US3197303A

US3197303A - Process for pretreatment of ores in rotary kiln

Info

Publication number: US3197303A
Application number: US173539A
Authority: US
Inventors: Collin Fredrik Christen
Original assignee: Elektrokemisk AS
Current assignee: Elektrokemisk AS
Priority date: 1961-02-24
Filing date: 1962-02-15
Publication date: 1965-07-27
Anticipated expiration: 1982-07-27
Also published as: ES274865A2; GB935353A; FI40291B; NO118749B

Description

y 27, 1965 F. c. COLLIN 3,197,303

PROCESS FOR PRETREATMENT OF ORES IN ROTARY KILN Filed Feb. 15, 19 62 2 Sheets-Sheet 1 a? /d 54 44 7?; z l T. 1

lg 51% 5; Z0 1 W m 46 wy yAw As mmkvw xy Av /i f FIG.

. un. "ml

JNVENTOR. FREDRIK CHRISTEN COLLIN ATTORNEYS.

July 27, 1965 F. c. COLLIN 3,197,303

PROCESS FOR PRETREATMENT OF ORES IN ROTARY KILN Filed Feb. 15, 1962 2 Sheets-Sheet 2 ATTORNEYS.

United States Patent 3,197,303 PROCESS FOR PRETREATMENT OF GRES IN ROTARY KILN Fredrik Christen Collin, Lian, Oslo, Norway, assignor to Elektrokemisk A/ S, Oslo, Norway, a corporation of Norway Filed Feb. 15, 1962, Ser. No. 173,539 Claims priority, application Norway, Feb. 24, 1961, 139,223 4 Claims. (CI. 75-33) This is a continuation-in-part of application Serial No. 18,158, filed March 28, 1960, and now abandoned.

This invention relates to the pretreatment of ores or other oxides or materials of the type which are customarily treated in a submerged arc smelting furnace. These include iron oxides, various oxides for producing ferro alloys such as ferro manganese or ferro silicon, and also can be used in the production of calcium carbide and phosphorus.

In accordance with this invention the material to be treated is preheated and preferably more or less reduced in a rotary kiln. The novel feature of the invention is found in the fact that carbonaceous material such as bituminous coal, lignite, peat or the like, containing a substantial portion of volatiles is introduced into the kiln in an intermediate zone comprising the longitudinal middle one third of the furnace where the charge has been brought up to such temperature that the coal will be given such a sudden carbonization that the plastic period of the coal is quickly passed and sticking to the lining is avoided. Such coal is introduced into the kiln in such a Way that it falls onto the charge rather than onto the sides of the kiln and air is supplied in the intermediate zone so that the volatiles driven out of the coal will burn with a luminous heat radiating flame in the central part of the kiln to extend the zone of high heat toward the feed end of the kiln.

In order to accomplish this result the kiln is provided in the central one-third of its length with a series of pipes that extend into the kiln and are provided on their outer ends with scoops that dip into a mass of the coal. As the kiln rotates the scoops pick up the coal and then as rotation continues the coal passes through the pipes to the inside of the kiln and drops onto the hot charge at the bottom of the kiln. The kiln is operated at a pressure slightly below atmospheric so that additional air will come in through the scoops after they have delivered their charge of coal.

The kiln is fired at the discharge end and any convenient fuel may be employed as oil or gas. If the charge from the furnace is to be introduced into an adjacent electric smelting furnace, gas from such furnace may advantageously be employed.

For the purpose of illustration we may consider what happens in the case Where iron ore such as Fe O is being preheated and partially reduced in a rotary kiln. In such case a reaction takes place in two stages illustrated by the following two equations:

In an ordinary kiln operation the charge layer which moves around on the bottom of the kiln has little contact with the gases passing over it and the effect of the CO is relatively low. However, some reduction takes place as new CO is formed continuously by reaction between CO and the carbon of the coke. The efiectiveness of this reaction depends in large part upon the reactivity of the carbon material and this determines the speed of the overall reaction.

especially favorable conditions of reaction.

3,197,303 Patented July 27, 1965 It is recognized that freshly formed carbon produced by the carbonization of raw coal, lignite, wood, peat or the like, has high reactivity and in accordance with my invention such material is thus carbonized to form a coke of high reactivity so that an increased reduction speed is obtained at a relatively low temperature for example, between 900 and 1000 C.

