Читать книгу Industrial Poisoning from Fumes, Gases and Poisons of Manufacturing Processes - Josef Rambousek - Страница 8

Manufacture.—Sulphur dioxide, generally obtained by roasting pyrites in furnaces of various constructions, or, more rarely, by burning brimstone or sulphur from the spent oxide of gas-works, serves as the raw material for the manufacture of sulphuric acid. Before roasting the pyrites is crushed, the ‘lump ore’ then separated from the ‘smalls,’ the former roasted in ‘lump-burners’ or kilns (generally several roasting furnace hearths united into one system), and the latter preferably in Malétra and Malétra-Schaffner shelf-burners (fig. 1) composed of several superimposed firebrick shelves. The pyrites is charged on to the uppermost shelf and gradually worked downwards. Pyrites residues are not suitable for direct recovery of iron, but copper can be recovered from residues sufficiently rich in metal by the wet process; the residues thus freed of copper and sulphur are then smelted for recovery of iron.

Fig. 1.—Pyrites Burner for Smalls (after Lueger)

Utilisation for sulphuric acid manufacture of the sulphur dioxide given off in the calcining of zinc blende (see Spelter works), impracticable in reverberatory furnaces, has been made possible at the Rhenania factory by introduction of muffle furnaces (several superimposed), because by this means the gases led off are sufficiently concentrated, as they are not diluted with the gases and smoke from the heating fires. This method, like any other which utilises the gases from roasting furnaces, has great hygienic, in addition to economical, advantages, because escape of sulphur dioxide gas is avoided. Furnace gases, too poor in sulphur dioxide to serve for direct production of sulphuric acid, can with advantage be made to produce liquid anhydrous sulphur dioxide. Thus, the sulphur dioxide gas from the furnaces is first absorbed by water, driven off again by boiling, cooled, dried, and liquefied by pressure.

The gaseous sulphur dioxide obtained by any of the methods described is converted into sulphuric acid either by (a) the chamber process or (b) the contact process.

In the lead chamber process the furnace gases pass through flues in which the flue dust and a portion of the arsenious acid are deposited into the Glover tower at a temperature of about 300° C., and from there into the lead chambers where oxidation of the sulphur dioxide into sulphuric acid takes place, in the presence of sufficient water, by transference of the oxygen of the air through the intervention of the oxides of nitrogen. The gases containing oxides of nitrogen, &c., which are drawn out of the lead chambers, have the nitrous fumes absorbed in the Gay-Lussac tower (of which there are one or two in series), by passage through sulphuric acid which is made to trickle down the tower. The sulphuric acid so obtained, rich in oxides of nitrogen, and the chamber acid are led to the Glover tower for the purpose of denitration and concentration, so that all the sulphuric acid leaves the Glover as Glover acid of about 136-144° Tw. Losses in nitrous fumes are best made up by addition of nitric acid at the Glover or introduction into the first chamber. The deficiency is also frequently made good from nitre-pots.

The lead chambers (fig. 2) are usually constructed entirely—sides, roof, and floor—of lead sheets, which are joined together by means of a hydrogen blowpipe. The sheets forming the roof and walls are supported, independent of the bottom, on a framework of wood. The capacity varies from 35,000 to 80,000 cubic feet. The floor forms a flat collecting surface for the chamber acid which lutes the chamber from the outer air. The necessary water is introduced into the chamber as steam or fine water spray.

The Glover and Gay-Lussac towers are lead towers. The Glover is lined with acid-proof bricks and filled with acid-proof packing to increase the amount of contact. The Gay-Lussac is filled with coke over which the concentrated sulphuric acid referred to above flows, forming, after absorption of the nitrous fumes, nitro-sulphuric acid.

