Chlorine is extensively used ill the arts as a bleaching and . oxidizing agent. It is chiefly employed in the form of a solution of "bleaching powder" or "chloride of lime," which contains calcium hypochlorite, and as chi orates or hypochlorites of the alkali metals. Liquid chlorine, compressed in steel cylinders, has recently become an article of commerce, and it appears probable that this form of shipment may be extended in the future.
Practically, all the chlorine used in the arts must be derived from the chlorides of sodium, potassium, or magnesium, which are found more or less abundantly in nature. A very large part of the hydrochloric acid made from salt is used for making chlorine. Since this acid is the chief by-product of the Leblanc process, a plant for making bleaching powder is always a part of those works. The important methods of making chlorine from the acid may be considered under two heads: those using manganese oxides for decomposing the acid, and those not using manganese for this purpose.
The function of manganese is to oxidize the hydrogen of the acid, forming water and liberating the chlorine. At the same time, the manganese is converted into chloride, and being somewhat expensive, its recovery in a form that permits of its return to the process is essential.
The oxides of manganese are found in natme as pyrolusite (1ln02), braunite (Mn203), manganite (Mn203• H20), hausmannite (11n304), wad and psilomelane, the last two of indefinite composition. The reactions occurring when manganese oxides are treated with hydrochloric acid are as follows:-
1) 1lnO + 2 HCl = 1ln012+ H20.
2) Mn02 + 4 HOI = MnOl2 + 2 H20 + 201.
3) 1ln203 + 6 HOl = 2 lIInOl2 + 3 H20 + 2 01.
4) 11n304 + 8 HOI = 31ln012 + 4 H,O + 2 01.
Thus it is readily seen that with pyrolusite, less acid is necessary for a given yield of chlorine, and a smaller quantity of manganous chloride must be treated to recover the manganese. This ore is purchased according to its content of 1ln02, which is estimatell by determining the "available" oxygen. '1'he presence of iron oxides, silica, calcium carbonate, etc., is disadvantageous.
In small works, especially where no attempt is made to recover the manganese, the process is carried on in simple stills of earthenware or sandstone. The earthenware stills (Fig. 43)* are cheap, but of limited capacity. They are heated by blowing free steam into the wooden casing in which they are set. The pyrolusite is put into the central perforated cylinder, and the acid runs through the pipe (A), chlorine escaping at (B). Sandstone stills (Fig. 44)* are made from single blocks of sandstone, or built up of slabs, the joints being made tight by a rubber packing, or by a lute of clay and linseed oil. The pyrolusite rests on a false bottom (A), and the acid is run in through (B), while steam is blown in through the sandstone pipe D (C). Chlorine escapes through (D).
These stills are larger than the earthenware ones, but do not utilize the acid so completely. The pipes through which chlorine is conducted are of lead or earthenware. Since valves in these pipes are rapidly corroded, a device shown in Fig. 45 * is used to shut off the flow of gas. AU-shaped bend is made in the pipe, and a small flexible tube attached at the lowest point of the U, connecting it with the vessel (A), filled with water. By raising (A), the water flows into and fills the U-pipe to the line (CD), cutting off the flow of gas. By lowering (A) to (A'), the water runs out of the U, and the flow of gas is uninterrupted.
The liquor remaining in the still contains much free acid, manganous chloride, ferric chloride, etc. It continues to evolve some chlorine for a long time, and is a very offensive and troublesome material to dispose of, since it pollutes the air, or the streams, into which it passes. Of the many attempts to recover the manganese, the two following are the most important;-
By Dunlop's method, the" still liquor" is neutralized cold, with powdered limestone, until all free acid is removed and the iron precipitated. The clear solution of manganous and calcium chlorides is then mixed with a carefully determined quantity of powdered limestone or chalk, and heated ttndel' pi'eSSIli'e by steam. This precipitates the manganese as carbonate, which is settled, and the solution of calcium chloride drawn off. The manganous carbonate is washed, and then calcined at about 3000 C. in a retort, while water spray and a current of ail' is introduced. This produces a mixture of Mn02, ~[nO, lItl203, etc., containing about 70 per cent of the dioxide. The process requires an expensive plant and consumes a large amount of fuel.
