Lead Chamber Process

Lead Chamber Process had its origin in the early preparation of sulphuric acid by the oxidation of sulphur dioxide with nitric acid, for which operation lead chambers were first introduced in 1746 by Roebuck of Birmingham. In 1793 Clement and Desormes showed that under proper conditions the nitric acid aids the oxidation, which is in the main effected by atmospheric oxygen, and the injection of steam having already been introduced in 1774 by de la Follie, the basal chemical process was much the same as to-day. Gay-Lussac's tower was first suggested by that chemist in 1827 and was first used in 1835, being introduced into Britain in 1844. J. Glover constructed his first tower at Newcastle in 1861.

Until 1838 the sulphur dioxide was obtained entirely from free Sicilian sulphur, but on account of increasing price iron pyrites was substituted, and by 1862 free sulphur was practically no longer used. The abundant output of American sulphur has of recent years revived the employment of sulphur, which now to an ever-increasing extent is displacing iron pyrites as the chief source of sulphur dioxide. Other metallic sulphides which necessitate a preliminary roasting in their metallurgical treatment, for example zinc blende, also serve as subordinate sources of sulphur dioxide, but with such materials it is necessary that fluorine should first be removed from the gases by passing the latter up a tower packed with quartz.

Of the total output of sulphuric acid in Great Britain and Northern Ireland during 1928 (see above), 49 per cent, was produced from pyrites, over 24 per cent, from " spent oxide," 17.56 per cent, from sulphur, and 9.26 per cent, from zinc concentrates.

In the lead chamber process the first chemical action is the oxidation of sulphur to sulphur dioxide by atmospheric oxygen. The iron pyrites (or free sulphur, " spent oxide " from the gas works, or other sulphides such as zinc blende, as the case may be) is placed on shelves or bars in a series of ovens of suitable type. When iron pyrites or sulphur is used, the combustion when once started proceeds to completion without further assistance by external heat:

4FeS₂ + 11O₂ = 2Fe₂O₃ + 8SO₂.

The supply of air, which is drawn through the " burners " by suction, is so adjusted that sufficient oxygen will be present to oxidise the sulphur dioxide subsequently to sulphuric acid and also finally to leave the oxides of nitrogen (see the following) in approximately the composition represented by the formula N₂O₃.

From the pyrites burners the gases pass through a flue for the collection of dust and then at a temperature of approximately 350° C. enter the bottom of the Glover tower. This lead tower, about thirty feet high and lined with stoneware, is packed with some resistant material such as flints, or more frequently in modern practice with a specially prepared filling, for example small hollow stoneware cylinders. By means of a distributing device, dilute sulphuric acid (about 65 per cent.) from the chambers ("chamber acid") and concentrated sulphuric acid containing dissolved oxides of nitrogen from the Gay-Lussac tower (see the following) are introduced at the top of the Glover tower and trickle down over the filling material, thus exposing a large surface to the ascending current of hot gases; the Gay-Lussac tower acid thereby gives up its oxides of nitrogen, whilst the dilute acid becomes concentrated; the acid which issues from the bottom of the Glover tower has a temperature of 120° to 130° C. and contains roughly 80 per cent, of pure acid.

Another purpose served by the Glover tower is that it relieves the lead chambers of the production of part of the acid, and it may account for as much as 16 per cent, or more of the total yield of acid, due to the conversion of sulphur dioxide into sulphuric acid inside the tower; indeed, a tower of similar type is sometimes interposed between two of the chambers with a view to the same result.

The capacity of the Glover tower is usually 2½ to 3 per cent, that of the chambers, and in order to obviate the need for inconveniently large towers, two are sometimes employed in series.

With a view to remedying any shortage in the supply of oxides of nitrogen from the Gay-Lussac tower, due to unavoidable loss, a little nitric acid is introduced into the gaseous mixture, usually down the Glover tower but sometimes, although less frequently, by placing pots containing nitre and sulphuric acid in the flue between the pyrites burners and the Glover tower. In some cases the oxides of nitrogen are supplied by the catalytic oxidation of ammonia, the platinum catalyst being electrically heated.

