US1534244A - Process of producing low-sulphur pig iron from high-sulphur furnace burdens - Google Patents

Description

Patented Apr. 21, 1925.

UNITED STATES 1,534,244 PATENT OFFICE.

SAMUEL PEACOCK, 0F WHEELING, WEST VIRGINIA, ASSIGN'OR '10 ALEC J. GERRARD,

' OF CHICAGO, ILLINOIS.

PROCESS OF PRODUCING LOW-SULPHUR PIG IRON FROM HIGH-SULPHUR FURNACE BURDENS.

No Drawing.

To all whom it may concern:

Be it known that I, SAMUEL Pnecoox, a citizen of the United States, residing at \Vlieelin in the county of Ohio and State of West Tirginia, have invented certain new and useful Improvements in Processes of Producing Low-Sulphur Pig Iron from High-Sulphur Furnace Burdens; and I do hereby declare the following to be a full, clear, and exact description of the invention, such as will enable others skilled in the art to which it appertains to make and use the same.

This invention relates 'to a process of making a low sulphur pig iron from a high sulphur furnace burden, and has for its object to accomplish this result in'a manner more eflicient and less costly than has been heretofore proposed.

With this and other objects in view, the invention'consists in the novel combination of steps constituting the process all as will be more fully hereinafter disclosed and particularly pointed out in the claims.

In order that the invention may bemore clearly understood, it is said As is-well known, the modern iron-making blast furnace involves a counter-current heating effect, in which iron oxides are reduced by carbon monoxide gas and solid carbon, and the non-ferrous components of the ore, fluxes and ash of the fuel are combined as fusible slags. It is essential that both the metal iron and slags thus produced take on and hold a free running molten condition in order that they may be separated from each other in the hearth and be readily withdrawn from the furnace as mobile liquids. Pure iron melts at, say 2800 F.-, and for the necessary molten fluidity it must be heated to 3000 F., which latter is a temperature not directly obtainable by burning carbon with air. The formation of iron by the reduction of iron oxide with carbon monoxide is strongly exothermic, and when such reduction takes place with the ore and monoxide gas previously heated to 1600 F. to 1800 F., the heat liberated by the reaction is found to be sufficient to melt the metal, notwithstanding that at the instant of its formation it may exist as substantially pure metallic iron. The molten iron thus formed, drips through the fuel to the hearth, it contacts more or less with the fuel ash Application filed March 14, 1924. Serial No. 699,314.

and white hot coke present as well as with the hot combustion gases, and it is partly reconverted to iron oxide by a reversion with the"carbon monoxide present. It takes up carbon, silicon, etc., and produces as an ultimate end-product a crude iron carbon compound having a melting point about 2500 F. and a high liquid mobility at about 2600 F. This product, known commercially as cast iron, has a number of well known uses, but its most important industrial ap plication is in the manufacture of steel, for

which purpose it is treated almost exclusively to remove wholl or in part, the impurities unavoidably a ded to the substantially pure iron in the blast furnace.

Unfortunately, the steel making methods,-

such as the Bessemer or open hearth treatments, require high temperature reactions in which air is used to burn out the silicon,

carbon, etc. And in carrying out these methods, nitrogen, carbon monoxide, metalloid oxycarbides, iron oxides and oxycarbides, hydrogen, nitrogen dissociation products, etc., are unavoidably more orless introduced into the metal. It therefore results that for steels of standard characteristics,. the metal must be further treated, usually in an electric furnace, to remove the ill effects of the first refining step.

If it were possible to disregard the sulphur and phosphorus present .in'theore and coke, afurnace burden could be proportioned for a slag composition of low free running temperature. The iron, it is true, would have a high melting point, for it would be low in silicon and carbon, but still by using an excess of fuel and slag, the iron would be sufficiently impure to maintain a tapping mobility in the furnace crucible or hearth. But, unfortunately, the sulphur present in the charge negatives such an operation. The furnace burden sulphur goes into the furnace almost wholly as iron sulphide, and in the fuel.

The usual furnace practice for sulphur removal is to proportion the fluxes to make a highly basic, or what is known as a limey slag, on the assumption that a molten basic silicate in contact with iron sulphide will effect a change over to the acid condition, making free iron and calcium sulphide. But as calcium is electro positive to iron, such a reaction cannot range beyond a very small 4 high temperatures used in modern blast furdissolves in molten iron,

equilibrium balance, hence to remove a substantial quantity of sulphur a large excess of such limey slag must be used, thus entailing increased fuel costs and a lessened metal output per unit of plant investment. The temperature also must be forced to the highest point obtainable, to bring the limey slag to a Working fluidity.

