Experiments have shown that the use of enriched air would be of particular benefit in the roasting of zinc ores for the manufacture of sulfuric acid. Enriched air increases capacity of furnace, decreases fuel consumption, and increases SO2 content of roaster gas.
The work here described had for its purpose the procuring of data from which some quantitative estimate might be made of the results obtainable by using oxygen-enriched air in roasting zinc ores on a commercial scale. The principal metallurgical advantages of using enriched air in roasting zinc ores would be:
- The rate of roasting would be increased, with consequent gain in the capacity of the roasting furnace.
- As less air would be required for roasting, the volume of hot gases leaving the furnace and the heat carried out of the furnace as sensible heat in these gases would be less per ton of ore roasted; partly for this reason and partly because of the increased quantity of heat generated in the furnace by the larger amount of ore that could be roasted, the consumption of fuel by the furnace would be lessened; and by the use of air sufficiently enriched with oxygen the necessity of using fuel might be entirely obviated.
- Roaster gas having a higher SO2 content could be produced; this would make possible greater capacity and more economical operation of the sulfuric-acid plant.
Certain phases of the application of enriched air to roasting can be worked out only by experimenting with a furnace of commercial, or at least semicommercial, size. Thus the precautions necessary to secure proper distribution of heat in the furnace; the volume of enriched air, and the proportion of oxygen in this air, necessary to give the desired increase in roasting capacity of the furnace and in SO2 content of the roaster gases; and the most desirable frequency of raking, thickness of ore bed, and rate of advance of the ore through the furnace can be definitely determined only after such large-scale experiments.
On the other hand, by drawing up suitable heat balances, the amount of additional heat, per ton of ore roasted, made available in the furnace by the use of enriched air can be calculated, and from that can be calculated, if it is assumed that proper distribution of the total heat in the furnace can be arranged, the additional amount of ore that must be roasted per unit of time in order to make the use of fuel unnecessary. Furthermore, data concerning the effect of enriched air on the ignition temperature and rate of oxidation of zinc ores can be obtained by means of laboratory experiments in which all other conditions (such as temperature, volume of air supplied, thickness of the ore bed, size of; the ore particles, and frequency of stirring) can be maintained constant. From these data, estimates may be made of the increased capacity that may be expected in a full-size furnace, and of the increase in SO2 content of the roaster gas that may be expected, as a result of the use of enriched air.
Heat Balances of a Hegeler Roaster Using Ordinary Air and Using Enriched Air
Heat Balance of a Hegeler Roaster Using Ordinary Air
When the suggested use of enriched air for roasting zinc ores was first called to the attention of the writers, they drew up a heat balance of a Hegeler roaster using ordinary air; this particular type of furnace was selected because it is the furnace generally used in this country for roasting zinc ores when the gases are to be utilized for making sulfuric acid. The heat balances of different Hegeler roasting furnaces will vary in detail, depending on the design of the furnace, composition of ore roasted, kind of gas producers used, quality of coal used, and manner of preheating the air for roasting, but the variations are in the minor items; the net result, as shown by the consumption of coal per ton of ore, is nearly the same in most furnaces of this type.
The heat balance here given is for a hypothetical case, in that the operating, data were not taken from the actual operation of any one particular furnace. The conditions assumed were, however, representative of actual practice, so that the heat balance is typical. A few simplifying assumptions were made, such as assuming an ore consisting entirely of ZnS, FeS, and SiO2, minor constituents being neglected. The effect of such simplifying assumptions on the accuracy of the calculations is negligible.
This heat balance of a Hegeler roaster, using ordinary air and roasting 45 tons of 60 per cent, zinc ore per day, is summarized in Table 1.
Heat Balance of a Hegeler Roaster Using Enriched Air
In the heat balance of a Hegeler roaster using enriched air, if it be assumed that this use of enriched air is to eliminate the use of fuel, several of the items enumerated in Table 1 will be absent. These are, from the debit side, the sensible heat in the preheated air (as there will be no waste combustion gases for preheating), the sensible heat in the producer gas, and the heat of combustion of the producer gas; and from the credit side, the sensible heat in the combustion gases. There remains then, as a source of heat in the furnace, only the oxidation of the ore, which must balance the heat lost as sensible heat in the roasted ore and in the roaster gases leaving the furnace, and that lost by radiation and conduction.