On leaving the Glover tower the gaseous mixture of sulphur dioxide, oxygen, nitrogen oxides and atmospheric nitrogen, now cooled to 90° C., passes into the lead chambers. These are constructed of sheet lead about 3 mm. thick; the walls and roof are sealed together by a blowpipe flame, the edges of the base being turned up so as to form a flat dish in which the acid collects and forms a gas-tight seal between the base and the sides. The whole chamber is supported in a wooden framework on pillars so that each part is accessible, the exposure to the atmosphere also aiding cooling. Chambers frequently exceed 40,000 cubic feet in individual capacity and a series of three or four is commonly used, the gases being conducted from one to the other as well as to and from the chamber system by lead pipes.

Much depends on the proper construction of the chambers; they may be rectangular or circular, the latter possessing the advantage of allowing more complete mixing with the steam, the gases being introduced at the side tangentially and then moving spirally until the central exit is reached. It is mainly on account of the success attained by the use of the more modern forms of chambers that the chamber process is still able to remain in vigorous existence.

Steam, or water in the form of very fine spray, is injected into the chambers, sulphuric acid being formed and falling as a drizzle to the floor, whilst the oxides of nitrogen, having exercised their influence on the reaction between sulphur dioxide, oxygen and water (see the following), pass away with the atmospheric nitrogen. Dilute sulphuric or nitric acid or a solution of nitrous oxides in sulphuric acid (density 1.59) may be sprayed into the chamber instead of water or steam; there are advantages, however, in using cold water, since besides effecting an economy in nitre or nitric acid, the lower working temperature is suitable for a satisfactory reaction. It is unnecessary to go to the expense of raising steam in order to condense it again in the chambers.

Great care is required in regulating the process in the chambers; the gases need to be well mixed; they must not be allowed to attain too high a temperature during the reaction, and they must not emerge too soon from the chambers. The supply of water or steam has also to be adjusted carefully, too much causing an unnecessarily dilute acid and also tending to induce an excessive reduction of the oxides of nitrogen to nitrous oxide or even to nitrogen, whereas too little will give rise to an acid so concentrated that the lead is seriously attacked and the life of the chambers, which should be from 10 to 20 years, considerably shortened. The concentration of the oxides of nitrogen present is also an important factor influencing the yield of sulphuric acid obtained.

Lead-covered towers ("plate towers") fitted internally with perforated stoneware plates are sometimes interposed between the chambers, or the latter may in part be replaced by a series of smaller absorption boxes divided longitudinally into three narrow compartments by means of perforated walls, the gases entering the middle compartment and leaving from the side compartments. Such arrangements not only aid the blending of the gases, but also allow more than an equivalent reduction in the capacity of the chambers.

On leaving the chambers the gases contain only traces of sulphur dioxide, and are red in colour on account of the presence of the oxides of nitrogen. The latter are removed by leading the mixture to a circular tower, or sometimes two towers, the height of which may be 26 to 65 feet and the capacity about 4 per cent, that of the chambers. For convenience in the arrangement of the works the Gay-Lussac towers are usually built near the Glover tower.

Diagram illustrating the " Lead Chamber Process." A, B, C, D, E, F, Pyrites Burners; G, Glover Tower; G.L., Gay-Lussac Tower; L, Lead Chambers.

Down the towers and over the filling of hard coke or other material there trickles sulphuric acid of 80 per cent, concentration, for example, acid from the Glover tower, which absorbs the mixture of nitrogen dioxide and nitric oxide from the ascending gases; the solution thus obtained is then pumped to the top of the Glover tower, in which the oxides of nitrogen are again taken up by the gaseous current and once more find their way to the chambers. With perfect working, the gas issuing from the Gay-Lussac tower will consist of almost pure nitrogen, and is led away into the factory chimney. A water scrubber is sometimes placed between the two Gay-Lussac towers; the water converts nitrogen dioxide into nitric acid (in solution) and nitric oxide, and the latter after reoxidation is absorbed in the last tower.

There is, however, some loss of the valuable nitrous gases, necessitating the introduction of small quantities of nitric acid at or before the Glover tower or into the chamber itself; the loss is probably due mainly to the reduction of the oxides to nitrous oxide and even nitrogen, which are not absorbed in the Gay-Lussac tower.

The natural draught of the factory chimney was at one time found sufficient for the movement of the gases through the plant, but to-day suitable fans are commonly installed to aid the circulation.

