Sulphuric acid burners
A number of burners for smalls have been invented, but that
most commonly used is the Maletra burner (Fig. 22). This consists of a series of shelves, about 5 by 8 feet in size, a1'l'anged in a tall furnace. At the top is a hopper, through which the smalls are introduced, falling on the top shelf, on which they are spread out by rakes, introduced through the door (A). At the front of the shelf is an opening (B), through which they can be made to fall upon the a time they are rakell down to the next shelf, and so on, each shelf being hotter than the one preceding. Finally, the spent cinders are raked out at the bottom at (C). For starting the furnace, the shelves
are heated by a coal fire on a special grate, but after the temperature is sufficiently high to ignite the charge, combustion continues from the heat given out by the burning pyrites, provided the furnace is properly regulated and raked, and fresh are is introduced as needed.
Another form of smalls burner is Spence's furnace, in which the fine pyrites is put into a long muffle, heated by a coal fire or by the waste heat from lump burners. Furnaces modelled after the Spence type are much used for roasting zinc blende and copper mattes, in which the per cent of sulphur is too low to support combustion without external heat. Eyen from poor are, muffle furnaces yield a fairly concentrated sulphur dioxide gas.
The Perret-Ollivier furnace is a shelf burner, in which the shelves are set above the grate of an ordinary lump burner. The smalls are spread upon the shelves and the hot gases from the lump ore pass over the surface of the flames, heating the latter to the combustion point.
The several shelf burners above described must be raked at intervals, and this is very hard labor if done by hand rakes, besides allowing the entrance of an undue amount of air. To remedy these defects, mechanical raking apparatus has been applied, some being very successful, though expensive to build and keep in repair.
The Gerstenhofer furnace (Fig. 23) is used somewhat in Germany. It consists of a tall tower containing a series of grates made of triangular bars (T, T) of fire-clay set horizontally. The smalls are introduced from a hopper (H) at the top, in a regular stream, and falling from one grate to the next, are exposed to the hot fumes from the burning pyrites on the lower grates. The cinders are raked out at the bottom of the tower. These furnaces are about 17 feet high and 4 by 2 feet inside diameter, Before starting, the burner is heated
to a white heat by a coal fire on the grate (G), but no more fuel is used after the pyrites is once ignited. A proper regulation of the flow of smalls will maintain the requisite temperature.
The Hasenclever-Helbig burner consists of a lump bnrner, joined to a smalls furnace, which is a tower containing shelves, set at an incline of 38°, the alternate shelves being parallel. The smalls, fed in at the top through a hopper, fall on the first
shelf, and then slide down from one shelf to the next. At alternate ends of the shelves are openings for the fumes to pass upwards. The hot gases are thus forced to travel back
and forth, under and over each shelf as they ascend the tower. The rate of the flow of the smalls through the burners is regulated by the rapidity of the discharge of cinders through an automatic discharging wheel at the bottom.
The Glover tower is placed next to the burners, and is now used in nearly all sulphuric acid works. Its functions are to set free the nitrogen oxides from the Gay-Lussac tower acid; to cool the burner gases to a temperature of from 50° to GO° C. before they enter the lead chambers; to furnish a large part of the steam necessary in the lead chambers; and in many works to concentrate the dilute acid from the lead chambers to a density of 1.75 specific gravity. Moreover, it doubtless increases the yield of sulphuric acid from a plant of given lead chamber capacity, for some acid is formed directly in the tower itself in addition to that condensed in the lead chambers. The tower (20 to 30 feet high and about 10 feet across) is made of
sheet lead, joined as described on p. 53, and supported on a framework of timbers or steel. It is lined with acid-resisting brick or tile, laid without mortar,* and is filled with broken quartz lumps or flint stones: At the top is an apparatus for distributing the acid, which is to run down through the tower. The burner gases enter at the bottom, and pass up and out at the top, through a pipe leading to the lead chambers. These form the most important part of a sulphuric acid plant, and it is in them that the reactions involved of the formation of the acid take place. They are immense boxes, made by joining sheets of lead, and are supported from a strong timber framework, by means of lugs 01' strips of lead, attached to the outside of the sheets. The joints cannot be made with solder, but the edges of the sheets arc fused together by means of an aerohydrogen
flame, and the process, called "lead burning" is both difficult
and slow. At several points, steam may be introduced into the
