US2265866A - Smelting manganese ore - Google Patents

Description

Dec. 9, 1941.

, P. H. RoYs'rI-:R v2,265,866

SMELTING MANGANESE ORE' Filed -March 9, 1939 2 Sheets-Sheet Dec. 9, 1941. P. H. RoYs'rER 2,265,855

SMELTING MANGANESE ORE Filed March 9, 1939 2 Sheets-Sheet 2 /apoo Patented Dec. 9, 1941 UNITED STATES PATENT OFFICE i 2,265,866 sMEL'rING MANGANESE ORE Percy n. myster, Bethesda, Ma.

Application March 9, 1939, Serial No. 260,867

` (ci. 'z5-131) This invention relates to the smelting, in the blast furnace, of natural ores', beneciated ores, and the like, containing reducible manganese compounds, whereby to metallize their manganese contents. 'Ihe invention is concerned more particularly with an improved smelting process for the production, in the blast furnace, of alloys of manganese and iron.

The improved smelting process of the invention is applicable to the metallization of manganese occuring in ores (or metallurgical equivalents thereof) containing reducible manganese compounds, with or without reducible compounds of other metals, commingled with much silica. Production, in the blast furnace, of ferro-manganese of 80% grade from ores (or metallurgical equivalents thereof) containing much silica and/or other acidic refractory oxides is a particular object of this process: although production of alloys containing 4from 50% to 90% of manganese is within the purview of the invention.

By the expression manganese ore or metallurgical equivalent thereof I mean to include .f

both naturally 'occurring and synthetically promanganese and iron, silica and alumina, in which compositions the manganese and iron are present in ratios adapted to yield ferromanganese of at least 50% grade. In the appended claims the expression ore" includes metallurgical equivalents of a naturally occurring ore. Y

In the smelting of an ore in which the silica is low with respect to manganese content. the ore does not melt to a iiuid fusion product until relatively hightemperatures are reached. For example, a manganese ore containing 48% to 52% Mn and 6% to 12% SiOz, when melted, forms a# fusion "product inthe binary system MnO-SiOz containing from 89.4 to 81.4 n'iolar per cent of MnO and 10.6 to 18.6 molar per cent of MnSiOa. While the eutectic in the binary system MnO-MnSiOa is low-melting and fluid, the great preponderance of MnO in this low silica ore in excess of said eutectic composition renders said ore high-melting and viscous. the blast furnace smelting of such an ore, a liberal supply of limestone or dolomite would be added to the furnace burden where, as the ore and stone descended through the shaft, incipient fusion initiallywould take place between the CaO and MgO (resulting from calcinaticn of the stone,'atabout 1,650 F.) and progressive fusion of the ore 'and bases would form reaction prod- Some of the silica would `be combined with CaO and MgO to form the metasilicates CaSiOs and MgSiOa, thereby reducing the relative amount of the low-melting and fluid MnSiOa formed. The success heretofore attained in the blast furnace production of ferro-manganese has depended in large measure upon the retention of the manganese oxide of the ore in a non-fluid state in the furnace shaft above the tuyres and in the path of the ascending heated gases, so that a large part of the heat necessary for effecting the reduction of the MnO to metal by solid carbon could be brought into a satisfactory approach to equilibrium. 'I'he reduction of MnO by solid carbon according to the equation is strongly endothermic, absorbing about 2,300 B. t. u. per pound of manganese.

The above outline of the conduct of a metallurgical grade of manganese ore, i. e., a manganese ore having a relatively low SiOz content, in its descent to the blast furnace hearth is given in order to explain the dierences between its pyrometallurgy and the pyrometallurgy of an ore (or metallurgical equivalent, containing much contaminatingy silica. I have found experimentally that such an ore-melt, when maintained at 2,100o F., could quite readily be stirred with a carbon rod, and that the temperature of the melt had to be reduced to below 2,050 before the same could be called sluggish" or slow owing.

When such a'mineral composition as that just described is subjected to treatment according to conventional ferro-manganese blast furnace practice, the results are metallurgically unsatisfactory and economically unacceptable.