The gases issuing from the chambers consist mainly of nitrogen dioxide, nitric oxide and atmospheric nitrogen. The two former are dissolved by the sulphuric acid in the Gay-Lussac tower with formation of a solution of nitrosylsulphuric acid in excess of sulphuric acid:

NO + NO₂ + 2H₂SO₄ ⇔ 2NO₂.SO₂.OH + H₂O.

Occasionally the resulting acid solution is violet in colour, due to the presence of the unstable sulphonitronic acid, and it then readily decomposes with effervescence of nitric oxide.

As the formation of nitrosylsulphuric acid is a reversible process, dilution of the sulphuric acid solution in the Glover tower tends to cause hydrolysis, which is aided by the high temperature; the oxides of nitrogen resulting from the decomposition pass on with the sulphur dioxide and excess of air to the chambers, whilst the sulphuric acid descends and issues at the bottom of the tower.

In addition to the foregoing process, however, the Glover tower actually produces sulphuric acid. This is brought about by interaction of the sulphur dioxide in the burner gases with nitrosylsulphuric acid, as follows:

2NO₂.SO₂.OH + SO₂ + 2H₂O = 3H₂SO₄ + 2NO.

The actual mechanism of the chemical process in the lead chambers has for years been a matter of much conjecture and controversy. The formation of unstable intermediate compounds which subsequently undergo decomposition with production of sulphuric acid is almost universally accepted, but unfortunately there is no general agreement as to the identity of the intermediate compound or compounds.

G. Lunge favoured a view, first put forward by H. Davy in 1812, that sulphur dioxide, water, oxygen and nitrogen trioxide (or a mixture of nitric oxide and nitrogen dioxide) interact with formation of nitrosylsulphuric acid, which subsequently undergoes decomposition by water producing sulphuric acid and reproducing "nitrogen trioxide"; the latter is then able once more to give rise to nitrosylsulphuric acid. The cycle of changes proceeds until all the sulphur dioxide and oxygen are consumed:

2SO₂ + NO + NO₂ + O₂ + H₂O = 2NO₂.SO₂.OH,
2NO₂.SO₂.OH + H₂O = 2H₂SO₄ + NO + NO₂.

In the first chamber, and also in the Glover tower, the gases are usually colourless, and it is possible that the excess of sulphur dioxide here causes a reduction to nitric oxide:

2NO₂.SO₂.OH + SO₂ + 2H₂O = 3H₂SO₄ + 2NO.

More recently this theory has been modified by the introduction of another intermediate product, namely the violet or blue sulphonitronic acid which is supposed to precede the formation of the nitrosylsulphuric acid. The series of changes is then represented as follows:



the sulphonitronic acid undergoing conversion by oxygen or water into nitrosylsulphuric acid, which is then decomposed by water giving sulphuric acid. This theory loses somewhat in attractiveness, however, on account of the uncertainty of the composition of the sulphonitronic acid.

Free nitrosylsulphuric acid actually occurs in the lead chambers only under abnormal conditions of working; its separation as "chamber crystals" is most undesirable from the manufacturer's point of view, as not only does it indicate improper regulation of the process, but it also causes the lead walls of the chambers to be attacked unduly. A solution of "violet acid" is sometimes obtained at the bottom of the Gay-Lussac tower.

An alternative explanation proposed by F. Raschig, who also regarded nitrogen trioxide or nitrous acid as the form in which the nitrogen oxides are active, is based on the primary formation of a hypothetical nitrososulphonic acid, as represented by the equations:

2NO + O + H₂O = 2HNO₂,
HNO₂ + SO₂ = NO.SO₂.OH.

The nitrososulphonic acid then reacts with a second molecule of nitrous acid, producing the purple sulphonitronic acid. As stated earlier, the composition of this acid is uncertain, but assuming the formula H₂SNO₅, the equation for the reaction is

NO.SO₂.OH + HNO₂ = H₂SNO₅ + NO

whilst the final reaction yielding sulphuric acid is H₂SNO₅ = H₂SO₄ + NO.

An advantage possessed by this series of changes is that it explains more satisfactorily than the preceding theory why much sulphuric acid can be formed in the Glover tower. In the lead chambers the acid formed is so weak that the nitrososulphonic acid stage is possibly absent from the series.