chambers to supply water vapor as needed. Each chamber is suspended above a large, flat, lead pan in such a way that the acid collecting in the pan forms a hydraulic seal- for the lower edge of the lead chamber. These pans are 6 or 8 inches wider than the chamber, and have sides from 14 to 24, inches high. There is much difference of opinion among acid makers as to the best size and number of the lead chambers. There are usually from 3 to 5, with a capacity of 140,000 to 200,000 cubic feet in the system. As a rule, the first chamber is the largest, and in it the greater part of the acid is formed. The individual chambers vary from 10,000 to 80,000 cubic feet (100 by 40 by 20 feet). In this climate they are unclosed in a building to avoid changes of temperature, which should not vary much from 50° to 65° C. in the first chamber, and 15° above that of the outside air in the last; and they are usually on the second floor, so that the acid may flow from them by gravity to the evaporating pans sometimes placed on top of the pyrites burners; and also that the bottoms may be better watched for leaks. In order to observe the working of each chamber, small lead dishes are fastened at various points on the inside of the chamber wall, and from these, pipes called "drips" lead to test glasses outside, where the density of the acid may be taken. A better method is to place the dish inside the chamber at a distance from the wall, supporting it above the level of the condensed acid, and connecting it by means of a pipe with a test glass outside. Panes of glass are sometimes
set at opposite points in the chamber walls, so that the color
of the gases may be observed. This is quite important as a means of controlling the process. In the first chamber the color is white and opaque, owing to the copious condensation of acid vapor, but in the succeeding chambers the color becomes more and more reddish, owing to the excess of nitrogen oxides. If the color becomes pale in the last chamber, there may be a deficiency of nitrous vapors; or too much or too little steam; or the draught may not be properly regulated, causing too much 01' too little oxygen to enter the chamber.
The standard remedy is to use, at once, more sodium nitrate
in the nitre pots, and then to locate the difficulty and gradually
bring the system to its normal working condition.
From the last lead chamber the gases pass to the Gay-Lussac tower, whose purpose is to recover the oxides of nitrogen which would otherwise be lost. The tower is usually about 50 feet high, and 8 feet across. It is built of lead, supported on a timber frame, in much the same way as the Glover towel'. (In many small modern plants, earthenware pipe of large diameter [2 or 3 feet] is often used instead of lead.) The tower is filled with metallurgical coke, the hardest and strongest kind . .A t the top is a distributing apparatus to spread the acid evenly over the coke. Sometimes two towers are used, the gases passing up through one and then to the
bottom of the other, and up through this to the chimney. The acid which flows down the Gay-Lussac tower is that which has been concentrated in the Glover tower to a density of from 150° to l55° Tw. (about 1.750 sp. gr.) Acid of this strength absorbs the nitrous anhydride (N203) and the nitrogen tetroxide (NO2 or N2O4,), but does not absorb nitric oxide (NO) or nitrous oxide (N2O). Hence, with normal working of the process, only that part of the nitrogen oxides is lost which is reduced to nitrous and nitric oxide. If there is an excess of oxygen present, some of the nitric oxide is converted to nitrous anhydride, and thus saved. The absorption of these nitrogen oxides takes place only when concentrated acid is led through the Gay- Lussac tower; if the acid is of less than 1.50 sp. gr., it will not absorb them; for best results it should be 1.75 sp. gr. The solution of nitrous gases, in concentrated sulpll11rie acid, is known in the works as "nitrous vitriol"; it is sent to the Glover tower, where it is diluted with water, or chamber acid, till its specific gravity is about 1.6. It then passes down the Glover tower, coming in contact with the hot sulphur dioxide from the burners and steam from the lower part of the tower. The high temperature causes the dilute acid to give out its absorbed nitrous gases, which mix with the sulphur dioxide and pass back into the lead chambers. This process is called denitration of the tower acid. The heat in the lower part of the Glover tower evaporates a considerable portion of the
water from the acid, thus concentrating it again to a strength sufficient for use in the Gay-Lussac tower, to which the required amount is returned, and the remainder is added to the acid which has been concentrated in the lead pans.
The hot burner gases are cooled by contact with the tower acid in the Glover tower to 50° to 60° C.; the best temperature at which to work the first chamber is 50° to 65° C.
If the nitrogen oxides are allowed to go to waste entirely, about 11 to 13 kilos of sodium nitrate must be used with each 100 kilos of sulphur burned. The recovery process by means of the Glover and Gay-Lussac towers reduces this consumption of nitrate to about 4 kilos per 100 kilos of sulphur, while a larger quantity of nitrous oxides is introduced into the chambers, causing the acid to form more rapidly and in greater quantities.