The Weldon process,'*' for manganese recovery, is the most successful, and is in general use in all large works, since it furnishes a continuous process for chlorine making and manganese recovery. The "still liquors" arc neutralized with just sufficient powdered limestone or chalk to remove free acid and precipitate the iron. This is done in the tank (A) (Fig. 46),t provided with a stirrer. The mixture is then pumped into settling tanks (B, B), where the precipitate deposits. The clear solution of manganous and calcium chlorides is then drawn into the" oxidizers" (C), where steam is blown in to heat it to 550 C. Milk of lime is made from pure lime, especially free from magnesia, and is added from (E) until tests forced into (C). The quantity of "milk" used is noted, and then from one-half to one-quarter more is added, and the air blast turned on at full strength. 'l'his addition of an excess of lime is necessary to hasten and complete the conversion of manganous hydroxide into the peroxide, and to prevent the formation of Mn304 (" red batch "). The total quantity of lime used should be such that the precipitate formed during the blowing contains approximately two molecules of manganese peroxide to one of calcium oxide. This is the so-called "acid calcium manganite" (CaO . :lln02) +(:llnO. Mn02), a mixture of mangallites of calcium and manganese. It forms a thin: slimy, black mass, and is called "Weldon mud." By adding a little more neutralized" still liquors " during the" blowing," some of the calcium oxide in the calcium manganite can be replaced by manganese from the manganous chloride of these liquors.
The calcium chloride liquor, in which the mud is suspended. is run into settling tanks (D, D), from which the supernatant solution is drawn off as waste. The Weldon mud is then run into the chlorine stills (F, F) as a thin paste; if of good quality, it contains about 80 per cent of its manganese as 11n02, and owing to its fine state of division, is readily decomposed by dilute hydrochloric agicl A small loss of manganese occurs in the precipitate from the first neutralization with marble or chalk dust; this loss is made up by decomposing some pyrolusite with hydrochloric acid in a small still (G), and adding this liquor to that from the stills (F, F). The ,I'eldon process works continuously and almost automatically, the materials being handled by pumps as liquids or slimes.
It is also very rapid, producing large amounts of chlorine, with but slight loss (2 to 3 per cent) of manganese oxide. But even at its best, only about one-third of the chlorine of the hydrochloric acid is obtained as gas, the remainder going to waste as calcium chloride in the liquor from the oxidizers.
Schlosing's process * for chlorine by the use of nitric and hydrochloric acids and manganese oxides depends upon the following reactions: -
2 H01 + 2 HN08 + 1ln02 = lfn(N08)2 + 2 H20 + 012,
The reaction is carried out by heating the mixture of acids and
manganese peroxide to 1250 C., using an excess of nitric acid. By
heating the manganous nitrate to 1800 to 1900 C., it ',s decomposed, and nitric acid may be regenerated from the vapors by treating them with air and steam, while manganese peroxide is recovered:-
lIn(N03)z = :lnOz + NZ04;
N204 + HzO + 0 = 2 HN03.
'iVischin, Just, and Alsberge have each patented modifications of the above process. Alsberge proposes to apply the method to the recovery of chlorine from the ammonium chloride liquors of the ammonia soda process, by employing the following equations:-
1) 2 NH401 + lIgO + llnOz=:lg012+lInOz+HzO +2 NH3'
2) MgC12+lIn02+4 HN03=llg(NOs)2+1Yn(N03)2+2 HzO +C12.
By evaporating to dryness and calcining the residue, the nitrates are decomposed thus: -
lIg(N03)2 + lI11(N03)2= :M:gO+ 1lnOz + 2 Nz04 + O.
The peroxide of nitrogen is converted to nitric acid by treatmentwith steam and air:-
N204 + HzO + 0 = 2 HN03.