Assuming the composition of the ore before and after roasting, the temperature of the green ore and of the air supply entering the furnace, and the temperature of the roasted ore and roaster gases leaving the furnace, to be the same as they were assumed for the purpose of calculating the heat balance of the roaster using ordinary air, and assuming that the loss of heat from the furnace by radiation and conduction would remain constant, the tonnage of ore that would have to be roasted per 24 hr. to maintain the furnace at roasting temperature without the use of other fuel was calculated for the following cases:
Case 1.—The enriched-air supply to contain 25 per cent, oxygen; the exit gases to contain two volumes of SO2 to one volume of O2 (in this case 13.3 per cent. SO2 and 6.65 per.cent. O2); 60 per cent, of the roaster gases to be recirculated and returned to the furnace at 200° C. to help in controling the rate of combustion and the distribution of the heat in the furnace and in procuring the desired high content of SO2 in the gases. The flow sheet under these conditions is shown in Fig. 1.
Without going into details, the calculations may be summarized as follows: From the combustion of 1000 lb. of the ore are obtained 1,049,650 lb.-cal. The heat leaving the furnace as sensible heat in the roasted ore (853 lb.; see flow sheet) at 800° C. is 106,350 lb. cal. and in the roaster gases (63,140 cu. ft.) at 600° C., 775,888 lb.-cal.; of the latter, 142,644 lb.-cal. are returned to the furnace in the recirculated roaster gases (37,884 cu. ft.) at 200° C. Thus 1,049,650 – 106,350 – 775,888 + 142,644 = 310,056 lb.-cal. are available per 1000 lb. of ore roasted, to balance radiation and conduction losses.
From the previously calculated heat balance of a Hegeler roaster using ordinary air, it was found that the loss of heat from the furnace by radiation and conduction was 111,696,200 lb.-cal. per 24 hr. From this, it follows that 111,696,200/2 x 310,056 = 180 tons of green ore must be roasted in the furnace per 24 hr. to maintain it at the usual roasting temperature, using enriched air under the conditions assumed in this case.
Case 2.—Conditions the same as in Case 1, except that none of the roaster gases are recirculated; or, what amounts to the same thing thermally, that the recirculated gases are returned to the furnace at the same temperature as that at which they leave. Under these conditions the flow sheet is as shown in Fig. 2.
The heat obtained from the combustion of 1000 lb. of the ore is, as before, 1,049,650 lb.-cal., and the heat leaving the furnace as sensible heat in the roasted ore is 106,350 lb.-cal., the heat leaving the furnace as sensible heat in the roaster gases is, however, only 40 per cent, of what it was in the former case, or 310,355 lb.-cal. The heat available to balance radiation and conduction losses is then: 1,049,650 — 106,350 — 310,355 = 632,945 lb.-cal., and 111,696,200/2 x 632,945 = 88 tons green ore must be roasted per 24 hr. to maintain the furnace at the usual roasting temperature, using enriched air under the conditions assumed in this case.
Case 3.—The enriched air supply to contain 50 per cent, oxygen; the exit gases to contain two volumes of SO2 to one volume of O2, which in this case means that the SO2 content will be 28.6 per cent, and the O2 content 14.3 per cent.; none of the roaster gases to be recirculated. The flow sheet under these conditions is shown in Fig. 3.
In this case, the heat leaving the furnace as sensible heat in the roaster gases at 600° C. is, per 1000 lb. of ore roasted, only 155,178 lb.-cal. and the heat available to balance conduction and radiation losses is
1,049,650 – 106,350 – 155,178 =788,122lb.-cal. Therefore, 2×788 122 = 71 tons of green ore must be roasted per 24 hr. to maintain the furnace at the usual roasting temperature.