A specially reactive coke is obtained by wholly or partly employing bituminous coal as a reducing agent. The resulting coke has high porosity and consequently high chemical reactivity with large surface area, and Heretofore the use of such coal has however been largely limited as it causes great problems sincethe coal goes through a plastic interval at temperatures between 400 and 700 C. and at this stage the coal will stick to the lining of the furnace.

By my present invention the coal which is fed more or less continuously so that it drops onto the preheated charge is heated with great rapidity and gives up its volatiles and passes through the plastic stage substantially without contacting the kiln walls. This supplies the coke of a particularly valuable nature and volatile constituents are caused to burn in the central part of the furnace so that the heating zone is extended. As previously brought out, the air for burning the volatiles from the coal may be introduced through the same pipes as introduced the coal or additional air pipes as frequently used may be supplied preferably toward the discharge end of the kiln. Also such air may be introduced by supplying excess air to the kiln burner.

Several important conditions must be met in carrying out the process of the present invention. First of all care must be taken to feed the coal or other carbonaceous material with high volatile content onto the charge and the charge must be at a temperature of not less than 800 C. In this way the heat of the charge will cause the coal or other carbonaceous material to pass so quickly through the plastic sticky state that when the particles of coal contact the walls of the rotary kiln there will be no objectionable sticking.

Secondly, there must be enough air present at the point of feed or immediately adjacent thereto to cause the volatiles liberated from the coal to ignite and burn with a luminous heat radiating flame. generating the heat necessary for extending the zone of high heat toward the feed end of the kiln and for maintaining the charge at the above specified temperature for driving volatile material out of the coal. It is also important actually to burn the volatiles with a luminous flame as otherwise sticky tars and other volatiles will condense and build up objectionable deposits on the wall of the kiln especially toward the relatively cool feed end of the kiln. If tar or other deposits are allowed to build up on the wall of the kiln it becomes necessary to shut the kiln down to remove the deposits which is time consuming and expensive. 7 i p Finally it is important to control the feeding of coal or other carbonaceous material into the intermediate zone of the rotary kiln in order to prevent the partially reduced charge from sticking to the wall of the rotary kiln. As 'is known, there is a temperature at which the metal oxides or other charge to be reduced will soften. For example in the case of low grade iron ores the softening point of the metal oxideoccurs within the range of about 800 to 100.0 C. and if the ore becomes soft it will stick to the wall of the rotary kiln and form objectionable deposits.

It has now been established that when the coal is fed into the intermediate zone of the kiln at a single point, only a part of the carbon required for the desired reduc- This is important for tion of the charge may be fed into the intermediate zone of the kiln. By use of coal with exceptionally high contents of volatiles, e.g. above 50%, it is preferable to introduce no more than about two thirds of the amount of carbon required for reduction of the charge into the intermediate zone of the kiln while at least about one third of the carbon required for reduction of the charge is introduced into the intermediate zone of the kiln. No significant advantage is achieved if less than about one third of the amount of carbon required for reduction of the charge is introduced into the intermediate zone of the kiln. The remaining carbon required for reduction of the charge is introduced into the feed end of the kiln along with the charge. The carbon introduced into the feed end of the kiln is in the form of coke or other carbonaceous material with a low volatile content of not more than about 25%.

While great advantage is realized in accordance with the present invention by control of heat balance to keep the temperature of the charge below softening point, subsequent work has resulted in the surprising discovery that when coal is fed into the intermediate zone of the "kiln at two or more separate feed points it is possible for the temperature of the charge to exceed the softening point without sticking to the wall of the kiln. The reason why the softened charge does not stick to the wall of the kiln is not now fully understood but it is believed that coal fed into the intermediate zone so eifectively cools the kiln lining that the softened charge does not stick to the wall. But regardless of the exact mechanism of the way in which the coal prevents the softened charge from sticking to the wall of the kiln the fact remains that the charge quite unexpectedly may be heated to a temperature of about 100 to 200 C. above softening point without any objectionable sticking to the wall of the kiln. For best results the distance between the spaced feed points in the intermediate zone of the kiln is not more than about three times the inside diameter of the kiln and preferably about twice the inside diameter of the kiln.