The charge material liquees too-early in its downward course through the furnace shaft, promptly owsv into the hearth where it kceases to .be in -contact with hot reducing gases, and resists reduction. l

This latter course of events probably explains the prior art conception that it was impossible to reduce to metal the manganese content of such an ore (or metallurgical equivalent) typiiied by manganese-bearing material containing 18% or more of SiO: or containing. 25% or more of gangue material, without at the same time reducing considerable (e. g.,` 10% to' 25%) of the SiO: content thereof. A

Asa result of conventional p'racticeapplied to such ores, the manganese content ot the reucts in the ternary system MnG-CaO-SiOa. 65 sulting slag is high, a. great increase in coke consumption over conventional practice is necessary (in order to maintain even unsatisfactory operation of the furnace), large additions of flux are required, daily tonnage of metal is low, and excessive losses of manganese and of heat result from the large volume of gases produced in the furnace, which gases leave the latter at an excessively high temperature, thereby adversely inuencing vaporization of manganese. The fraction of total manganese charged which is lost in the final slag is great, due to two causes: first, the actual manganese content of the nal slag is high, and second, the slag volume (i. e., pounds of slag per long ton of metal produced) is very much greater than are the slag volumes met with in smelting standard manganese ores.

From the foregoing discussion is will be appreciated that economical metallization, in the blast furnace, of manganese from such highsilica ores (or metallurgical equivalents) cannot be effected by carrying out the ferro-manganese blast furnace procedure which is conventionally considered good practice."

It is an object of the present invention to provide an improved process for the economical metallization, in a blast furnace operation, of

relatively small amounts of solid fuel (e. g., ma-

terially less fuel than ore. by weight).

These, and other, inventive objects are realized by the process of the present invention, according to which at least the major part of the manganese contained in a siliceous manganese ore of relatively low Mn-to-SiOz ratio ofthe types just described, is metallized in a blast furnace operation conducted under chemical and thermal conditions adapted to increase the quantity of available heat supplied to the furnace hearth and to promote the reactionl of reduction of MnO in solution in the slag by contact of the latter with solid carbon.

v In smelting a manganese-SiO: mixture (e. g., an ore of the sort above described), I charge the blast furnace with appropriate amounts of the mixture, solid fuel (e. g., coke), and a suitable basic uxing agent (e. g., a limestone and/or dolomite, natural or calcined), and blow the charge with a preheated blast. For increasing the quantity of available heat supplied to the furnace hearth I may:

I. vIncrease the blast temperature, and/or II. Decrease the criticaltemperature ofreduction of the manganese, and/or III. Decrease the partial Apressure of the gaseous product of the reduction of MnO by solid carbon, and/or IV. Increase the activity of the manganese compound existent in solution in the slag, and/or V. Increase the time of contact of slag, metal and carbon, and/or Y VI. Decrease the dimensions of the oxidizing zone adjacent the blast entrance.

.of the carbon monoxide in the hearth:

A fuller explanation of these six measures follows:

I. By increasing the blast temperature beyond that which has heretofore been employed in the blast furnace production of ferro-manganese I desirably increase the total sensible heat of the blast gases (oxygen and nitrogen) entering the furnace, and increase the amount of available heat per unit of fuel consumed without changing the amount of unavailable heat. In general it is a feature of the process of the present invention to preh'eat the blast to a temperature of at least 1600 or 1650 F. Increasing the blast temperature tends to increase the temperature of the gases produced in the combustion zone without concurrently increasing the critical temperature (J. E. Johnson, Jr., Principles. Operation and Products of -the Blast Furnace) of reduction of MnO.

II. The critical temperature of the reduction of manganese may, when the blast temperature is maintained at a sufficiently elevated value, be decreased by increasing the non-oxidizing constituents of the blast (i. e., operate with a blast containing a lesser proportion of oxygen than has ordinary atmospheric air). This 'expedient is effective for reducing Tc", since dilution of the blast with nitrogen decreases the partial pressure As the partial pressure of the carbon monoxide is decreased the content of MnO in the slag decreases in direct linear proportion.

III. I may decrease the partial pressure of the gaseous product of reduction of NnO by solid carbon,-and hence decrease the critical temperature of the reduction,either by (a) Lowering the hydrostatic pressure inside the furnace hearth, or by (b) Decreasing the percentage of CO present in the furnace hearth gases.