Fig. 2a.—Lead Chamber System—Section through X X (after Ost)

Fig. 2b.—Lead Chamber System—Plan

 A Pyrites Burner

 B Glover Tower

 C Draft Regulator

 D, D´ Lead Chambers

 E Air Shaft

 F, F,´ F,´´ F´´´ Acid Reservoirs

 G Acid Egg

 H Cooler

 J Gay-Lussac Tower

As already stated, two Gay-Lussac towers are usually connected together, or where there are several lead-chamber systems there is, apart from the Gay-Lussac attached to each, a central Gay-Lussac in addition, common to the whole series. The introduction of several Gay-Lussac towers has the advantage of preventing loss of the nitrous fumes as much as possible—mainly on economical grounds, as nitric acid is expensive. But this arrangement is at the same time advantageous on hygienic grounds, as escape of poisonous gases containing nitrous fumes, &c., is effectually avoided. The acids are driven to the top of the towers by compressed air. The whole system—chambers and towers—is connected by means of wide lead conduits. Frequently, for the purpose of quickening the chamber process (by increasing the number of condensing surfaces) Lunge-Rohrmann plate towers are inserted in the system—tall towers lined with lead in which square perforated plates are hung horizontally, and down which diluted sulphuric acid trickles.

To increase the draught in the whole system a chimney is usual at the end, and, in addition, a fan of hard lead or earthenware may be introduced in front of the first chamber or between the two Gay-Lussac towers. Maintenance of a constant uniform draught is not only necessary for technical reasons, but has hygienic interest, since escape of injurious gases is avoided (see also Part III).

The chamber acid (of 110°-120° Tw. = 63-70 %) and the stronger Glover acid (of 136°-144° Tw. = 75-82 %) contain impurities. In order to obtain for certain purposes pure strong acid the chamber acid is purified and concentrated. The impurities are notably arsenious and nitrous acids (Glover acid is N free), lead, copper, and iron. Concentration (apart from that to Glover acid in the Glover tower) is effected by evaporation in lead pans to 140° Tw. and finally in glass balloons or platinum stills to 168° Tw. (= 97 %). The lead pans are generally heated by utilising the waste heat from the furnaces or by steam coils in the acid itself, or even by direct firing.

Production of sulphuric acid by the contact method depends on the fact that a mixture of sulphur dioxide and excess of oxygen (air) combines to form sulphur trioxide at a moderate heat in presence of a contact substance such as platinised asbestos or oxide of iron. The sulphur dioxide must be carefully cleaned and dried, and with the excess of air is passed through the contact substance. If asbestos carrying a small percentage of finely divided platinum is the contact substance, it is generally used in the form of pipes; oxide of iron (the residue of pyrites), if used, is charged into a furnace. Cooling by a coil of pipes and condensation in washing towers supplied with concentrated sulphuric acid always forms a part of the process. A fan draws the gases from the roasting furnaces and drives them through the system. The end product is a fuming sulphuric acid containing 20-30 per cent. SO₃. From this by distillation a concentrated acid and a pure anhydride are obtained. From a health point of view it is of importance to know that all sulphuric acid derived from this anhydride is pure and free from arsenic.

The most important uses of sulphuric acid are the following: as chamber acid (110°-120° Tw.) in the superphosphate, ammonium sulphate, and alum industries; as Glover acid (140°-150° Tw.) in the Leblanc process, i.e. saltcake and manufacture of hydrochloric acid, and to etch metals; as sulphuric acid of 168° Tw. in colour and explosives manufacture (nitric acid, nitro-benzene, nitro-glycerine, gun-cotton, &c.); as concentrated sulphuric acid and anhydride for the production of organic sulphonic acids (for the alizarin and naphthol industry) and in the refining of petroleum and other oils. Completely de-arsenicated sulphuric acid is used in making starch, sugar, pharmaceutical preparations, and in electrical accumulator manufacture.

Effects on Health.—The health of sulphuric acid workers cannot in general be described as unfavourable.