Some manufacturers prefer to supply the nitrogen oxides in the
form of liquid nitric acid, introduced into the chambers. This is
easily regulated, admits no excess of air, and causes no loss of sulphur dioxide, such as may happen during the introduction of the "nitre." But much care must be taken that the nitric acid does not run down the sides of the lead chamber, nor collect in the acid on the floor, for then the lead is rapidly corroded. Sometimes the nitric acid is introduced into the Glover tower with the tower acid. The cost of making and condensing the nitric acid must be balanced against the advantages gained by its use.
When sodium nitrate is decomposed by sulphuric acid in the
nitre pots, the nitric acid vapor enters the bottom of the Glover tower with the sulphur dioxide. The vapors here coming in contact with steam begin to react at once, probably as follows:
2 S02 +2 HN03 + H20 = 2 H2S04 +N203;
or, 3 S02 +2 HN03 +2 H20 = 3 H2S04 +2 NO.
Thus the process of acid making begins in the Glover tower, and is continued in the chambers according to the reactions already given.
The nitre pots are fixed under a hopper through which the nitre
is introduced. The sulphuric acid for decomposing the nitre is run into the pots through a pipe, sufficient being
used to form the acid sodium sulphate (NaHS04). This is liquid at the temperature of the flue, and after the reaction is ended
can be easily run out through a pipe attached to the bottom of the pot. On cooling, this acid sulphate solidifies, forming "nitre cake".
Compressed air is employed to force the concentrated acid from the Glover tower to the top of the Gay-Lussac, and the nitrous vitriol from the Gay-Lussac to the top of the Glover tower. The acid collects in a large oval vessel of cast iron called the acid egg (Fig. 24), and the compressed air from (B) forces it out through the pipe (A) to the Glover or Gay-Lussac tower. The "air-lift" pump (Fig. 25) is now used to some extent to raise the acid to the top of the towers. A pipe (P) is sunk into the ground to a depth equal to the height to which the acid from (5) is to be raised; the air from (R) is forced in near the bottom of the pipe, the pressure causing a rush of air up the pipe, carrying before it some of the acid, which is thus thrown out into (T) in "slugs," and not in a continuous stream.
The manufacture of chamber acid is shown in the diagram in Fig. 26. The acid condensed in the lead chambers varies
from 1.5 to 1.62 sp. gr. If more concentrated, it will absorb oxides of nitrogen present in the chambers, and its action
on the lead becomes serious. If the chamber acid is weak, much concentration is necessary to produce a commercial
acid. As a rule, all chamber acid is evaporated
to a density of 1700 sp. gr. (600 Bé.) in shallow lead pans placed over and heated by the waste heat from the pyrites burners or platinum stills. Since acid stronger than 1.70 attacks the lead, "oil of vitriol" is concentrated in glass balloons or in platinum stills.
Glass stills placed on sand baths and heated by a fire are used somewhat, and yield a very pure, colorless, and strong acid; but owing to breakage there is much loss and some danger.
Platinum stills (Fig. 27) are usually shallow platinum dishes
(S, S) covered with a lead hood or bell (B), which is kept cool by a water jacket. The vapors condensing in this hood as a dilute acid do not fall back into the still, but collect in a narrow trough around the lower edge of the bell, and are usually returned to the lead pans or concentratecl in the Glover tower. 'When the acid in the still reaches a density of 1.835 sp. gr. (660 Bé.), it is drawn off through a platinum or lead cooling apparatus (C), and thence into the carboys as "oil of vitriol."
Sometimes platinum stills having a spiral partition in the pan
are used; in these the dilute acid is compelled to flow a considerable distance over the hot still-bottom before it escapes through a tube leading from the central compartment. The rate of flow through the still determines the density of the acid.
The platinum stills are set directly over coke or coal fires on
the grate (G), and are not allowed to cool except for repairs. If
the chamber acid contains any nitrous vitriol, the stills are rapidly destroyed. To prevent this, it is customary to add ammonium sulphate to the acid during the concentration in the lead. pans. This destroys the nitrogen oxides, thus:-
N203 +(NH4)2S04 = 3 H20 +H2S04 +4 N.
Sometimes the platinum is alloyed with iridium to render it more
resistant to the action of nitrous vitriol.
A still in vented by Heraeus consists of platinum lined with a
layer of pure gold rolled with the platinum, and not electroplated. This resists the action of the concentrated acid better than the platinum.
When chamber acid is concentrated by running through the
Glover tower, it is contaminated with iron from the flue dust of the burners. It is better to further concentrate such acid in cast-iron stills, since, when the density reaches 640 or 650 Be., a precipitate of ferric sulphate forms, which may cake upon the platinum and cause it to crack. The acid intended for oil of vitriol is usually drawn from the lead pans, while that which has been through the Glover tower is frequently not further concentrated.