Deacon's process t seems to be the most successful method of producing chlorine without the use of manganese. It depends on the oxidation of hydrochloric acid gas, by the oxygen of the air. This is done in the presence of certain metallic salts, which may act as "contact" substances, or as carriers of oxygen from the air to the acid, the apparent reaction being:-
2 HCl + 0 = HzO + 2 C1.
The most satisfactory" contact" or "catalytic" substance for this purpose is copper chloride. When cupric chloride is heated to 4000 C., it dissociates into cuprous chloride and free chlorine. Then, on exposing the cuprous chloride to oxygen, cupric oxide is formed and more chlorine set free. But the cupric oxide, reacting with hydrochloric acid gas, forms water and cupric chloride. The following are the reactions involved:-
1) 2 CuClz = Cu2C12+ 012,
2) Cuz01z+ O2= 2 CuO + Cl2.
3) 2 CuO + 4 HCI = 2 CuCl2 + 2 H20.
Thus the catalytic substance is regenerated and the cycle of changes begins anew.
Heat to the amount of 32 calories is absorbed during the dissociation of cupric chloride, but in the other reactions GO.4calories is evolved. Since the sum of the reactions represents a positive gain of 28.4 calories, theoretically when the process is once under way no addition of heat is necessary. But, in fact, some heat must be supplied; this is done by heating the mixture of ail' and hydrochloric acid gas to 4000 C. before admitting it to the" decomposers." Theoretically, too, all the chlorine of the hydrochloric acid should be recovered, but practically the reaction is far from complete. The plant for the process (Fig. 47)* is quite extensive. The gases from the salt-cake pan (A),t together with air, are passed through cooling pipes and drying tower (B) to condense moisture; then they go through the" superheater" (C), where the temperature is raised to 4000 C. The hot gases then pass into the" decomposer" (D), a tall cast-iron cylinder, containing bits of brick or other porous material which have been soaked in a solution of cupric chloride. Here the above reactions take place, and the resulting mixture of chlorine, hydrochloric acid, nitrogen, steam, and oxygen, passes through a condensing apparatus (E, E) to remove the hydrochloric acid, and then through a coke tower (F, F) sprinkled with concentrated sulphuric acid to remove all the moisture; finally, the dry chlorine gas (with the nitrogen and oxygen) goes to the chambers where bleaching powder is made.
The catalytic substance in the decomposer becomes inactive after a time (it seldom lasts more than four months) and must be replaced by fresh material. To accomplish this without interrupting the process the decomposers are now built in separate compartments, each holding about six tons of broken brick; every two weeks one compartment is emptied and recharged without discontinuing the flow of gas through the others. This loss of activity in the catalytic substance is attributed* to the presence of sulphuric acid ill the gases from the salt-cake furnace. To overcome this difficulty, Hasenclever has devised a method t by which an aqueous solution of impure hYllrochloric acid, made in the bombollllcs and coke towers, is run into hot, concentrated sulphuric acid (1.402° Tw.) while a blast of ail' is forced through the mixture. The sulphuric acid absorbs the water and generates pure lIOl gas, which mixes with the ail' in proper proportion for use in the decomposer of Deacon's process. By this method, S-! per cent of the hydrochloric acid gas is decomposed according to the reaction: -
2 HCl +0 = H20 + 2 CL
The diluted sulphuric acid is concentrated and returned to the process. The dilute hydrochloric acid which passes through the apparatus is recovered by washing the chlorine gas, and is mixed with the strong acid from the roasters.
Owing to the admixture of nitrogen with the chlorine, the latter is weaker than that furnished by the 'Yeldon process. It is well suited for making chlorates, but for making bleaching powder a special form of absorption chamber must be used.
When the hydrochloric acid gas is taken directly from the saltcake pan or from the mullie furnace, there is apt to be some difficulty in working Deacon's process, owing to the variation in the rate of liberation of the gas. Much care in the regulation of the air supply is necessary.
The hydrochloric acid gas from the Hargreaves process is too dilute for direct use in the Deacon apparatus.