The SO2 content of the roaster gases in the examples just discussed was purposely assumed to be very high. If it should be impracticable to obtain such a high SO2 content in the roaster gases, a larger amount of ore would have to be roasted to produce the same amount of available heat in the furnace. Even with the SO2 content assumed as high as it has been, the increase in the amount of ore that must be roasted per 24 hr. in order to dispense with the use of file] is considerable. It is open to question whether such high SO2 content in the roaster gases, with simultaneous large capacity of the furnace, could be accomplished except by the use of enriched air containing a very high percentage of oxygen.
On the other hand, it might seem possible that the ignition temperature of zinc blende in enriched air would be so much lower than in ordinary air that the roasting furnace would not have to be run at so high a temperature when enriched air is supplied as when only ordinary air is available; this would permit a saving in the heat lost by radiation and conduction, and as sensible heat in roaster gases leaving the furnace. To obtain experimental evidence bearing upon these questions, the series of experiments here described was undertaken.
Ignition Temperatures of Sphalerite in Air Enriched with Various Proportions of Oxygen
In giving data on ignition temperatures, it is necessary to define exactly what is meant by the term ignition temperature. The usually accepted meaning is the temperature at which the oxidation of a substance becomes so rapid that the heat liberated counterbalances the heat radiated or conducted away, thus maintaining a visible spontaneous combustion. The temperature at which this can take place varies with the rate at which heat is radiated or conducted away from the substance; this in turn is affected by the heat conductivity of the walls of the vessel in which the substance is contained and by the volume and temperature of air circulated over it. ”
Oxidation of sphalerite exposed to the air, no doubt, takes place at an extremely slow rate, even at ordinary atmospheric temperatures. It is conceivable, if a pile of finely divided zinc blende could be so insulated that radiation and conduction from the pile would be nil and the air supply so regulated that the heat carried off by it as sensible heat would be as small as possible, that the slow oxidation of the blende would, of itself, cause the pile to become sufficiently hot for active combustion to take place. This would be analogous to the spontaneous ignition of large coal piles. In such a case, it would be difficult to say just what should be called the ignition temperature.
In outlining the series of determinations of the ignition temperatures of sphalerite in air enriched with various proportions of oxygen, it was at first planned to heat slowly a sample of the mineral in an electrically heated tube, passing the enriched air over it at a fixed rate; to read the temperature of the sample at intervals by means of a thermocouple, the junction of which was placed in the sample; and then to plot a curve of the rate of temperature rise. It was thought that at the temperature of ignition there might be a sufficient increase in this rate to cause a noticeable deflection in the curve; it was found, however, that the oxidation of the sphalerite began so gradually that no such deflection could be detected.
It was then decided to determine the temperature at which sufficient sulfur dioxide was formed to cause to turn blue a solution of potassium iodate and starch placed at the exit of the tube containing the sample. The ignition temperature, even as determined by this method, varied according to the rate at which the sample was heated, the rate at which the air was passed over the sample, etc., but by heating very slowly and keeping the rate of heating and other variable factors the same in all the experiments, comparative results were obtained that show clearly the effect on ignition temperature of increasing the oxygen content of the air supply.
Apparatus and Procedure
The oxygen-enriched air for use in the experiments was made by mixing commercial oxygen with ordinary air in a large gas-storage bottle. The pressure in this storage bottle was regulated by raising or lowering a pressure bottle of the same size filled with water, which was placed on a small elevator and connected to the storage bottle by a flexible siphon. Before the gas was passed over the sphalerite; it was passed through two washing bottles containing, respectively, sodium-carbonate solution and distilled water, and through two drying tubes containing anhydrous calcium chloride.
The sample of sphalerite was placed in a pyrex glass combustion tube 20 mm. in diameter, which could be heated by means of a nichrome-wound electric-resistance furnace. There were two sections of this furnace, one of which was used to heat the sphalerite and the other to preheat the air so that it would have about the same temperature as the sphalerite before coming into contact with it. The temperature of the sphalerite was read by means of a thermocouple, the junction of which, protected by a thin quartz tube, was placed in contact with the surface of the sample. The temperature of the preheated air was read by means of a second thermocouple. One end of the combustion tube was connected to the supply of enriched air; the other (exit) end to a small washing bottle containing a few cubic centimeters of a solution of potassium iodate and starch, to serve as an indicator for sulfur dioxide. Beyond this bottle of indicator solution, there was attached a flow meter for measuring the flow of air through the system.
