or chalk, crushed to the size of a small pea is mixed with the crushed salt-cake and coal in order to carry out the third reaction. If the limestone contains magnesia or silica there is a consequent loss as insoluble residue. Usually 100 pound of salt-cake, 100 pounds of limestone, and 50 pounds of coal dust form a charge. This is an excess of limestone, the purpose of which is explained below.
The reactions are carried out in a "black-ash" or "balling furnace," which may be worked either by hand or mechanically.

The hand-worked furnace is a long reverberatory (Fig. 38), having two platforms on the hearth. The charge is introduced on the back platform (A) nearest the flue, where the heat is not very high. ,when thoroughly dried and well heated, it is raked down onto the front platform (B), which is a few inches lower than (A). Here the temperature is high, usually about 1000° C., and the surface of the mass soon begins to fuse. It is then raked over, thoroughly exposing it to the direct heat until it becomes a thick, pasty mass, from which carbon dioxide is escaping freely. After the salt-cake is all decomposed, the charge begins to stiffen, and the evolution of carbon monoxide is shown by the appearance of jets of blue flame, known to the workmen as "candles." The charge is then raked together into a" ball," which is drawn out of the furnace into an iron barrow. The evolution of carbon monoxide continues for a few minutes after the "ball " is removed, and the bubbles escaping from the pasty mass cause it to become porous. The formation of this gas is due to the action between the coal and the excess of limestone according to reaction (4). The caustic lime formed here slakes during the lixiviation of the black-ash, and swells, thus disintegrating the mass.
   Although the heavy tools are suspended by chains, their operation is still so difficult, and the temperature is so high, that a man cannot handle much more than 800 pounds at one time. In order to work larger charges, without the expensive hand labor, revolving

black-ash furnaces (Fig. 89) are much used. These are similar to the revolving furnaces described earlier; the flame from the furnace (A) passes through the cylinder (B). The charge is introc1ucedthrough the manhole (P), and the finished product discharged through the same opening, into the wagon, at the end of the operation. The cylinder is about 16 feet long by 10 feet in diameter, and is revolved by a gear (E) connected with an engine. Projections are fixed in the lining to help mix the contents. The charge is usually about two tons of salt-cake, with proportionate amounts of coal and limestone. It is customary to introduce only the limestone and a part of the coal at first, and to rotate the cylinder until some caustic lime is formed; then the remainder of the coal, together with the salt-cake, is introduced; and the rotation continued until the reactions are completed. The speed varies from one revolution in three or four minutes, at first, to four or five revolutions per minute during the last part of the process. The hot gases from the black-ash furnace, whether hand-worked or mechanical, pass through the dust box (N), and then through the long flue over the pan (J, J) on their way to the chimney (D). In this shallow pan, the liquor obtained by lixiviating the black-ash is evaporated. When crystallized, the salts are removed through the small doors (J).
Black-ash is a brownish black or dark gray substance of a pumice-like texture, containing about 45 per cent sodium carbonate, 30 per cent calcium sulphide, 10 per cent caustic lime, and from 10 to 12 per cent of other impurities, - sulphate, silicate, aluminate, and chloride of sodium, calcium carbonate, coal, and iron oxide, with traces of cyanides and of sulphides of sodium.
  The next stage in the process is the lixiviation of the black-ash. This presents some difficulties: if the black-ash is put directly into cold water, it often agglomerates in hard lumps, which dissolve exceedingly slowly; the free lime present forms calcium hydroxide, which reacts with the sodium carbonate solution, forming some caustic soda; the solution of sodium carbonate, especially if hot and dilute, reacts on any calcium sulphide present, forming some sodium sulphide; moreover, moist calcium sulphide oxidizes rapidly to sulphate in the air, and this reacts with the sodium carbonate. Hence the lixiviation must be done as rapidly as possible, at a low temperature, and without exposing the wet black-ash to the air.
   Shank's process gives the most satisfactory results. The lixiviation is carried on in a series of tanks, each haying a false bottom perforated with small holes .. Because of its density, the solution of sodium carbonate sinks, and passing through these perforations, is drawn off by means of a pipe which delivers it at the top of the next tank. There must always be sufficient liquor in each tank to keep the black-ash entirely submerged. The process is continuous, sufficient fresh water being admitted to the nearly exhausted ash to give an unbroken flow of strong liquor (above 45° Tw.) from the last tank of the series. When the liquor from the last tank falls to 450 Tw., it is turned into a tank which has just been filled with new ash. The exhausted ash is washed until the wash water has a density of only 10Tw. Then the residue of calcium sulphide and hydroxide, coal, ashes, and other insoluble matter, which constitutes the "tank waste," is sent to the dump. The tank is then refil1ed with black-ash and made the last of the series, to receive the strong liquors from the preceding tank.
  Since the black-ash contains caustic lime, sufficient heat is generated by its slaking during the lixiviation to warm the concentrated liquor to about 50° C., which is the best temperature for complete extraction. The temperature of the dilute lye from the first tank of the series is not allowed to rise above 38o C., in order to prevent the above-mentioned interaction between the calcium sulphide and the sodium carbonate.
  Good tank liquor has approximately the following composition:-

Na2COa (+ NaOH).........23.60
NaCl ..........................   .50
Na2S ..........................  .13
Na2S202......................  .30
Na2S04........................  .23
Na2SiO3 ..............traces
NaCN     ..............traces
NaCNS   ..............traces
FeS (in solution)....traces