The preceding views have been subjected to much criticism, both mutual and independent. Manchot regards the "blue acid" as an oxide of composition between NO_1.5 and NO. The formula H₂SNO₅ suggested by Raschig is improbable, as the absorption spectrum is not in any way similar to that of the compound CuSO₄.NO or FeSO₄.NO. A modification of Raschig's theory, whilst avoiding the two foregoing hypothetical acids, postulates the transient formation of yet another unknown acid. Nitrous acid and sulphur dioxide are supposed to condense to a "nitroxysulphuric acid," , which immediately decomposes into sulphuric acid and nitric oxide; the latter by uniting with more oxygen and water yields nitrous acid, which reenters the cycle of changes:

SO₂ + 2HNO₂ =,
= 2NO + H₂SO₄,
2NO + O + H₂O = 2HNO₂.

At the present time there is a tendency to a reversion to a simpler conception of the changes involved in the production of sulphuric acid. Already in 1844 Peligot had suggested that the oxides of nitrogen act simply as "oxygen carriers," thus:

2NO + O₂ = 2NO₂,
2NO₂ + SO₂ + 2H₂O = 2H₂SO₄ + 2NO,
2NO + O₂ = 2NO₂, etc.

It appears quite likely that the more complicated explanations referred to may be due to some confusion of less simple side reactions with a simple main reaction. Even at temperatures below 60° C., nitrogen dioxide is able to oxidise sulphur dioxide to sulphur trioxide, whilst nitric oxide readily combines with the latter to give the compound 2SO₃.NO, which is readily decomposed by water giving sulphuric acid and nitric oxide. Until stronger and more convincing evidence is forthcoming as to the formation of the major portion of the sulphuric acid from unstable nitrogen-sulphur oxyacids in the chambers, the only really satisfactory course for general purposes is to adhere to the simple view formulated in the immediately preceding series of equations. Even if these should prove not to show accurately the actual intermediate stages, they do at least represent truly not only the first and last stages of the process with respect to sulphur dioxide and oxides of nitrogen, but also the general nature of the change by which the sulphuric acid is produced.

The acid from the Glover tower generally contains flue dust (largely ferric oxide), on which account it is used up in the acid factory itself, part being returned to the Gay-Lussac tower and part being used for the preparation of sodium sulphate. For this reason it is not essential that pure nitric acid should be introduced at the Glover tower, and frequently an aqueous solution of the cheaper sodium nitrate is used in its place.

The remaining " chamber acid," containing 63 to 70 per cent, of H₂SO₄, is concentrated in a series of lead pans (heated on the "counter-current" principle), when water vapour passes off. When a concentration of 77 to 80 per cent, has been attained it is inadvisable to proceed further with the lead pans as the metal begins to reduce the acid. Until the beginning of the present century the subsequent concentration was carried out in porcelain, glass or platinum (gold-lined) retorts, or in a Kessler apparatus. Glass retorts have fallen into disfavour on account of their easy fracture on contact with the hot fire gases, whilst platinum is very expensive. The Kessler method, which is very successful in plants of moderate output, consists in passing hot gases from a coke producer over the acid contained in a shallow flat dish made of volvic stone, and then through a series of superimposed trays luted with acid; the weak acid passes downwards through the trays, meets the hot gases, and arrives at the bottom tray in a highly concentrated condition.

In 1906, Gaillard introduced a system of acid concentration, now extensively used, which has proved efficient and can be adapted (by multiplication of units) to any scale of acid production.

Gaillard Concentration Plant. A Furnace. B Main tower. C Becuperator. D Scrubber. E Tank. F Fans. G Exit pipe. H Eggs receiving acid from tanks and passing it to atomisers.

The weak acid (70 per cent.) is atomised in a fine mist down a tower built of obsidianite or acid-proof bricks (see fig.), up which hot gases from the combustion of producer gas are ascending. These gases enter at a temperature of 750° to 800° C., and after passing through the tower, which acts as a flue, pass on to a smaller tower of similar design (a recuperator), down which atomised acid undergoes a preliminary concentration before being fed to the main tower. The gases leave the recuperator at about 120° C. and pass to a scrubber, generally a lead-lined rectangular chamber or tower, the bottom of which is paved with acid-proof bricks. The scrubber is packed with coke, the resistance of which to the passage of the gas causes condensation of the acid carried o^er and removes any foreign matter, which forms a slime on the surface of the coke. The condensed acid is conveyed to the feed tank of the main tower; the gases pass to the exit pipe, the draught through the system being produced by high pressure fans.