Arsenic in the sulphuric acid used in the salt-cake pan, or for drying the chlorine gas, causes a loss, - in the first case by rendering the copper salt inactive, and in the Recond, by forming hydrochloric
As203 +<1CI + 2 H20 = ARP5 +<1HCL
Part of this hydrochloric acid combines with the AS205 to form a solution which condenses in the pipes between the drying tower and the bleaching powder chambers. But some of the acid is left in the chlorine and attacks the bleaching powder, causing it to be " weak"
The cost of a Deacon plant is rather more than of a ·Weldon plant of the same capacity; and while it is theoretically a superior process and requires less labor, it is not yet in general use. Several processes for the preparation of chlorine by the use of nitric and sulphuric acids have been proposed.
Dunlop's nitric acid-chlorine process depends * upon one or the other of the following equations:-
2 NaCI +2 NaNOa +<lH2S0~ = 4:NaHSO~ +N20~ +C12+2H20;
4 :NaCI +2 Na:N03 +6 H2S01 = 6 NaHSO, +N20a +2 Cl2+3 H20.
The mixture of salt, sodium nitrate, and sulphuric acid is heated in an iron cylinder which is surrounded by the flames of the fire. The vapors leaving the retort are passed through concentrated sulplmric acid which retains the nitrogen oxides, and the chlorine is then washed with water to remove any traces of hydrochloric acid. The nitrous vitriol obtained may be used in the sulphuric acid manufacture. The process was worked on a large scale at St. Rollox, England, but has been abandoned.
Donald's process t consists in passing the hydrochloric acid vapor from a salt.-cake furnace through sulphuric acid to dry it, and then through a mixture of nitric and sulphuric acids kept at 0° C., when the following reactions take place:-
2 HCl + 2 HiOa = 2 Hp + N204 + Cl2•
The gas mixture thus formed is led through dilute nitric acid, when the following takes place: -
N20~ + H20 = HN03 + HN02·
By passing through concentrated sulphuric acid, the nitrous acid and nitrogen oxides are absorbed, while the chlorine is sent to the bleaching powder chambers.
The Sadler-Wilson:l: nitric acid-chlorine process consists in reacting upon hydrochloric acid with nitric acid, in the presence of sulphuric acid; the operation is carried on in a heated "decomposer" built of flagstones. The hot, dilute sulphuric acid is again concentrated, and the gases from the decomposer are cooled and passed through a Gay Lussac tower to recover the nitrous vapors. The chlorine is washed to remove free hydrochloric acid, dried and led into the bleaching powder chambers.
Many attempts have been made to recover the chlorine from the waste liquors of the ammonia soda process, but no one of them has yet proved a commercial success. Several of them are, however, interesting, and deserve a few words.
Solvay conducted elaborate experiments in which he tried to realize the reaction:-
CaC]2+ Si02 + 0 = CaSiOj +C]2'
But calcium chloride is very stable, and its decomposition in this way is incomplete, and requires enormous expenditure of heat, besides that used in evaporating the solution of calcium chloride to dryness.
Magnesium chloride is more easily decomposed than calcium chloride, and several processes have been devised, based on the use of this salt. It is proposed to use magnesium oxide or hydroxide instead of lime for decomposing the ammonium chloride solution of the ammonin. process; by this, magnesium chloride is formed n.nd the ammonin. gas set free. Both Solvay and "Weldon, within a few dn.ys of each other, patented methods for carrying out this iden.. But the reaction between ammonium chloride and magnesia i~ not complete, and the solution of magnesium chloride obtained is dilute. Viewed as a method for chlorine, more promising results were obtained by using the concentrated magnesium chloride mother-liquors from the Stassfurt industries, or from other manufacturing operations. The magnesium chloride solution is evaporated to dryness at n. very low temperature, and the dried chloride is decomposed by passing air or steam over it while heated to a red heat. The reactions are as follows: -
1) lIgCI2 + 0 = iIgO + C12.
2) IIgC12 +H20 = )IgO +2 HCI.