The sphalerite used was a hand-picked specimen of the massive mineral. It had the following analysis: zinc, 65.17 per cent.; sulfur, 32.36 per cent.; iron, 0.48 per cent.; insoluble, 0.79 per cent. As the size of the particles of the sphalerite has a marked effect on the ignition temperature, the crushed sample was separated by screening into four sizes: through 20, on 28 mesh; through 28, on 35 mesh; through 35, on 100 mesh; and through 100 mesh; and a separate series of experiments run on each size.
A 15-gm. sample of sphalerite was used for each experiment. It was placed in the combustion tube, the thermocouple placed in position, the gas train made tight, and enriched air of the desired oxygen content passed until the apparatus was filled with it. The gas flow was then adjusted to a rate of 13.5 liters per hour, which had been selected as a standard for the experiments. The current was turned on in the furnaces for heating the sphalerite and for preheating the air; these were heated rapidly up to 30° to 40° C. below the expected temperature of ignition and then at the rate of 1° C. per min. until the temperature of ignition was reached, as indicated by the potassium iodate-starch solution turning blue. The temperatures of the sphalerite and the preheated air were at all times held approximately the same.
The ignition temperatures as determined by the above method are tabulated in Table 2; Fig. 4 shows curves plotted from the values given in this table.
The results of these experiments show that the ignition temperature of sphalerite is appreciably lowered by increasing the oxygen content of the air supply. This lowering is, however, very, small, the ignition temperature in pure oxygen averaging less than 25° C. below that in ordinary air containing only 21 per cent, oxygen; therefore, the effect of enriched air on ignition temperature would be of very slight practical importance in the roasting of zinc ores.
Rates of Oxidation of Sphalerite in Air Enriched with Various Proportions of Oxygen
Apparatus and Procedure
The apparatus used for this series of experiments was similar to that used for the determination of ignition temperatures, which has been described, except that the bottle of potassium iodate-starch solution was omitted. The sphalerite used was also the same and, as before, separate experiments were run on the following sizes through 20, on 28 mesh; through 28, on 35 mesh; through 35, on 100 mesh; and through 100 mesh.
A 5-gm. sample of sphalerite was used for each experiment. It was spread over the bottom of an alundum boat in a layer about 1/8 in. thick, and was not stirred during the roasting. When starting an experiment, the apparatus was filled with air of the desired oxygen content. The furnace for heating the sample and that for preheating the air were then started and raised rapidly to a temperature of 750° C. The gas flow was adjusted to a rate of 5 liters per hour, and the temperature held constant at 750° C. for 1 hour. The furnace was then allowed to cool rapidly and the partly roasted sphalerite was analyzed for total sulfur and water-soluble sulfur.
The results obtained in the experiments are tabulated in Table 3, and plotted as a series of curves in Fig. 5.
Theoretically, other conditions being equal, the rate of oxidation of zinc blende should vary directly with the partial pressure of oxygen in the air to which it is exposed. The curves in Fig. 5 show that this is borne out fairly well by the experiments in which the oxygen content of the air supplied was less than 50 per cent. With higher concentrations of oxy¬gen, the elimination of sulfur did not increase in the same ratio as the oxygen content of the air. In these experiments with air of high oxygen content, however, the sulfur was reduced to such a low point in 1 hr. that the surface of the blende particles was, no doubt, much reduced and was covered with a coating of zinc oxide sufficient to retard the rate of oxidation decidedly. In the experiments with the -35- + 100-mesh, and the —100-mesh sphalerite, the sulfur elimination was less in pure oxygen than in 50 per cent, oxygen-air; also in 50 per cent, oxygen-air and in pure oxygen the sulfur elimination was less from the —100-mesh than from the -35- + 100-mesh size. This is explained by the fact that the finer sizes, when roasted in air of high oxygen content, tended to sinter and form a cake. No doubt the surface temperature of the sphalerite, because of the rapid oxidation in oxygen, was considerably higher than the temperature of the furnace.