Measure (a) may be effected by operating the furnace under slow blowing conditions, permitting time for the CO (produced from the MnO reduction) to diffuse from the carbon-manganese oxide interface and for dilution of the gases at the interface With non-reactive nitrogen to take place; or, by shortening the charge column of the furnace to a height materially less than 'conventional for said furnace (whereby vhearth pressure is reduced) or, by decreasing th'e vertical height of the slag bath (whereby the hydrostatic pressure of the slag, and its MnO content, is reduced); or, by mechanically removing of CO from the slag displaces to. the right the equilibrium in the reaction expressed by Equation l and hastens the conversion of the MnO of the slag to metallic manganese and decreases the only as the blast temperature is so greatly elevated as to approach the critical temperature of reduction of MnO.

IV. The activity of the manganesecompound existing in the slag may be increased by increasing the basic oxide content of said slag, whereby the chemical combination between CaO and/or MgO and the A1203, CrzOs, SiOz, TiOz and other acidic oxides of the slag (forming silicates, alumi- `nates, alumino-silicates, etc., of the bases) isjincreased, and chemical fixation of the manganese as silicates, aluminates, or alumina-silicates of manganese is decreased.

As appears from the foregoing, the smelting of high-silica manganese ores in whichthe ratio of manganese to silica is 3to1 or less, e. g., a

manganese-iron ore (or equivalent) in which the.

ratio of manganese to silica is less than 3-to-1 and'in which the ratio of iron to manganese is less than 1-to-8, by conventional blast furnace practice is uneconomical for the following reasons:

(a) Increased fuel consumption (i. e.,pounds of fuel per long ton of metal) is necessary, due to the increased weight of material to be smelted per unit of metallized product;

(b) Increased addition of basic fluxing agent is required because of the greater weight of acid gangue oxides in the lower grade ,ore and because of the increase in fuel ash by reason of (a) above;

(c) Greater slag loss\of manganese is encountered because of the increased slag volume (i. e., pounds -of slag per long ton of metal), since the measure of the slag loss" of manganese is the product of the slag volume multiplied by the weight per cent of manganese retained in the Slag; f'

(d) The reducibili of the MnO in the charge is low because of dilution of the MnO with a large volume of the gangue oxides contained in such ore; and i (e) The reducibility of the MnO in the charge is further decreased by chemical combination of MnO with acidic gangue oxides to form silicates, aluminates, and aluminosilicates of manganese.

The increase in amount of uxing agent employed is a necessary result of the composition of the siliceous ore and cannot be obviated. In fact, in the process of the present invention the increase in amountf fluxing agent is greater than the proportional increase in acidic Sangue oxides, since it is desirable to increase the basicity of the slag-i. e., ratio of' (CaO plus MgO) to (A1203 plus SiOz) by weight. o

The principles of the process of the` present been employed either in pig iron practice or in ferro-manganese practice, e. g., a basicity ratio as high as 1.5.

For a blast furnace with a hearth pressure `of 7 lbs/sq in. (gauge) producing`80%, ferromanganese, in a case where the slag. basicity is sufficiently high, e. g., 1.5, the critical temperature Tc," in degrees F., may be plotted against the weight per cent of metallic manganese in the slag as shown in curve A of the graph designated Fig. 1 of the accompanying drawings.

On the contrary, when coke is relatively cheap and limestone is relatively expensive the basicity of the slag must, for economical reasons, be deinvention will be described with greater parn vention I employ an extremely basic slag whenever the same is` economically feasible. Thus, when limestone is relatively cheap (e, g., less than $1.00 per ton) and when coke'is relatively expensive (e. g., more than $5.00 per ton), I employ a basicity ratio higher than hitherto has creased, and can -be madeflessfthan unityy with resulting economy. With decrease in the basicity of the slag, the critical temperature of reduction of manganese is shifted upwardly. Thus, in the case of an extremely acid slag (e. g.,

cao+Mgo (Anth-sion :0.35

orless) Tc may be plotted as curve B in- Fig. 1.