The volume of acid passed down the recuperator is approximately half that fed to the main tower. The acid flows out of the recuperator at about 150° to 160° C., runs through a water-cooled trough to a lead cooler, where the temperature is reduced to 50° C., and then flows to the feed tanks of the main tower.

Instead of the foregoing type of recuperator, a packed tower working on the Glover principle is sometimes used.

The acid leaving the main tower runs through a mud-catcher to a cooler and thence to the storage tanks. The concentration normally attained is 94 to 95 per cent.

A method due to Strzoda consists in passing the acid, after concentration in open lead pans, downwards through a series of vertical pipes heated externally by producer gas. The vapours evolved are passed through a cooling tower packed with suitable material, and the dilute acid recovered is passed back to the concentrating pipes.

The limit of concentration attainable by any of the foregoing methods is about 98 per cent. If acid of higher concentration than this is required, acid of 97 to 98 per cent, strength can be partially frozen, when the colourless prisms which separate contain 99.5 to 100 per cent. H₂SO₄ and constitute the frequently so-called, but misnamed, "monohydrate."

Owing to the success of the "contact process" for the preparation of sulphur trioxide and of fuming sulphuric acid, the production of highly concentrated sulphuric acid has been rendered so simple a matter that concentration of the lead chamber acid is of diminished importance. The main value of the chamber process lies in its economical production of a not necessarily very concentrated acid. There are indications, however, that by a suitable combination of the two processes, an even more economical production of concentrated acid may be obtainable.

At one time sulphuric acid for special purposes was purified from non-volatile foreign substances by distillation, but the process was so expensive, particularly as regards breakage of the retorts if of glass or porcelain, or their depreciation if of platinum, that distillation of the acid has now been abandoned as a technical process.

Arsenic, arising from the fumes of arsenious oxide produced in the combustion of impure pyrites, is the commonest impurity in the chamber acid. As the presence of this element is especially dangerous in acid destined for use in the preparation of foodstuffs, as for example glucose, the acid is purified usually by dilution and treatment with hydrogen sulphide or a metallic sulphide, for instance barium sulphide, or with a thiosulphate, for example barium thiosulphate; the arsenic is precipitated as sulphide and removed before the final concentration. An alternative but less frequently adopted procedure is to remove the arsenic as the volatile arsenious chloride by adding hydrochloric acid or a chloride and heating gently. In the concentration of sulphuric acid by freezing, practically the whole of any arsenic present remains in the liquid.

A recently adopted method for minimising the amount of arsenic in the acid is to pass the gases through a closed vessel placed between the Glover tower and the chambers and containing an oxidising agent such as nitric acid, which will retain the arsenic.

Nitrogen Oxides form another common impurity in crude sulphuric acid; their presence may be detected by the formation of a deep blue coloration on the addition of a little diphenylamine, or by the production of an azo-dye coloration on treating the diluted acid with suitable reagents such as m-phenylenediamine or a mixture of sulphanilic acid and α-naphthylamine. In the presence of dissolved nitrogen oxides a "brown ring" test can also be made by carefully adding ferrous sulphate solution. Oxides of nitrogen which, if the acid is not too weak, are present in the form of nitrosylsulphuric acid, can be removed by the introduction of the requisite quantity of ammonium sulphate, as first recommended by Pelouze; during the subsequent concentration the nitrogen is rapidly eliminated in the free state by a reaction analogous to the decomposition of ammonium nitrite:

NO₂.SO₂.OH + NH₄.HSO₄ = N₂ + 2H₂SO₄ + H₂O.

In the laboratory sulphuric acid can be freed from nitric acid by agitation with mercury in a Lunge nitrometer.

Lead Sulphate, which is soluble to a considerable extent in concentrated sulphuric acid, can be removed almost completely by dilution, when it is slowly precipitated.

The preceding impurities, as also others such as antimony and selenium, can all be eliminated by dilution and treatment with hydrogen sulphide, followed by a short heating of the resulting clear acid with a sufficient quantity of ammonium sulphate; in special cases this treatment might be followed by distillation or by recrystallisation of the "hydrate" H₂SO₄. The presence of selenium in sulphuric acid may be detected by the addition of aspidospermine, when an intense violet coloration develops, no colour being given by the pure acid. At the present day, however, acid of an extremely high degree of purity is not expected from the lead chamber process.