A typical rotary kiln for commercial production of 'iron sponge or prereduction of iron ore and subsequent electric smelting may have an inside diameter between 2 to m. and a length of 50 to 100 m. The distance between the feed points in the intermediate zone for these kilns will be from about 4 to In. A plurality of feeders may be employed in the intermediate zone of the kiln. For example excellent results have been achieved in a kiln 3 m. in diameter and 100 m. long by spacing five feeders in the intermediate zone of the kiln arranged 30, 36, 42, 48 and 54 meters away from the discharge end of the kiln. The desired high temperature in the intermediate zone of the kiln is best achieved by feeding approximately the same amount of coal into the kiln at each of the five spaced feed points and if the coal is fed at two or more feed points all the carbon required for reduction of the charge may be introduced into the intermediate zone of the kiln. Feeding coal into the intermediate zone of the kiln at two or more spaced feed points is of particular advantage in the production of carbide. In such case the temperature in the kiln may be kept sufiiciently high for calcination of the limestone without building up objectionable deposits on the Wall of the kiln.

Additional details and advantages of the process of the present invention may be readily understood by reference to the accompanying drawings in which:

FIG. 1 shows a side view of the kiln.

FIG. 2 is a section on line 2- 2 of FIG. 1.

FIG. 3 shows the kiln of FIG. 1 with a plurality of feeders positioned in the intermediate zone of the kiln.

In this drawing 10 is the kiln properwhich turns on rollers 12. The charge may be introduced from a storage hopper 14 through a pipe 16, which. is provided with a control valve 18. The smoke and exhaust gases pass into a smoke chamber 20 and then are taken to a flue by pipe 22.

At the discharge end the kiln is provided with a discharge pipe 24 and a usual burner 26 for oil or gas. It is understood that air will also be introduced through the burner 26. If desired, the discharge pipe 24 may pass directly into an electric smelting furnace as is well understood in the art and gas from such a smelting furnace may be used as fuel for the burner 26. Up to this point the kiln is conventional.

At an area in the central portion of the kiln I provide a series of pipes 28 which extend into the furnace and at their outer end are bent in the direction of rotation of the kiln so as to form scoops 50. A container 32 runs under the kiln in the zone of the scoops 30 and has an upwardly extending portion 34 which follows the contour of the kiln.

A hopper 36 is provided for the coal which may be connected with feeder 38 and discharge pipe 40 which discharges the coal, or other non-coked fuel into the upwardly extending portion 34.

It will be obvious that as the furnace rotates, the scoops 30 will pick up coal from the container 32 and then as the rotation continues this coal will be dropped onto the charge indicated at 42. This charge will already have been heated up to a temperature which normally will be about 800 C. and the coal falling in small quantities on such charge will promptly give up its volatile matter and be brought through its plastic state. Air will enter the furnace through the scoops 30 which have already discharged their coal and will thus cause the volatile ingredients to burn.

In this way it is possible to use relatively low grade coal for the smelting operation and to obtain all the good of the coal by directly burning the volatiles and using the coke elements for carrying out the reaction.

In one case I employed a kiln 8.3 meters long having an internal diameter of .55 meter. Powdered Hematite ore having an Fe content of about 60% was used. For part of the test the ore was mixed with the full quantity of gas coke and passed through the kiln in the usual manner with the pipe 28 closed off. In the other case one-half of the carbonaceou material was mixed with the charge in the form of gas coke and the other half was made up of powdered coal taken from the Longyear Mine in Spitzbergen, with 60% fixed carbon and 40% volatiles. This was introduced at about the central zone of the kiln and after the charge had reached a temperature of about 800 C. In each case the kiln was run at the same feeding rate and a temperature of 900 to 1000 wa maintained between the point where the coal was fed in and the discharge end of the kiln. The kiln in this case was oil fired from the discharge end and the rate of feeding was about 300 kg. of charge per hour.

An examination of the charge that had passed through. this kiln showed that with of carbon sup-plied as gas coke mixed with the charge, between 20% and 24% of the oxygen Was removed from the ore. In the other case, using 50% low grade coal, as much as 40% and in some instances up to 60% of the oxygen was removed from the ore. Similar results were had using ores for the production of ferro-manganese.

The kiln illustrated in FIG. 3 is identical to the kiln shown in FIG, 1 with the exception that two of the feeders illustrated in FIG. 2 are located in the intermediate zone of the kiln. As shown in FIG. 3 the distance between the spaced feed points is about twice the interior diameter of the kiln. As a result of feeding coal into the intermediate zone of the kiln at a plurality of spaced points the temperature of the charge may exceed the softening point without building up objectionable deposits on the wall of the kiln.