For slag basicities intermediate 0.85 and 1.5 there exist a family of curves ,lying between curve A and curve B. This family of curves is repre sented by the following equation:

m=Mn in slag, in weight per cent, M=Mn in metal, in weight per cent, P=hearth pressure, in lbs./sq. in., absolute, Tc=critical temperature, in F. l Tc`|460=critical temperature, absolute, and V and W are parameters whose values are given by the equations, V=7.23+0.6G, and W=28,240-200G, where CaO MgO A12O3-i-'Si02 A considerable decrease in critical temperature of reduction of manganese can be effected by lowering the hearth pressure.` Hearth pressure may be reduced as low as, say, 1 lb./sq. in. by one or another, or by a combination of two o r more, of the following measures:

1. Employing a larger diametered furnace of standard height;

2. Employing a shorter furnace shaft, with standard diameter;

3. In a standard furnace, carrying the stock line lower than customary;

4. Blowing the furnace at a lower rate than cusinduced draft blower,` operating upon the" outlet gases. Illustrative of the effect of this measure vis the following; For the case 0f a hearth pressumption, in producing ferro-manganese, by increasing the fraction of the heat, supplied to the combustion zone, which is available for manganese reduction. For example, when one pound of coke-carbon is burned at the blast entrance by air (60% humidity, 60 F.) to form CO and N2 (plus 1.2% H2), there are generated 3,990 B. t. u. If the blast be preheated to 2,000 F., its sensible heat is 2,940 B. t. u., making a total (gross) value of 6,930 B. t. u., from which latter must be subtracted 117 B. t. u. for the endothermic reaction of blast moisture with carbon and 630 B. t. u. lost to the tuyre and bosh water-cooled devices and for conduction through the brick work, leaving a net combustion zone heat of 6,183 B. t. u. per pound of carbon burned by the O2 of the blast. If the reaction of Equation 1 is carried out at a reduction temperature of, say, 2,500 F. (6.9% Mn in the slag,-curve A of Fig. 1), the combustion zone gases cannot be cooled below 2,500 F. by'absorption of heat in carrying on the endothermic reduction reaction (Equation l); therefore, these gases carry away from the reduction zone the sensible heat which was required to heat them up to 2,500", i. e., 4,525 B. t. u. per pound of carbon burned to CO. In other words, 73% (4.525/6,183) of the net combustion Zone heat is unavailable for manganese reduction. This situation is made even worse when Tc-is raised to 2,7 F., in which latter event the unavailable heat is 4,925 B. t. u./lb. of carbon, or 79.7% of the net heat.

In order to make completely clear the relationship between the critical temperature Tc, the blast temperature and the coke consumption, I

have computed the thermal requirements for pro- Percent Mn (as metal) 35.5 Fe (as metal) 3.2 SiOz 21.5 A1203 5.3 MgO 2.5 CaO 0.8 BaO 0.7

This ore would, under conventiona1 practice, be considered non-merchantable because of its high concentration of the acidic gangue' oxides S102 and A1203.

A standard grade of ore operable under current practice for production of ferro-manganese has an analysis, for example, of: Mn-49.25%, Fe2O3-4.5%, Sim-8.0%, Al2O3-3.0%, CaO- 1.5%, MgO--1.25%, and BaO-0.85%. In smelt-f `ing this latter ore in accordance with conventional best practice, one would charge into a blast furnace 4,375 lbs. of the ore, 950 lbs. of limestone and 4,100 lbs. of coke, blow the furnace charge with air at 1,550 F., and produce 1 long ton (2,240 lbs.) of 80% ferro-manganese and 1,300 lbs. of a slag analyzing 8.0% manganese. With manganese at 33 per unit, coke at A$3.50

per ton, and limestone at $1.75 per ton, thesmelting cost would be $9.45, including $7.18

for coke, $0.74 for flux and $1.53 for manganese lost in the slag.

