US3206300A - Process for reducing ore - Google Patents

Description

United States Patent 3,206,300 PROCESS FOR REDUCING ORE Murray C. Udy, Niagara Falis, N.Y., Nicholas J. Themelis, Montreal, Quebec, Canada, and Leonard E. (lids, Grand Island, N.Y., assignors, by mesne assignments, to Independence Foundation, Philadelphia, Pa, 21 corporation of Delaware, and Koppers Company, Inc., Pittsburgh, Pa., a corporation of Delaware No Drawing. Filed Dec. 31, 1962, Ser, No. 248,219 2 Claims. (Cl. 75-11) This invention relates to metallurgy, in general, and more particularly to the treatment of ores and like materials in a rotating kiln to effect dehydration and substantial prereduction thereof prior to smelting. The invention is specifically concerned with improved operating techniques whereby the rate at which materials may be treated within such kilns may be substantially increased or even doubled.

M-any advances have been mad-e in recent years in the design, construction and operation of rotating kilns, and particularly those employed for the prereduction of ores. Starting from a simple rotating, refractory-lined inclined tube having a gas-fired burner at the discharge end, advancements have included the injection of carbonaceous reducing material at intermediate points from the middle towards the discharge end, injection of side air to burn carbon monoxide and other volatiles issuing from the bed and the important discovery that with the use of volatile coals and like materials a substantial amount of hydrogen reduction will occur within the bed. A further advance in operating techniques is that for optimum reduction when employing a deficiency of reductant the load factor of the kiln should not exceed about 15%. Several of the foregoing advances are described in detail in copending application of Franklin C. Senior and Chester E. Shaffer, Serial Number 145,865, filed October 18, 1961.

In spite of the foregoing advances, it has been determined that under most operating conditions reduction is generally confined to that one-fourth of the kiln nearest to the discharge end. That is, the conditions of a substantially reducing atmosphere and a temperature sufficient to let reduction proceed at a substantial rate are generally confined to this area. While it is, of course, necessary to devote a portion of the kilns length to the dehydration of the ore charge and to oxidation of such materials as sulfur and arsenic, it was believed that very material improvements in kiln operation would result if the effective reducing zone within the kiln could be enlarged. Of course, this had to be done in a manner which did not involve safety hazards and which was economic in practice. That these ends have been accomplished by means of the present invention is obvious from the detailed example data set forth hereinbelow.

As is well known, the reduction of iron oxides in the solid state will take place from a temperature of about 500 C. up to the point of fusion, but subject to varia tion depending on composition and so forth. It is also known from thermodynamic and kinetic studies that reduction by hydrogen is favored at the lower temperature in this range while carbonaceous reduction is most efficient at the higher temperatures. With these considerations in mind and with the end result of increasing the length of the reduction zone in view, it was found that the result did come about when the kiln was operated by injecting an excess of natural gas or other reducing gas at the discharge end of the kiln and the kiln was operated at a slight positive pressure to keep outside air from coming in; also, high volatile coal was added at or near the feed end of the kiln, and side air was injected quite near the feed end so as to burn the excess gas and raise the temperature at this point much higher than had heretofore been possible. Moreover, by raising the bed temperature as early as possible in its traverse of the kiln length and by providing the high-volatile coal at or before this point was reached, it was found that by the time the bed reached the temperature required for reduction to proceed, it was substantially permeated with hydrogen and hydrocarbons resulting from volatilization of the coal. The hydrogen was thus present at the temperature range where its efficiency was highest. The coal was effectively coked and the carbon reduction proceeded at the higher temperatures as the bed moved towards the discharge end. Under these operating conditions, it was found that the throughput of the kiln could be effectively doubled while still maintaining the desired prereduction of approximately Retention time was substantially reduced, and it was not necessary to exceed the desired maximum load factor of 15%.

The following detailed discussion of modified kiln oper-ation was carried out on an eighty-foot kiln having an inside diameter of approximately four and one-half feet. The kiln had three coal feeders and five side air injectors,

but none within 35 feet of the feed end. Firstly, the seal between the discharge end and its housing was renewed and tightened. It was found, however, that the kiln was somewhat out of round, and even with the seal tightened there was not direct contact of all points. By running the kiln under pressure it was possible to insure that gas would leak out rather than air leak in.

