US2357198A - Sintering - Google Patents

Description

P ntented Aug. 29, 1944 umrso STATES PATENT ,o F f snvraamc C. Hooey, South Orange, N. J., asslgnor to The New Jersey Zinc Company, New York, N. Y., a corporation or New Jersey No Drawing.

4 Claims.

-a more porous and hence more desirable sinter.

The invention is particularly useful in the sintering of finely-divided flotation concentrates or calcines derived therefrom, containing substantial amounts of sulfide sulfur, although it may also be employed advantageously in the treatment of other material. Fine oxidic material, such as hematite concentrates and flue dust may be agglomerated by blast roasting in accordance with the invention, provided that it contains sufiicient fuel, say fine coal, to supply'the'heat required for sintering.

In preparing some-metallurgical charges for subsequent treatment, they are subjected to a sintering operation to bring about agglomeration of the particles in the charge and thus produce a coarser aggregate. Thus, it is customary to subject some sulfide ores and concentrates to blast roasting, air being forced through a bed of the ignited material sothat roasting and agglomeration are accomplished simultaneously. In some instances, as in the case of zinc sulfide concentrates-the material is, first subjected to partial roasting in a furnace in which agglomeration does not occur. The roasted concentrate or calcine produced in this operation contains a substantial proportion (say 8% to 10% by weight) of residual sulfide sulfur. This calcine is subjected to sintering by blast roasting,.jfor example in a straight line machine of the Dwight-Lloyd type, although other equipment, such for example as an up-draft Huntington-Heberlein blast roasting pot or down-draft apparatus, such as the drum orhorizontal-table type of Dwight-Lloyd machine, may be employed. I

In the more usual practice of sintering by blast roasting. the charge (containing an adequate amount of combustible material to serve as fuel during the sint ring operation) is placed upon a hearth layer on a traveling grate. In the sintering or roasted sulfide concentrates, such for example as calcines produced from zinc blende, the fuel consists'in part oiresidual sulfur, say 8% Application February 20, 1922, Serial No. 431,665

' draft.

sulfide sulfur, and in part of added coal or 'coke. Such a charge is ignited by passing it on the traveling grate below an igniter; The traveling grate carrying the'ignited chargeothe n. passes over suction boxes that provide adequate down- A .blast of air is sucked downwardly through the ignited charge to burn the sulfide sulfur and coal therein. Burning progresses gradually through the charge from top to hottom,the operation being .so conducted that immediately after the combustion has been completed in the, bottom layer of the charge, the resulting sinter cake is discharged from the machine.

The sinter is a more or less porous mass which can be crushed readily to a granular form appropriate for subsequent reductionoperations.

Sulfides that are unconsumed during the passage of the charge over the sinterihg machine are, for

the most part, not sintered together with the rest of the charge and remain in the form of fines. These unroasted sulfides remaining in the sinter cake can be and usually are separated therefrom by screening and returned to the sintering operation so that they will be roasted and agglomerated.

It has been found-that it is desirable sintered sulfides, and to mix both with fresh charge to be sintered. Thus, it is customary to screen the sinter cake on a screen with a mesh substantially .coarser than that through which the unsintered sulfides would pass, removing in this way from 25% to 45% of the sinter. Re-

circulation of this large proportion of charge tosintering lay-rolling it in a cement mixer or the like with'water and a. binder such as zinc oxide or clay. However, nodullzation of the charge produces only a coarse porosity between individual pellets or nodules. Satisfactory sintering is dependent upon a satisfactory degree of porosity in the individual pellets. Nodulizing does not bringaboupa mb tantialincrease in the porosity of individual pellets in the charge, and

to remove some sintered fines along with the unized by the capacity of imbibing water freely with pronounced swelling at room temperature, and pronounced shrinkage upon evaporation of the imbibed water. The presence of the wet and intimately distributed argilloid substance in the charge during sintering results in the formation of a series of fine pores in the charge. These pores permit the hot blast to be distributed throughout the charge to an increased degree. Combustion in the charge is thus improved greatly, with resultant increase in capacity and efflciency of the sintering machine. The increase injcapacity is due partly to the fact that the sintering progresses more rapidly and partly to the fact that the proportion of sintered fines required to be recirculated to increase porosity is reduced.

The argilloid water-imbibing substance is distributed throughout the charge in the finelydivided condition. Prior to heating, the particles thereof are wet 'and swollen, but the water is evaporated from the particles as the heating progresses and shrinkage occurs with resultant formation of a series of fine voids or pores, which are more or less intercommunicating throughout the charge. These fine pores provide avenues for the passage of the blast, with the result that the pressure required to force the blast through the charge is reduced while the contact of theblast and fire with the charge is more intimate.

Any electrolyte (say a water soluble metal salt such as zinc sulfate) that is present in the charge may tend to interfere with the absorption of waamounted to about 2% on the weight of the te-r by the bentonite. This is undesirable, for bentonite that has not imbibed water freely, due to the interference of the electrolyte, is less effective as a pore-producing agent in sintering. Conblast roasting on a Dwight-Lloyd straight line sintering machine. Prior to the application of the invention, the practice was to mix a partially roasted zinc blende concentrate containing about 7% to 12% of residual sulfide sulfur and 50% to 55% zinc with about 6% (by weight of the concentrate) of fine coal (minus -8 or 10 mesh) and 25% to 40% of recirculated fines. This mixture was sintered and treated on a screen having openings about 0.3 inch wide. The undersize of the screening operation, i. e., 25% to 40% of the total, was recirculated after admixture with the fresh feed.