It would probably be safe to state, as a result of these experiments, that under similar conditions, the rate of oxidation of sphalerite varies very nearly directly as the partial pressure of oxygen in the air to which it is exposed, at least for all concentrations of oxygen likely to be used in roasting on a large scale.
A second fact is the large amount of water-soluble sulfur in the calcine from roasting in air of high oxygen content. This indicates that the tendency to form zinc sulfate in the preliminary stages of roasting would be greater with enriched air than with ordinary air. It would be necessary to break these up in the final stage of roasting; this might require a higher temperature or a longer time at a high temperature at the end of the roast than present practice requires.
This series of experiments concerning the effect of enriched air on the rate of oxidation of sphalerite is incomplete. In the experiments just described, the sphalerite was roasted for a definite time in all the experiments, consequently in the experiments with the finer sizes and with enriched air of high oxygen content the sulfur in the sample was reduced to a much lower point than in the experiments with the coarser sizes and with air of lower oxygen content. The rate of oxidation naturally decreased as the sulfur content of the sample decreased, and this effect counterbalanced to a certain extent the effect that the increased oxygen content of the air had of increasing the rate of oxidation. It appears now that a better method of experiment would have been to roast all samples to the same content of sulfur and compare the time required to do this with enriched air of various oxygen contents. It was considered unnecessary, however, to carry this series of experiments any further, as more reliable information is given by the experiments next to be described, which were carried out with a roaster capable of taking a charge of several pounds of ore.
Experiments with a Mechanically Rabbled Laboratory Roasting Furnace
Apparatus and Procedure
The laboratory roasting furnace used in these experiments was an electrically heated, mechanically rabbled furnace, patterned after one used by C. A. Hansen for experiments in the roasting of zinc ores for leaching. It is shown in Fig. 6. A sheet-iron cylinder 30 in. in diameter and 18 in. high was set on timber skids, as a foundation, and a layer of heat-insulating brick laid in the bottom. A heavy sheet of iron was laid level on the layer of brick and on this, concentric with the outer sheet-iron cylinder, was set a thin cast-iron cylinder 16 in. in diameter and 9 in. high. Inside of this inner cylinder was laid a layer of firebrick, covered with about ½ in. of crushed firebrick. On this was set the heating unit, which was a fireclay disk with shallow grooves running transversely across the upper surface in which was wound the heavy chromel wire that served as a resistor. On top of the heating unit a thin fireclay disk was placed. Above this hearth bottom the cast-iron cylinder was lined with a fireclay cylinder ¾ in. thick. On top of the cylinder rested a sheet of asbestos and a heavy iron plate, with a hole in the center for the shaft to which the rabble arms were keyed. All joints in the furnace lining were sealed with alundum cement. An opening 4½ in. wide by 3½ in. high was left in one side of the furnace as a door; it was closed with a firebrick plug.
The rabble arm and rabbles were formed from a single piece of heavy strap iron. The rabbles were so arranged that the ore was thoroughly stirred and, at the same time, maintained at uniform depth over all the hearth. The rabble arm was driven by a small motor and worm gears at a speed of 0.95 r.p.m. The driving mechanism for the rabble arm was supported on a slab of hard asbestos board resting on top of the furnace.
The space between the inner cylinder, forming the roasting furnace proper, and the outer sheet-iron cylinder, was filled with infusorial earth for heat insulation.
The shaft carrying the rabble arm was hollow, and a carefully calibrated platinum-platinum rhodium thermocouple, with silica protecting tube, was inserted through it so that the end rested upon the floor of the roasting hearth. The power input to the furnace was controlled by a voltage regulator; in this way the temperature of the furnace could be regulated to within ±10° C. The temperature of the roasting ore was difficult to determine accurately; occasional readings, taken with a ther-
mocouple thrust into the layer of ore while the rabble arm was stopped, averaged about 20° C. higher than the temperatures read with the thermocouple in the central shaft.