Piping for excess fuel gas was installed in the discharge-end housing at the floor line, about 2- /2 feet below v the bottom of the kiln. Gas was admitted at two points. Open-end %-ll'lh pipes were used to bleed in the gas. Two extra side-air ports with the necessary fans were installed, one nearer to the discharge end and the other near the feed end. The latter was of larger size than the others. An air-operated hydraulic cylinder was installed on the discharge gate of the sinter-receiving hopper to allow quick opening and closing and reduce gas leakage out or air leakage in. Finally, it was necessary to install a water spray on the blades of the flue-gas exhaust fan, since it was realized that the flue gas would be much hotter than before.

The kiln was heated slowly to a dull red heat at the discharge end using the (old) primary gas burner only. After a day, the primary gas was increased, and as soon as the kiln was red at the first air port, about 15 feet from the end, auxiliary gas was turned on and the air port opened. This gave a flame of air burning in gas which heated the kiln further upstream. The process was repeated at succeeding air ports, using more and more auxiliary gas, until the kiln was bright red along almost its entire length. It required about 14 hours for this preheating step.

Due to safety considerations, more air then necessary for complete combustion was admitted at the air ports, and the oxygen in flue gas during the period was 0.9 to 2.0% with no carbon monoxide, no hydnogen and methane. The flue-gas temperature out of the kiln was about 900 F. at the end of the period.

Iron ore at 2000 pounds per hour was started, and when the ore reached the side-coal feeders, high-volatile coal was fed in at each feeder. Side air was increased and fuel gas reduced to provide air to burn the volatile matter of the incoming coal. Two hours after start of feeding, the ore feed was increased to 4000 pounds per hour, and this rate was maintained throughout most of the run.

A small amount of coal was fed with the ore throughout the run, in order to provide reductants in the feed of the kiln (the first coal feeder downstream is 35 feet from the feed end). The quantities of materials fed during the entire runare listed in Table I. Ore and other materials 3 were fed continuously for 96 /2 hours with the exception of 1% hours on the first day when trouble was had with the kiln ore feeder. The feeding time therefore was 95 During the first hours of the run, at 4000 pounds per hour ore feed rate, no metallic iron was found in the sinter, indicating something less than 33% reduction. After cutting the rate slightly metallics began to appear in sinter in a small amount (up to 3%). However, it was 26 hours before conditions in the kiln could be set to obtain good reduction. Then followed a period of 10 consecutive hours wherein the metallic iron ran from 7 to 13%, averaging 9.4%. This is equivalent to 44.3% reduction. For two hours during this period the metallic iron averaged 13.6%, equivalent to 50.0% reduction. A summary of the reduction results is shown in Table II.

T ABLE II.REDUCT IO'N The particular ore treated during the test was selected because previous tests thereon indicated it was very difficult to prereduce. The excellent results obtained testify to the efficacy of the modified practice of the invention. It is to be noted that during the run, with efforts being concentrated to increase the feed-end temperature, the temperature at the discharge end was neglected, and averaged only about 1700 F., or 150 to 200 lower than the mid-point temperature. It is known that the discharge temperature should be as high as is practical, to obtain high reductions, but, of course, longer retention time at a lower temperature produces an equivalent effect and reduces the change for sintering.

On three occasions during the run, doughnut-shaped rings built up on the walls some distance upstream from the feed end, usually near one or both of the coal feeders. In all cases the rings fell from the walls after some hours without any action by the operators. They were softly fused cakes.

The formation of rings indicate that the kiln was operated at/ or slightly above the maxi-mum temperature for this ore at certain areas in the kiln, but this can readily be adjusted to prevent ringing at any point, and yet .have a sufi'iciently hot kiln throughout.

It should be noted that when operating with an excess of gas, temperatures must be controlled with air input; in normal combustion in excess air, temperature is controlled by gas input.

An average of 9.3% carbon was detected in the sinter during the entire run, and 10.6% during the 10-hour higherreduction period. This is a sufficient quantity for complete reduction in the smelting furnace.

The kiln was operated at 0.46 r.p.m. during the entire run, and retention time was about 2 hours, 55 minutes. The load factor (percent loading) varied between 7.5

and 8.9%. As noted hereinabove, load factor is important because a bed which is too deep is difiicult to heat through and to circulate reducing gases through, thereby decreasing reduction. As noted in the aforementioned copending application, load factor should be kept below 15% and the optimum range is about 6l1%.