In some modem works the method of making very concentrated acid has been radically changed. A strong acid of 1.6 sp. gr. (550 Bé.) is made in the lead chambers and passed through the Glover tower, where it reaches a density of 1.77 sp. gr. (63° Be.). It is then concentrated in cast-iron stills (Fig. 28) * to a density of 1.835 to 1.842 sp. gr. (66° to 66.3° Bé.). The pan acid enters through the pipe (A) in a regulated stream. The concentrated acid passes
out through (B) into the vessel (C), in which any sediment (sulphates, etc.) deposits. A concentrated acid (08 per cent H2S04) flows from the spout at the top of (C) into the cooling vessels (E, E). The still is heated with gaseous fuel entering through (F). Acid over 1.75 sp. gr. has very little action on cast iron, and the stills stand from two to six months' constant use. Sometimes instead of pans or stills two Glover towers are used, the acid being denitrated in that next the chambers, and further concentrated to a density of 63° to 66° Be. in that next the burners.
Chamber acid is often concentrated in lead pans by allowing the flame from a coal fire or gas producer to pass directly over the surface of the acid. But it may be thus contaminated with soot and its color become dark brown.
Open lead tanks containing lead coils, heated by steam under
pressure, are sometimes used for acid concentration to GO° Be. This gives a clean product, but is not so economical as evaporation by the waste heat from the burners.
To secure the very intimate mixing of the gases essential in the lead chambers, Professor Lunge invented his plate tower. This is a tall tower lined with lead, and divided into narrow chambers by transverse stoneware plates (Fig. 20) perforated by small holes, and so placed that the holes are not in line. By this arrangement the gases and liquids are brought into very close contact, and it is claimed that the chamber space for a given yield of acid can be much reduced. It is recommended that such a towel' be placed between each pail' of adjoining chambers, and that the plates be used in the Gay-Lussac tower also. They are not practicable for the Glover towel', because the heat is liable to crack them, and the
small holes become clogged with dust. It may be noted here that these Lunge-Ruhrmann "plate towers" have found some favor for condensing hydrochloric acid, but are said to obstruct the draught in sulphuric acid making.
The "pipe column" invention of Gilchrist and Hacker carries
out the same idea of mixing the gases more thoroughly. It consists of towers containing a number of small earthenware or lead pipes set horizontally, and open to the air at each end.
Another recent innovation in sulphuric acid making is Barbier's tower system, in which the lead chambers are abolished and a series of towers substituted (Fig. 30). Sulphur dioxide is led through the towers, which are joined by pipes (A, A) leading from the top of one to the bottom of the next.
The towers are filled with perforated pottery vessels, affording a large surface exposure. Dilute nitric acid is supplied to each tower by a sprinkler at the top. Beneath the towers are pans (B, B) for collecting the acid as it forms. These are placed over a flue leading from a grate at the lower end of the series, and so arranged that the overflow from one pan passes into the next. The acid thus flows toward the fire, becoming more concentrated in each pan. The water vapor and nitrous gases from each pan go directly back into the tower above it. The last two or three pans are not under towers, and in these the acid is concentrated to about 6O° Be. by direct heat from a fire, the denitration being effected at the same time. The last tower of the series is a Gay-Lussac for saying the nitrous vapors. The chief advantages claimed for this system arc: it occupies less ground and is cheaper to build than the lead chambers; it works at 90° C., hence atmospheric changes have less influence on the process, and it is suitable for use in hot or cold climates; and that it gives a greater yield of acid per cubic meter of space than does the chamber system. Although some unfavorable results with this method have been reported, it does seem that great developments in tower systems Illay be expected in the near future. A substitute for the cumbersome and expensive lead chamber is much to be desired.
When zinc blende or copper mattes arc roasted, muffle furnaces
externally heated, sometimes by generator gas, are used, as the amount of sulphur in the ore is insufficient for the combustion.
These furnaces contain shelves, and are a modified form of shelf burner, the she 1ves being in the muffle. Sometimes mechanical rakes are used to draw the charge down from shelf to shelf.
Very concentrated acid is now made by artificially cooling oil of, vitriol of 66.3° Be. considerably below 0° C. Under such conditions crystals of sulphuric acid (monohydrate) separate. These are quickly freed from mother-liquor by means of a centrifugal machine. The crystals melt at 10° C., and yield an acid of 99.5 per cent H2SO., containing only a trace of moisture.
Organic Chemistry for the industry
Inorganic Chemistry for the industry
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