Air for roasting, either atmospheric or enriched, as the case might be, was admitted to the furnace through a ½-in. pipe, curved to direct the incoming air away from the gas outlet and sample tube. The roaster gas left the furnace chiefly through the small cracks around the plug that was inserted in the door of the furnace. Samples of the gas for analysis were drawn off through a silica tube not far from the gas outlet.
Sulfur dioxide in the roaster gases was determined by absorption in potassium-hydroxide solution, and oxygen in the roaster gases and in the air supply by absorption in alkaline potassium-pyrogallate solution, in an Orsat apparatus.
The method of controlling the volume and composition of the air supply is shown in Fig. 7. The atmospheric air was supplied by a small
laboratory blower; a large glass carboy was placed in series with this to act as an accumulator to diminish the effect of minor fluctuations in pressure. Oxygen was supplied from a cylinder of the compressed gas. Flow meters were inserted in both the air and the oxygen supply lines to indicate directly the rates of flow of air and oxygen. By maintaining a constant reading on each of these flow meters, the volume and composition of the air supplied to the furnace could be maintained constant within about 1 per cent. In series with the flow meters were wet gas meters, serving as integrating meters on which could be read the total volume of gas passed in any given interval of time. The oxygen and air supply lines led into a large glass bottle, which served as a mixing chamber; the outlet from this bottle was fitted with a three-way stop cock, of which one outlet led to the roasting furnace and the other to the gas analysis apparatus. The complete equipment is shown in Fig. 8.
The ore used was Joplin concentrate, screened through a 10-mesh screen to give a product of fairly uniform size. Its composition was: zinc, 62.07 per cent.; lead, 1.29 per cent.; iron, 1.41 per cent.; sulfur, 31.58 per cent.; insoluble, 1.84 per cent.; CaCO3, 0.59 per cent. Its
screen analysis is given in Table 4. For each experiment, 7 lb. of this ore was used; this made a layer in the furnace about ¾ in. thick.
When starting an experiment, the furnace was heated to the temperature at which the experiment was to be run the air, of the desired oxygen content, was turned into the furnace; and the charge of ore was placed in the furnace and spread evenly over the hearth. The introduction of the cold ore produced a temporary cooling of the furnace, but within about 15 min. it would again be up to the desired temperature. The temperature of the furnace and the volumes of air and oxygen supplied to the furnace were read every 15 min. The average volume of air supplied (ordinary air + oxygen) in all but one of the experiments was 30.8 cu. ft. per hr. In the one experiment referred to, for which enriched air containing 42 per cent, oxygen was used, one-half the usual volume was supplied, or 15.4 cu. ft. per hr. Samples of the air supply, when enriched air was being used, were taken occasionally and analyzed for oxygen; the variation in the oxygen content was never more than a fraction of a per cent, during the course of an experiment. Samples of the, roaster gas were taken every half hour and analyzed for SO2. Samples of the ore were taken, usually, at intervals of 1¼ or 1½ hr. These were analyzed for total sulfur and water-soluble sulfur; the latter is approximately equivalent to the sulfur present as normal zinc sulfate.
Data Obtained from the Experiments
In Figs. 9, 10, and 11 are plotted the data obtained from a series of roasts made at the constant temperature of 800° C. This series includes one roast with ordinary air, one with enriched air containing 28 per cent, oxygen, one with enriched air containing 42 per cent, oxygen, in all of which the volume of air supplied was 30.8 cu. ft. per hr., and one roast with enriched air containing 42 per cent, oxygen, in which the volume of air supplied was 15.4 cu. ft. per hr. Fig. 9 shows the variation of the SO2 content of the roaster gas as the roasts progressed; Fig. 10 shows the progressive decrease in total sulfur content of the ore; and Fig. 11 the variation in water-soluble sulfur content of the ore. It should be noted that the vertical scale in Fig. 11 is ten times that in Fig. 10.
Theoretically, if the volume of air supplied is the same, the rate of the oxidation reaction, and consequently the SO2 content of the roaster gas, should vary directly as the partial pressure of oxygen in the air supplied for roasting. When air containing 28 per cent, oxygen is supplied, the SO2 content of the roaster gas should be 33 per cent, greater than when ordinary air containing 21 per cent, oxygen is supplied; and with air con¬taining 42 per cent, oxygen, the SO2 content of the roaster gas should be doubled. The time required for roasting should be in inverse ratio to the partial pressure of oxygen in the air supplied. As shown in the first three columns of Table 5, this is borne out approximately by the experimental data.