The importance of hydrogen reduction is attested to by the fact that during the 10-hour period during the run when reduction was highest, hydrogen reduction within the bed was equivalent to 82 pounds of carbon.

The economic benefits accruing through use of the process of the invention are attested to by the fact that, in spite of using as much as 10 times more reducing gas than had previously been used, the increase in production resulted in a saving of $3.20 per ton of ore treated, when compared to the best previous results with the same ore being treated in the same kiln.

It is believed that a better understanding of the invention will be gained by referring to the appended examples which are intended to be illustrative only and not in a limiting sense.

Example I A 20 in. I. D. x 20 ft. long kiln was modified to allow the practice of the invention. Two air fans were installed along the length of the kiln to allow controlled combustion of the excess gas. The discharge end was sealed to retard .air inleakage. The discharge drum was also tightly sealed to the discharge hopper. The kiln was inclined 2. 5" and was operated at /2 r.p.m. The temperature of the kiln bed in the first four feet from the feed end increased from 600 C. to 900 C.; the rest of the kiln was maintained between 900 C. to 1050 C.

After heating up, the kiln was operated under stable conditions for 44 hours using two different iron ores. Feed rate was 180 pounds of ore. This gave a kiln loading of about 10%. The reductant was added with ore at the feed end of the kiln in an amount to give 10% carbon in excess of that to completely reduce all of the iron to metallic iron.

Anthracite fines were used with both ores. A medium volatile bituminous coal was also studied with one of the ores. Sinter discharge averaged 165 pounds of sinter per hour.

The main burner supplied 3600 s.c.f.h. of air and 400 s.c.f.h. of natural gas. An auxiliary burner at the discharge end supplied an additional 900 =s.c.f.h. of natural gas. Thus the air to gas ratio at the discharge end was 2.8 to 1. During the run, the side fan nearest the discharge end supplied 2,500 s.c.f.h. of air and the second fan an additional 4,000 to 6,000 s.c.f.h. of air.

When the kiln reached stable conditions, prereduction of the first ore ranged from 50.2% to 60.3%. Prereduction for the second ore using anthracite was from 45 to 54% and using the low volatile bituminous from 40.8% to 47.5%.

Example II A rn'agnetitehematite calcine was processed in an ft. kiln using the kiln practice of the invention. The carbonaceous reductant was a finely divided anthracite low in volatile content. For the run, the kiln was already hot having completed a previous phase of operation.

The k-iln was maintained under an excess of gas pres sure and the ore and coal were fed together through the feed end of the kiln. Since there was no interruption in feeding between phases, the kiln was operated for eight hours while feeding the new mixture in order to completely drain out the materials from the first operation. During the next 40 hours of operation, the following materials were fed to the kiln:

54 tons of calcine 11.6 tons of anthracite 22,900 s.c.f. gas

Feed rate averaged 2,700 lbs. of calcine.

A total of 37.2 tons of sinter was discharged from the kiln at an average temperature of 1010 C. Prereduction of the ore ranged from 45.2% to 69.9% throughout the 40 hour production period. The average prereduction for this period was 60.2%. Metallic iron in the sinter was 26% during optimum operating conditions.

In comparison, using previous kiln techniques, it has not been possible to exceed a feed rate of from about 1300 to 1500 lbs. of ore or calcine per hour. Even at this lower feed rate, prereductions seldom exceeded 50%.

The heat balance calculated for the run indicated that even with the excess gas in the kiln and the gas leakage at the nose ring, the kiln had operated at 37.4% thermal efficiency. If the gas leakage were eliminated as in the case of a new kiln, the thermal efficiency would have been 42.3

It will be understood that various changes in the details, materials, steps and arrangements of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.