In comparison with chemical workers they have, it is said, relatively the lowest morbidity. Although in this industrial occupation no special factors are at work which injure in general the health of the workers, there is a characteristic effect, without doubt due to the occupation—namely, disease of the respiratory organs. Leymann’s figures are sufficiently large to show that the number of cases of diseases of the respiratory organs is decidedly greater in the sulphuric acid industry than among other chemical workers. He attributes this to the irritating and corrosive effect of sulphur dioxide and sulphuric acid vapour on the mucous membrane of the respiratory tract, as inhalation of these gases can never be quite avoided, because the draught in the furnace and chamber system varies, and the working is not always uniform. Strongly irritating vapours escape again in making a high percentage acid in platinum vessels, which in consequence are difficult to keep air-tight. Of greater importance than these injurious effects from frequent inhalation of small quantities of acid vapours, or employment in workrooms in which the air is slightly charged with acid, is the accidental sudden inhalation of large quantities of acid gases, which may arise in the manufacture, especially by careless attendance. Formerly this was common in charging the roasting furnaces when the draught in the furnace, on addition of the pyrites, was not strengthened at the same time. This can be easily avoided by artificial regulation of the draught.

Accidents through inhalation of acid gases occur further when entering the lead chambers or acid tanks, and in emptying the towers. Heinzerling relates several cases taken from factory inspectors’ reports. Thus, in a sulphuric acid factory the deposit (lead oxysulphate) which had collected on the floor of a chamber was being removed: to effect this the lead chambers were opened at the side. Two of the workers, who had probably been exposed too long to the acid vapours evolved in stirring up the deposit, died a short time after they had finished the work. A similar fatality occurred in cleaning out a nitro-sulphuric acid tank, the required neutralisation of the acid by lime before entering having been omitted. Of the two workers who entered, one died the next day; the other remained unaffected. The deceased had, as the post mortem showed, already suffered previously from pleurisy. A fatality from breathing nitrous fumes is described fully in the report of the Union of Chemical Industry for the year 1905. The worker was engaged with two others in fixing a fan to a lead chamber; the workers omitted to wait for the arrival of the foreman who was to have supervised the operation. Although the men used moist sponges as respirators, one of them inhaled nitrous fumes escaping from the chamber in such quantity that he died the following day.

Similar accidents have occurred in cleaning out the Gay-Lussac towers. Such poisonings have repeatedly occurred in Germany. Fatal poisoning is recorded in the report of the Union of Chemical Industry, in the emptying and cleaning of a Gay-Lussac tower despite careful precautions. The tower, filled with coke, had been previously well washed with water, and during the operation of emptying, air had been constantly blown through by means of a Körting’s injector. The affected worker had been in the tower about an hour; two hours later symptoms of poisoning set in which proved fatal in an hour despite immediate medical attention. As such accidents kept on recurring, the Union of Chemical Industry drew up special precautions to be adopted in the emptying of these towers, which are printed in Part III.

Naturally, in all these cases it is difficult to say exactly which of the acid gases arising in the production of sulphuric acid was responsible for the poisoning. In the fatal cases cited, probably nitrous fumes played the more important part.

Poisoning has occurred in the transport of sulphuric acid. In some of the cases, at all events, gaseous impurities, especially arseniuretted hydrogen, were present.