In iron-making, it is well known that it is economically important to reduce the iron oxides in the upper part of the furnace, by means of the carbon monoxide gases made at the tuyere zone. Carbon monoxide reduces iron oxide freely at 1000 F. But, however, if a limey slag must be used in order to eliminate sulphur, the temperature at thetop of the bosh must reach or exceed 2600 F., and at this temperature thefree iron made above the bosh attacks carbon monoxide, producing iron oxide and free carbon, thus:

Fe-l-CO FeO-f-C.

That is, iron already reduced is reconverted in the furnace to iron oxide, and such iron oxide in the furnace hearth reacts with white hot coke to form iron carbide. It is at this point that the iron silicids are formed.

While it is true that these conditions are desirable in making certain grades of cast iron, yet for steel making it is obvious that energy is'being freely expended to no useful end.

Phosphorus is usually present in iron ores, fluxes, etc., as phosphates of iron, limwnd aluminum. These phosphates have a comparatively low melting point and when fused prior to contact with red hot carbon, they are reduced to phosphides. Iron phosphide but aluminum and calcium phosphides do not. These latter phosphides therefore react with iron oxides to form iron phosphides, but they do not react in contact with molten iron only. At the naces,.iron oxides are always present, from the top of the bosh to the tuyere zone, due to the high temperature reaction and reversion above mentioned; hence nearly all of the phosphorus in the furnace burden ultimates in the hearth as a phosphide which dissolves in the molten iron. The silicon and carbon forced into the iron is a somewhat simpler reaction, due to the conditions imposed on the sulphur elimination involving a very high hearth temperature, and avery hot slag.

This invention, on theother hand, avoids many of the foregoing objections by proceeding as follows: To the furnace burden there is added a quantity of sodium chloride NaGl of at least twice that required to chemically combine with all the sulphur and phosphorus present.

It thus results that even after a considerable proportion of said sodium chloride has been lost throughvolatilization, there will still remain a sufficient quantity to eliminate The slag, after the addition of the said sodium chloride, is found to be free running at a temperature at or below 2000 F., and the hearth temperature need not exceed 2400 F. Further the iron oxide of the charge is reduced to metallic iron in or above the bosh, and the temperature is too low to cause a reversal of the reducing reaction, or a reproduction of iron oxide as in the prior procedures above discussed. The sulphur and phosphorus present in the burden in contact with molten alkali chloride, being converted to sulphur and phosphorus chlorides, in accordance with the above equations and both being volatile even at the temperature at the furnace throat, they readily escape with the combustion gases. Further the very fluid slag produced carries the iron sponge formed down to the furnace hearth. Although the alkali employed suffers more or less volatilization in the process, as above stated, Fyet as its boiling point is well above 1800 a very considerable portion of the alkali vapor condenses on the relatively cold burden a ove the bosh and is returned to the process, ultimating as a silicate and forming part of the slag. The reduced metal will contain from 1.5% to 4.00% carbon, with or phoshigh grade of pig iron either free from silicon, phosphorus and sulphur, or containing only traces or very small quantities of the same, notwithstanding the fact that a furnace burden high in sulphur was employed.

What is claimed is 1. The process of making pig iron low in its sulphur content from an iron ore having a high content of sulphur which consists in adding to the usual furnace burden sodium chloride in a quantity of at least twice that required to chemically combine with all the sulphur and phosphorus present; smelting said ore in the presence of said chloride; and recovering the iron thus produced.

2. The process of making pig iron low in its sulphur content from an iron ore having a high content of. sul hur which consists in adding to the usualurnace burden .sodium chloride in a quantit of at least twice that required to chemical y combine with all the sulphur and phosphorus present; smelting said ore in the presence of said chloride at I, a temperature not substantially above 2400 pig iron from a high sulphur iron ore which consists in providing the usual blast furnace charge containing said ore to which has been added more than twice the quantity of sodium chloride than would be necessary to react with all the sulphur present; smelting said charge without permitting the hearth temperature to greatly exceed 2400 F.; and discharging the molten slag and iron produced in constant streams from said furnace. 20

SAMUEL PEACOCK.