By halving the volume of air supplied, keeping its composition the same, the SO2 content of the roaster gas can be increased considerably., but the time required for roasting is also increased by about 50 percent., as will be seen by comparing the last two columns of Table 5, and the curves in Figs. 9 and 10.
In the roasting of this ore, made up of fairly evenly sized particles, the SO2 content of the roaster gas was fairly constant until most of the sulfur was eliminated from the ore, especially when air of moderate oxygen content was supplied. This would probably not hold true when roasting an ore made up of a mixture of fine and coarse particles. The total sulfur content of the ore decreased at a uniform rate in all the experiments, until it was reduced to between 1 and 2 per cent., after which it decreased very slowly; this agrees with the usual experience in roasting in practice. In the roast in which half the usual volume of air was supplied, the sulfur content of the ore, when sulfur elimination stopped, was over twice what it was when the larger volume of air was supplied.
The curves in Fig. 11, showing variation of the water-soluble sulfur content of the ore, are interesting. This sulfur remained fairly constant at between 0.1 and 0.2 per cent, in all experiments until the total sulfur
content of the ore became very low. It then increased sharply to a maximum and later decreased again, first sharply and then more slowly, with continued heating. The height of this maximum, and the amount of water-soluble sulfur remaining in the ore at the end of the roast, increased with increasing oxygen content of the air used for roasting. This agrees with the observation made, as a result of the preliminary laboratory experiments concerning the effect of oxygen on the rate of oxidation of sphalerite. Decreasing the volume of air supplied per hour greatly increased this tendency to form zinc sulfate.
Before running the above experiments at 800° C., some similar roasts were made at 750° C., but in this series the mistake was made of charging the ore in the cold furnace and heating the latter up to roasting temperature afterward. Thus, a variable amount of sulfur was eliminated before the furnace reached 750° C. and, while the results were similar to those obtained in the roasts at 800° C., the separate experiments are not strictly comparable with one another. For that reason the analyses of SO2 in the roaster gas are not given, but the curves showing the rate of sulfur elimination from the ore in the final stages of the roast are of such interest that they are given in Fig. 12. The water-soluble sulfur is here plotted on the same scale as the total sulfur, as it runs considerably higher than in the roasts at 800° C.
Noticing first the curves showing the variation of the water-soluble sulfur content of the ore as the roasts progressed, it will be noted that, as in the roasts at 800° C., the water soluble-sulfur remained very low until most of the sulfide sulfur was eliminated from the ore, and then increased sharply to a maximum that was considerably higher than in the roasts at 800° C. Instead of again decreasing rapidly, as at 800° C., it remained stationary at the maximum or at least decreased only very slowly with continued heating. Thie tendency for zinc sulfate to be formed is greater at 750° C. than at 800° C., and the sulfate is not so readily broken up again at the lower temperature. At this temperature, as at 800° C., the formation of zinc sulfate was greater in the roasts with enriched air of high oxygen content.
The curves show that the total sulfur content of the ore decreased at a uniform rate until it was reduced to a few per cent. Then the rate of sulfur elimination became slower, at the same time that the water-soluble sulfur began to increase. Finally, the total sulfur in the ore actually increased and followed along parallel with the water-soluble sulfur. The explanation of this would seem to be about as follows:
The rabbles used in these earlier experiments, though they kept the ore spread evenly over the hearth and thoroughly mixed, for the most part, left a small amount of ore caked in the corner formed between the floor of the muffle and the circular wall. This ore roasted more slowly than the rest and continued to give off SO2 after the rest of the ore was almost completely roasted. This SO2, together with the oxygen of the air, especially in the roasts with enriched air, reacted with the zinc Oxide in the main portion of the ore to produce zinc sulfate, to such an extent that the total sulfur content of this main portion of the ore increased.