With the siliceous ore given hereinbefore, the slag volume (in the immediately preceding example) is increased to about 4,500 lbs. In order to hold the manganese slag loss to the 104 pounds of the above example, or to a lesser ligure, when employing the above siliceous ore, it is necessary so to operate the blast furnace that the slag contains not more than 2.3% Mn. Reference to Fig. 1, however, shows that the critical temperature of manganese reduction for a 2.3% manganese slag is 2,650 F. Reference to Fig. 2 shows that with 1,550 F. hot blast the coke consumption at Tc=2,650 F. is 7,000 pounds per ton of metal, which figure represents an increase in fuel cost of $5.25 per ton of metal. For a neutra slag, the siliceous ore requires 3,300 pounds of limestone per ton of metal, which figure represents an increase in flux cost of $1.86. VThe total increased materials cost called for by use of the siliceous ore under the furnace conditions just mentioned is $7.11.

But when the furnace is blown with air prehea-ted to 1,870 F. the coke Vconsumption is reduced to 4,100 pcunds,-a figure equal to the standard ore's competition in this respect,- and when the furnace is blown with air preheated to 2,300 F. the coke consumption (corresponding to Tc=2,640 F.) is reduced to 3,050 pounds. This latter reduction in coke consumption cancels the added expense of the flux, thereby showing that the metallurgica1 penalty on the siliceous ore can be obviated by raising the blast temperature from the conventional figure of 1,550 F. to 2,300 F.

It is to be noted that if, in the above example, the loss of manganese into the slag is maintained at, say, 4.3%, instead of at 2.3%, the temperature of the hot blast must be elevated to about 2,600 F. in order to make the smelting of this ore competitive with smelting a standard ore.

Another example of application of the concepts of the present invention to smelting of an ore containing 21% SiOz in competition with an ore containing only 8% SiOz is as follows, in which example cake is valued at $5.00 per ton, limestone at and ore at 26 per unit: When smelting an ore containing 8% SiOz, using a blast temperature of 1,550 F., 4,100 lbs. of coke and 950 lbs. of limestone are employed. The slag volume is 1,300, and the slag contains 8% Mn. When employing the siliceous ore (21% S102), I use a blast temperature of 2,000 F., a coke consumption of 3,600 pounds (corresponding to Tc=2,610 F.,-Fig. 2) and a flux consumption of 3,300 pounds. Under these conditions the slag volume is 4,500, and the content of manganese in the slag is 2.95%. The materials cost is substantially identical with that of the just mentioned standard practice applied to ore of 8% SOz.

Substantially the same end can be attained by 'reducing fuel consumption to 3,400 pounds and holding the blast temperature at 2,000 F. (thereby raising the content of Ymanganese in the slag to 4% and lowering Tc to 2,572" F.)

Reference to Figs. 1 and 2, and knowledge of prevailing unit costs of ore, coke and flux, will .enable the operator to adjust the operation of the blast furnace for smelting aV given siliceous ore under the economically most favorable conditions.

Iclaim:

1. Process of producing ferrofmang'anese containing at least 50% of manganese, in the blast furnace, from a manganese-bearing cre containing more than 18% silica, which comprises feeding the furnace with a charge consisting essentially of the ore, a basic fluxing agent and a solid carbonaceous fuel; and blowing the charge with a gas, containing oxygen and nitrogen, preheated to a temperature equal to the critical temperature of reduction of manganese, the blown gas containing a lesser proportion of oxygen than does atmospheric air.

2. In the process of producing, in the blast furnace, ferro-manganese of at least 50% grade from a manganese-bearing ore, the improvement which consists in blasting a furnace charge, consisting essentially of the ore, a basic uxing agent and a solid carbonaceous fuel, with a blast preheated to a temperature in excess of l,600 F., and simultaneously suicientlydecreasing the hydrostatic pressure of the CO within the resulting slag bath, by reducing the hearth pressure to a value of the order of 1 lb./sq. in. gauge, as

materially to lower the critical temperature of reduction of manganese.

the furnace, and the particle size and propor- Y tions of the charge ingredients, as to maintain the static pressure of gases within the furnace hearth at not to exceed 5 lbs/sq. in. above atmospheric pressure.

4. In the process of producing manganese alloys in the blast furnace, the improvementV which consists in maintaining the furnace hearth at a static pressure less than atmospheric pressure by accelerating the exhaust of furnace gases by means of an induced draft.

PRCY H. ROYSTER.

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