The lye obtained by the lixiviation has a specific gravity of about 1.25, and is lll11elely from suspended impurities. It is purified by settling and then pumped to the top of the "carbonating towers," which are filled with pebbles 0r coke, or have numerous chains or wire ropes suspended from the top and weighted at the lower ends. The tank liquor trickles over the porous material or chains, and comes into intimate contact with a strong current of carbon dioxide t entering at the bottom and passing up through the tower. The carbon dioxide and oxygen which pass through the tower, .convert the caustic soda to carbonate, decompose the ferro-sodium sulphide (sollution of ferrous sulphide in sodium sulphide), converting the sodium sulphide into bicarbonate, and. precipitating the iron, together with any silica and all1mina which may be present.
 The reactions involved were supposed to be the following: -

1) 2 NaOH + CO2 = Na2COa + H20.
2) Na2S + CO2 + H20 = NaHCO3 + NaSH.
3) NaHCO3 + NaSH =Na2COa + H2S.

But Lunge has shown that reactions (2) and (3) may not be fully
realized, and hence that the decomposition of sodium sulphide is very difficult. Some manufacturers complete the purification of the
tank liquor by adding zinc hydroxide, which precipitates the sulphide: -

  Na2S + Zn (OH)2 = ZnS + 2 NaOH.

If air is blown through the tank liquor, the sodium sulphide is converted into thiosulphate:-

   2NaS+ 2 Oz + H2O = 2 NaOH + Na2S2O2

The tank liquor may be better purified according to Pauli's process, in which a little "Weldon mud " is added to the liquor, and air and steam blown through it. This oxidizes the sodium sulphic1c very completely, besides precipitating ferric oxide, silica, and alumina in the sludge. If the "Weldon mud" be regarded as manganese dioxide, for brevity the reactions may be written as follows:-

   2 Na2S + 4 Mn02 + 5 H20 = 2 NaOH + Na2S203 + Mn(OH)2
                4 Mn(OH)2 +2O2= 4 Mn02 + 4 H2O.

used repeatedly, until it becomes very much contaminated with ferric oxide, silica, alumina, etc.

  After settling, the purified and carbonated tank liquor is drawn directly into the evaporating pans, which are usually large shallow iron tanks, the liquor being heated by surface contact with the waste gases from the black-ash furnace. Sometimes deep pans, heated from below, are used, since surface evaporation gives a product contaminated with dust from the furnace. The liquor is evaporated directly to dryness, and the "black salt" (chiefly monohydrated sodium carbonate, Na2CO3 · H2O) is calcined by heating it to a red heat. Sometimes sawdust is mixed with the uncarbonated liquor before evaporation, and then on calcining, the soda-ash is carbonated by the carbonaceous matter from the wood; but the charge is very liable to cake in this operation. The caustic soda and sodium sulphide of the tank liquor are thus converted to sodium carbonate, and, after all the sawdust is burned out, the ash becomes white or light brown.
  Or the liquor is evaporated till a crystalline mass separates: then the mother-liquor ("red liquor") is drawn off, and the black . salt is raked out of the pan. Much care is necessary to prevent the formation of a crust or the burning on of the precipitated carbonate. In most large works, a semicircular evaporating pan is used, provided with mechanical scrapers, to prevent the black salt from adhering to the pan. The best form of this apparatus is Thélen's pan (Fig. 40). In this, the scrapers (R, R) move the salts towards the end of the pan as they deposit, and a scoop lifts them to the

draining apron. The beam (B) carrying the frame from which the scrapers are suspended, is rotated by the gear (J).
    For a very light colored product, the cru de soda-ash is dissolved in water, and a little bleaching powder solution added; the precipitated iron and other impurities settle out, and the clear solution is evaporated until a thick mass of crystals separates, when the mother-liquor is drawn off to remove any soluble impurities. The monohydrated salt remaining is then calcined without the addition of carbonaceous matter, to remove its crystal water, and the product is called "white alkali" or "refined alkali." A little sodium chloride is forn1ed by the addition of the bleaching powder, so that refined alkali is not quite so strong as soda-ash. It is chiefly used for glass making and other purposes where iron and sulphides would be detrimental. Good Leblanc soda is nearly white or pale yellow, and should contain but few black specks. It usually contains a little caustic soda, a trace of sulphides and sulphites, some chloride and sulphate, and not over 1 per cent of insoluble matter. It should be finely ground before packing.

  Soda crystals or sal-soda (Na2C03 . 10 H20) is made by dissolving
soda-ash in warm water, allowing the hot solution to stand quietly until all sediment deposits, and drawing off the clarified liquor into crystallizing tanks, where it is cooled to the atmospheric temperature. Large crystals of sal-soda, very nearly pure, are deposited. They contain over 60 per cent of water, and are thus very bulky and not ecol1omical to ship; but they are still preferred to soda-ash by same manufacturers. They do not dissolve so readily as soda-ash. They are sometimes used for making sodium bicarbonate, by exposing them on a grating, to an atmosphere of carbon dioxide:

- Na2C03 . 10 H20 + CO2 = 2 NaHC03 + 9 H20.

The water resulting from the reaction drips through, leaving the
bicarbonate on the grating.