The hydrochloric acid obtained is used in the Weldon or Deacon process. 1'he Weldon-pechiney * process was the most successful of the magnesia methods, though none of them can be said to be profitable. In this, magnesium chloride solution (made by dissolving the oxide in hydrochloric acid, or obtained from waste liquors) is concentrated until it contains six molecules of water for each molecule of magnesium chloride; then it equivalents of magnesium oxide is stirred into the solution. The pasty mass heats and soon hardens to n. solid cake of magnesium oxychloride, which is broken into lumps about the size of a butternut, and screened to remove the dust. The presence of dust causes the mass to cake badly during the subsequent drying. The lumps are dried at a temperature not exceeding 300° C., by passing a current of hot air over them while spread in a thin layer on gratings. Too high temperature causes a loss of chlorine as such. If not thoroughly dried, chlorine is lost as hydrochloric acid. The dried oxychloride is quickly decomposed in a special form of retort, which has been heated by producer gas to a temperature of 1000° C. before the charge is introduced. Air is passed into the retort to assist in the decomposition, which must be rapid, or the yield of chlorine is reduced. Magnesium oxide is left in the retort, while a mixture of chlorine, hydrochloric acid, and nitrogen escapes. The hydrochloric acid is recovered by washing the gases with water, and is used to dissolve part of the oxide from the retort. The chlorine, mixed with nitrogen, is used for bleaching powder, or for chlorate making, preferably the latter.
The residue of magnesium oxide from the retorts is returned to the first stage of the process.
The yield, including the hydrochloric acid recovered, is about 88 per cent of the whole amount of chlorine in the magnesium chloride. About 40 per cent is obtained as free chlorine, and 48.5 per cent is returned to the process as MgC12 and HOL Of the several methods that have been devised for the direct production of chlorine from the ammonium chloride formed in the ammonia soda industry, Mond's process,* which provides for the recovery of the ammonia, has been most carefully developed, but its practical success is as yet problematical. It is based on the dissociation of ammonium chloride into ammonia and hydrochloric acid, at a temperature of 350°-360° C.; the hydrochloric acid being then combined with some metallic oxide, to form a non-volatile chloride, to be later decomposed with liberation of the chlorine. Oxide of nickel was used at first, but was later abandoned in favor of magnesium oxide. The reactions are:-
1) MgO + (2 NHs + 2 HCl) = 1IgCl2 + Hp + 2 NHs.
2) MgCl2 + 0 = MgO + Cl2•
Since an excess of magnesia is present, it is very probable that considerable magnesium oxyehloride is also formed, according to the reaction: -
2 :i.IgO+ 2 HCl = 1IgO . lIgCl2 + H20.
Then this is decomposed by the air (reaction 2), thus:-
:i.IgO·2 :i.IgC12+ 0 = 2 :i.IgO+ C12•
The liberated ammonia passes from the apparatus to the scrubbers of the ammonia recovery process. The complete recovery of this ammonia is the first essential to the success of this method. The ammonium chloride is crystallized from the liquors of the Solvay carbonating towers (p. 87), by cooling them to about 00 C. The dry crystals are then vaporized by introducing them into melted zinc chloride, contained in an iron vessel lined with an antimony alloy.
The magnesium oxide, mixed with some potassium chloride, china clay, and lime, is made into balls ("pills "), about one-half inch in diameter, and baked. The decomposer is then filled with the "pills" and heated to 3600 C., when vapors of ammonium chloride are passed through the apparatus. The reaction between the ammonium chloride and magnesia raises the temperature in the decomposer above ,1000 C. Next, inert gases, such as those from limekilns, heated to 5500 C., are passed into the apparatus to drive out the ammonia and water vapors; these also heat the charge above 5000 C. Air, heated to 8000 C., is then admitted to break up the magnesium chloride (reaction 2) and regenerate the oxide; it also sweeps out the chlorine formed. After cooling to 3600 C. ammonium chloride vapors are again introduced and the cycle of operations is repeated. To secure uninterrupted working, there are usually four decomposers in each plant .
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