' This operation was modified by mixing with the charge a suspension of finely-divided northern bentonite (minus 200 mesh). The bentonite charge, and about 7 pounds of water were employed per pound of bentonite. The charge also included a small proportion of recirculated fines.

Among the argilloid water-imbibing substances that may be employed advantageouslyin the practice of my invention are bentonite and related argilloid substances, such as sapom'te which isfound in a form sometimes called hectorite. I prefer to employ bentonite, specifically the type of bentonite that imbibes water most freely with swelling, i. e., the northem bentonite or alkali bentonite.

The bentonite should be finely-divided, say all minus 200 mesh, and may be employed in proportions ranging from 1% to 5% on the weight of the charge. For most operations, it is preferable to employ about 2.5% of bentonite on the weight of the charge. Bentonite in proportions in excess of 2.5% tends to produce a friable sinter cake that yields a relatively high proportion of .fines when the cake is discharged from the machine. In some operations, however, a friable sinter may be desirable and in such cases it is desirable to employ more than 2.5% of bentonite.

Water should be employed in proportions sufficient to bring about complete swelling of the pore-producing agent, say the starch or bentonite. The amount of water should also bring about adequate bonding of the particles in the charge. Thus, the proportion of water to be employed may vary from about 10% to 20% 'on the weight of thecharge, or say 5 to 10 times the weight of the water-imbibing substance.

I prefer to disperse the argilloid substance in the water prior to addition of the two to the charge, However, good results are obtained by adding the water and the substance (say, bentonite) together, or at least at points not widely separated so as to provide the substance with an adequate opportunity to imbibe the water.

It was treated on the Dwight-Lloyd machine as in the case described above, but with improved results. It was discovered that the presence of hentonite permitted a substantial reduction in the proportion of coal employed, the coal being replaced by unroasted zinc blende concentrate containing a substantial proportion of loose atom or volatilizable sulfur. It is believed that the presence of coal in the charge contributes a degree of porosity thereto and that with the bentonite, it is no longer necessar for the coal to perform this function, provided that another easily ignited material (such as the loose atom sulfur) is available. Thus, the invention permits substantial savings in the proportion of carbonaceous fuel required, since the bentonite or other water-imbibing agent assures adequate porosity while the loose atom sulfur takes the place of part of the coal in aiding ignition of the charge and speeding blast roasting.

The 0.3 inch screen on which the prior sinter was treated was replaced by one having openings only 0.03 inch wide. The material passing through this screen consisted in large part of unsintered sulfide particles and the proportion of sintered material in the fines was greatly decreased, as was the total quantity of fines which amounted to only 10% to 20% of the feed instead .Of the 25% to 40% previously produced. The smaller proportion of fines was recirculated through the machine.

In addition to reducing the proportion of fines to be recirculated, the presence-of the bentonite also brought about a marked decrease, of the order of 35%, in the degree of suction required to force the blast through the charge, in spite of the increase in the proportion of fresh material in the charge and the decrease in the proportion of recirculated material. Because 01' the reduced resistance to gas passing through the charge, the blast and therefore the fire traversed the charge much more rapidly. In view of the decreased suction required, there was a great decrease in leakage of air around the charge atthe edges of the grate and consequently an improvement in the uniformity of the sinter. There was also a reduction in the time required for sintering and for elimination of the sulfide sulfur, so that the machine could be operated at increased speed with the same or better sulfur elimination. The sinter contained substantially less than 1.5% total sulfur.

The net result of all these improvements was an increase in the capacity of the sintering machine of about 75% and the production of a more porous and consequently more satisfactory sinter with a lower consumption of coal.

Although I prefer to employ bentonite, argilloid materials related to bentonite, such as saponite including the form of saponite known as hectorite, may also be used in about the same proportions, say 1% to 5%, and preferably about 2% for most types of sinter. T

The charge need not contain sulfide sulfur provided that there is enough other combustible material to bring about adequate sintering. Thus, iron ores and smelter by-products, such as flue dust, that are substantially sulfur-free may'be sintered in accordance with my invention, em-

ploying a larger proportion of fuel, say coalor coke.

I claim:

In a sintering operation in which a blast of oxidizing gas is passed through a non-agglomerated pervious solidmetal-bearing charge .having an oxidizable constituent distributed therein to burn this constituent and heat the charge to a high temperature with resultant agglomeration of the particles of the charge to form a sinter, the improvement which comprises forming a series of fine intercommunicating pores in the charge as the sintering progresses by incorporating in and distributing throughout the charge prior to heating a finely divided water-swollen argilloid substance, said substance being selected from the group consisting of bentonite and saponite characterized by the capacity of imbibing water freeiywith pronounced swelling at room temperature and pronounced shrinkage upon the evaporation of the imbibed water, and heating the charge to the high temperature of the sintering operation and' thereby evaporating the water imbibed in swelling the argilloid substance and shrinking the substance with the resultant formation of a series of fine intercommunicating pores throughout the charge. 1

2. The sintering operation of claim 1, in which the dry weight of the argilioid substance incorporated in the charge is from l to 5%on the weight of the charge and the imbibed water for swelling the substance is from 10 to 20% on the weight of the charge.

3, The sintering operation of claim 1, in which the dry weight of the argilloid substance incorporated in the charge is about 2.5% on the weight of the charge and the imbibed water for swellin the substance is from 5 to 10 times the dry weight of the substance.

4. The sintering operation of claim 1, in which the dry weight of the argilloid substance incorporated in the charge is about 2% on the weight of the charge and the imbibed water for swelling the substance is about '1 pounds per pound of the substance.-

WILLIAM C. HOOEY.