Having thus described the subject matter of our invention, what it is desired to secure by Letters Patent is:

1. Process for maintaining reducing conditions over approximately two-thirds of the length of a rotating kiln for use in the continuous treatment of a metallurgical charge that comprises:

charging a mixture of ore and carbonaceous rediictant into a rotary kiln,

'sa-id mixture occupying no more than fifteen (15%) percent of the cross-sectional area of said kiln;

at least partially oxidizing said mixture at an elevated temperature within the one-third of said kiln nearest the feed end to elfect dehydration thereof and the removal of volatile and oxidizable components therefrom including sulfur and arsenic;

maintaining a higher than atmospheric pressure of reducing gas adjacent the discharge end of said kiln;

injecting sufiicient air at a first air-injection point no more than about one-fourth the length of said kiln from said feed end to burn all reducing gas between said point and said feed end;

injecting a volatile carbonaceous material between the feed end of said kiln and said first air-injection point;

injecting additional carbonaceous material and air at a plurality of points between said first air-injection point and the discharge end of said kiln, the quantity of said additional carbonaceous material being suffic-ient to establish the total residual carbon content of the mixture as discharged from said kiln at no more than about a 10 percent excess of the stoichimetric quantity required for effecting the degree of total metallization desired upon subsequent smelting of said kiln discharge, and

supplying said kiln discharge while hot to an electric smelting furnace and smelting the same therein to totally reduce to the metallic state the desired metallic values thereof.

2. The process as claimed in claim 1, wherein said charge mixture occupies between 6 and 11 percent of the cross-sectional area of said kiln.

References Cited by the Examiner UNITED STATES PATENTS 1,760,078 5/30 Newkirk 753 6 1,937,822 12/33 Jones 75-36 2,754,197 7/56 Wienert 7536 2,829,042 4/ 58 Moklebust 75-36 2,941,791 6/ 60 Wienert -36 3 029,141 4/ 62 Sibakin 75-34 DAVID L. RECK, Primary Examiner.

WINSTON A. DOUGLAS, Examiner.

Claims (1)

1. PROCESS FOR MAINTAINING REDUCING CONDITIONS OVER APPROXIMATELY TWO-THIRDS OF THE LENGTH OF A ROTATING KILN FOR USE IN THE CONTINUOUS TREATMENT OF A METALLURGICAL CHARGE THAT COMPRISES: CHARGING A MIXTURE OF ORE AND CARBONACEOUS REDUCTANT INTO A ROTARY KILN, SAID MIXTURE OCCUPYING NO MORE THAN FIFTEEN (15%) PERCENT OF THE CROSS-SECTIONAL AREA OF SAID KILN; AT LEAST PARTIALLY OXIDIZIG SAID MIXTURE AT AN ELEVATED TEMPERATURE WITHIN THE ONE-THIRD OF SAID KILN NEAREST THE FEED END TO EFFECT DEHYDRATION THEREOF AND THE REMOVAL OF VOLATILE AND OXIDIZABLE COMPONENTS THEREFROM INCLUDING SULFUR AND ARSENIC; MAINTAINING A HIGHER THAN ATMOSPHERIC PRESSURE OF REDUCING GAS ADJACENT THE DISCHARGE END OF SAID KILN; INJECTING SUFFICIENT AIR AT A FIRST AIR-INJECTION POINT NO MORE THAN ABOUT ONE-FOURTH THE LENGTH OF SAID KILN FROM SAID FEED END TO BURN ALL REDUCING GAS BETWEEN SAID POINT AND SAID FEED END; INJECTING A VOLATILE CARBONACEOUS MATERIAL BETWEEN THE FEED END OF SAID KILN AND FIRST AIR-INJECTION POINT; INJECTING ADDITIONAL CARBONACEOUS MATERIAL AND AIR AT A PLURALITY OF POINTS BETWEEN SAID FIRST AIR-INJECTION POINT AND THE DISCHARGE END OF SAID KILN, THE QUANTITY OF SAID ADDITIONAL CARBONACEOUS MATERIAL BEING SUFFICIENT TO ESTABLISH THE TOTAL RESIDUAL CARBON CONTENT OF THE MIXTURE AS DISCHARGED FROM SAID KILN AT NO MORE THAN ABOUT A 10 PERCENT EXCESS OF THE STOICHIMETRIC QUANTITY REQUIRED FOR EFFECTING THE DEGREE OF TOTAL METALLIZATION DESIRED UPON SUBSEQUENT SMELTING OF SAID KILN DISCHARGE, AND SUPPLYING SAID KILN DISCHARGE WHILE HOT TO AN ELECTRIC SMELTING FURNACE AND SMELTING THE SAME THEREIN TO TOTALLY REDUCE TO THE METALLIC STATE THE DESIRED METALLIC VALUES THEREOF.