By this process it is possible to use low grade reducing agents such as lignite, peat or even wood, as well as ordinary coal and these materials with high volatile content of 20% or more produce a charge that forms a luminous heat radiating flame in the central part of the kiln. This helps to extend the Zone of high temperature in the kiln where the reduction takes place. Also, as brought out above, an increased reduction of the ore is had, apparently due to the high reactivity of the carbon which reacts rapidly with the CO generated, to maintain CO in the furnace. It will also be noted that the gases are given oif in a part of the kiln where the temperature of the ore is sufiiciently high to permit the volatile constituents including any hydrogen contained, to react with the metal oxides.

I claim:

1. A process of heating in a rotating cylindrical kiln metal oxides of the type which are smelted and reduced in an electric furnace which comprises introducing into the feed end of said kiln a charge of said metal oxide and from about one third to about two thirds of the amount of carbonaceous reducing agent required for reducing all of said metal oxide, such carbonaceous reducing agent having a relatively low volatiles content up to about 25% by weight, heating said charge to a temperature of at least about 800 C., dropping upon said heated charge, while the same is located in a zone corresponding to the longitudinal middle third of said kiln, the remaining two thirds .to one third or" carbonaceous reducing agent required for complete reduction, said second-mentioned carbonaceous reducing agent having a relatively high volatiles content greater than about 25% by weight and being dropped upon said heated charge from a plurality of feed points spaced apart from each other a distance not greater than about three times the inside diameter of said kiln, and introducing suflicient air in said dropping zone to ignite and burn with a luminous flame the volatiles liberated from said second-mentioned carbonaceous reducing agent, thereby increasing the zone of high temperatures in the longitudinal middle third of said kiln.

2. A process as in claim 1 wherein an equal amount of said second-mentioned carbonaceous reducing agent is dropped from each of said plurality of feed points.

3. A process as in claim 1 wherein said charge is heated to a temperature within the range from about 800 C. to

a about 1000" C.

4. A process as in claim 1 wherein said charge is heated in said dropping zone to a temperature from about 100 C. to 200 C. in excess of its softening point substantially without adhering to the kiln wall.

References Cited by the Examiner UNITED STATES PATENTS 890,563 6/08 Jones 7333 2,484,911 10/49 Seil -a 26-3-33 2,593,398 4/52 Kalling 75-36 2,754,197 7/56 Wienert 75--36 3,029,141 4/62 Sibakin 26333 X DAVID L. RECK, Primary Examiner. WINSTON A. DOUGLAS, Examiner.

Claims

1. A PROCESS OF HEATING IN A ROTATING CYLINDRICAL KILN METAL OXIDES OF THE TYPE WHICH ARE SMELTED AND REDUCED IN AN ELECTRIC FURNACE WHICH COMPRISES INTRODUCING INTO THE FEED END OF SAID KILN A CHARGE OF SAID METAL OXIDE AND FROM ABOUT ONE THIRD TO ABOUT TWO THIRDS OF THE AMOUNT OF CARBONACEOUS REDUCING AGENT REQUIRED FOR REDUCING ALL OF SAID METAL OXIDE, SUCH CARBONACEOUS REDUCING AGENT HAVING A RELATIVELY LOW VOLATILES CONTENT UP TO ABOUT 25% BY WEIGHT, HEATING SAID CHARGE TO A TEMPERATURE OF AT LEAST ABOUT 800*C., DROPPING UPON SAID HEATED CHARGE, WHILE THE SAME IS LOCATED IN A ZONE CORRESPONDING TO THE LONGITUDINAL MIDDLE THIRD OF SAID KILN, THE REMAINING TWO THIRDS TO ONE THIRD OF CARBONACEOUS REDUCING AGENT REQUIRED FOR COMPLETE REDUCTION, SAID SECOND-MENTIONED CARBONACEOUS REDUCING AGENT HAVING A RELATIVELY HIGH VOLATILES CONTENT GREATER THAN ABOUT 25% BY WEIGHT AND BEING DROPPED UPON SAID HEATED CHARGE FROM A PLURALITY OF FEED POINTS SPACED APART FROM EACH OTHER A DISTANCE NOT GREATER THAN ABOUT THREE TIMES THE INSIDE DIAMETER OF SAID KILN, AND INTRODUCING SUFFICIENT AIR IN SAID DROPPING ZONE TO IGNITE AND BURN WITH A LUMINOUS FLAME THE VOLATILES LIBERATED FROM SAID SECOND-MENTIONED CARBONACEOUS REDUCING AGENT, THEREBY INCREASING THE ZONE OF HIGH TEMPERATURES IN THE LONGITUDINAL MIDDLE THIRD OF SAID KILN.