It may be concluded, from the data obtained from these roasting experiments, that temperature, volume and composition of air supply, rate of rabbling; and other such conditions being equal, the rate of oxidation of a given zinc ore increases approximately in direct proportion with the oxygen content of the air supply; consequently that the SO2 content of the roaster gas varies approximately directly, and the time required for roasting varies inversely, as the oxygen content of the air supply. If air containing a high percentage of oxygen is supplied, but in reduced volume, roaster gas very high in SO2 can be produced, but in this case the time required for roasting is considerably greater than when air of the same composition is supplied in the usual volume. In other words, the use of enriched air in roasting can be expected to give a proportionate increase in both SO2 content of the roaster gas and rate of roasting, but an extremely high SO2 content in the roaster gas can only be obtained by sacrificing the gain in the rate of roasting, and vice versa.
The tendency to form zinc sulfate is greater with enriched air than with ordinary air.
Results that may be Expected from Application of Oxygen Enriched Air to Zinc Roasting in Practice
It is in the roasting of zinc ores for the manufacture of sulfuric acid that the use of enriched air would be of particular benefit and, at least as far as we can foresee at present, the possibility of the practical application of enriched air to zinc roasting is not great except where the sulfur dioxide in the gas is to be made use of in some way. In this country, the Hegeler kiln is almost universally used for roasting zinc ores when the roaster gas is to be used for making acid; hence it is chiefly the application of enriched air to roasting in Hegeler kilns that will here be considered.
The possible advantages to be derived from the use of oxygen-enriched air in zinc roasting are an increase in the capacity of the roasting furnace, a decrease in the fuel consumption of the roasting furnace, and an increase in the SO2 content of the roaster gas. From the increased SO2 content of the roaster gas would follow increased capacity and more economical operation of the acid plant.
Our experiments in roasting with enriched air in a laboratory roaster show that with equal temperature, rate of rabbling, and volume of air supplied, the SO2 content of the roaster gas and the rate of roasting increase in the same ratio as the oxygen content of the air supply. The heat balances given in the first section of this paper (Cases 2 and 3) show that increases in the rate of roasting of 95 per cent, when enriched air containing 25 per cent, oxygen is supplied, and 58 per cent, when enriched air containing 50 per cent, oxygen is supplied, are necessary to obviate the necessity of using fuel. In obtaining these figures, the SO2 contents of the roaster gases were assumed as 13.3 per cent, and 28.56 per cent., respectively. Our experiments indicate that the SO2 content of the roaster gases cannot be raised this high except by greatly reducing the volume of air supplied; and if this is done, the capacity of the roasting furnace is correspondingly reduced. It would seem then that large roasting capacity and roaster gas with high SO2 content cannot be simultaneously obtained except by the use of enriched air of very high oxygen content and that, therefore, the use of fuel cannot be done away with except by the use of such highly oxygenated air.
It should be borne in mind, however, that rabbling in a Hegeler kiln is done only at very infrequent intervals and that the ore is therefore very inefficiently exposed to the current of air passing over it. If it could be arranged to use enriched air and rabble, let us say, twice as frequently, the rate of roasting and SO2 content of the roaster gas would be much increased and roasting without the use of fuel would be more nearly within the realm of possibility. This question can only be decided by experiments with a roaster having a capacity approaching that of a full-size furnace.
The possibility of applying enriched air to Wedge furnaces, such as those in which the autogenous roasting of zinc ore is now being attempted, should also be mentioned. Roasting can be carried on autogenously in these furnaces as long as everything goes just so, but the margin of heat is so small that any disturbance of conditions in the furnace is apt to upset the balance. The use of air only slightly enriched in oxygen would increase the margin of safety so that no provision would be necessary for burning fuel in these furnaces.
In conclusion, the writers wish to state that, while they believe that the experimental data and the heat balances which they have given are reasonably accurate, they realize that their interpretation of them is not the only possible one and that from the same data, other metallurgists may draw different conclusions as to the effect that the use of enriched air may have on roasting zinc ores in practice. It is hoped that the data given may be of help to others who are working on the application of oxygen-enriched air to the same or similar phases of metallurgy, and serve to stimulate further thought on the subject.