Thus, in the reports of the German Union of Chemical Industry for the year 1901, a worker succumbed through inhalation of poisonous gases in cleaning out a tank waggon for the transport of sulphuric acid. The tank was cleaned of the adhering mud, as had been the custom for years, by a man who climbed into it. No injurious effects had been noted previously at the work, and no further precautions were taken than that one worker relieved another at short intervals, and the work was carried on under supervision. On the occasion in question, however, there was an unusually large quantity of deposit, although the quality of the sulphuric acid was the same, and work had to be continued longer. The worker who remained longest in the tank became ill on his way home and died in hospital the following day; the other workers were only slightly affected. The sulphuric acid used by the firm in question immediately before the accident came from a newly built factory in which anhydrous sulphuric acid had been prepared by a special process. The acid was Glover acid, and it is possible that selenium and arsenic compounds were present in the residues. Arseniuretted hydrogen might have been generated in digging up the mud. Two similar fatalities are described in the report of the same Union for the year 1905. They happened similarly in cleaning out a sulphuric acid tank waggon, and in them the arsenic in the acid was the cause. Preliminary swilling out with water diluted the remainder of the sulphuric acid, but, nevertheless, it acted on the iron of the container. Generation of hydrogen gas is the condition for the reduction of the arsenious acid present in sulphuric acid with formation of arseniuretted hydrogen. In portions of the viscera arsenic was found. Lately in the annual reports of the Union of Chemical Industry for 1908 several cases of poisoning are described which were caused by sulphuric acid. A worker took a sample out of a vessel of sulphuric acid containing sulphuretted hydrogen gas. Instead of using the prescribed cock, he opened the man-hole and put his head inside, inhaling concentrated sulphuretted hydrogen gas. He became immediately unconscious and died. Through ignorance no use was made of the oxygen apparatus.

Another fatality occurred through a foreman directing some workers, contrary to the regulations against accidents from nitrous gases, to clean a vessel containing nitric and sulphuric acids. They wore no air helmets: one died shortly after from inhalation of nitrous fumes. Under certain circumstances even the breaking of carboys filled with sulphuric acid may give rise to severe poisoning through inhalation of acid gases. Thus a fatality1 occurred to the occupier of a workroom next some premises in which sulphuric acid carboys had been accidentally broken. Severe symptoms developed the same night, and he succumbed the next morning in spite of treatment with oxygen. A worker in the factory became seriously ill but recovered.

A similar case is described2 in a factory where concentrated sulphuric acid had been spilt. The workers covered the spot with shavings, which resulted in strong development of sulphur dioxide, leading to unconsciousness in one worker.

The frequent observation of the injurious effect of acid gases on the teeth of workers requires mention; inflammation of the eyes of workers also is attributed to the effects of sulphuric acid.

Leymann’s statistics show corrosions and burns among sulphuric acid workers to be more than five times that among other classes. Such burns happen most frequently from carelessness. Thus, in the reports of the Union of Chemical Industry for 1901, three severe accidents are mentioned which occurred from use of compressed air. In two cases the acid had been introduced before the compressed air had been turned off; in the third the worker let the compressed air into the vessel and forgot to turn off the inlet valve. Although the valves were provided with lead guards, some of the acid squirted into the worker’s face. In one case complete blindness followed, in a second blindness in one eye, and in the third blindness in one eye and impaired vision of the other.

Besides these dangers from the raw material, bye-products, and products of the manufacture, lead poisoning has been reported in the erection and repair of lead chambers. The lead burners generally use a hydrogen flame; the necessary hydrogen is usually made from zinc and sulphuric acid and is led to the iron by a tube. If the zinc and sulphuric acid contain arsenic, the very dangerous arseniuretted hydrogen is formed, which escapes through leakages in the piping, or is burnt in the flame to arsenious acid.

Further, the lead burners and plumbers are exposed to the danger of chronic lead poisoning from insufficient observance of the personal precautionary measures necessary to guard against it (see Part III). Those who are constantly engaged in burning the lead sheets and pipes of the chambers suffer not infrequently from severe symptoms. Unfortunately, the work requires skill and experience, and hence alternation of employment is hardly possible.

Finally, mention should be made of poisoning by arseniuretted hydrogen gas from vessels filled with sulphuric acid containing arsenic as an impurity, and by sulphuretted hydrogen gas in purifying the acid itself. In the manufacture of liquid sulphur dioxide, injury to health can arise from inhalation of the acid escaping from the apparatus. The most frequent cause for such escape of sulphur dioxide is erosion of the walls of the compressor pumps and of the transport vessels, in consequence of the gas being insufficiently dried, as, when moist, it attacks iron.

Sulphur dioxide will come up for further consideration when describing the industrial processes giving rise to it, or in which it is used.