US3254987A - Method of operating an iron-refining basic converter and for refining iron into steel - Google Patents

Description

June 7, 1966 R EF METHOD OF OPERATING AN IRON-REFINING BASIC CONVERTER AND FOR REFINING IRON INTO STEEL Filed April 19, 1963 512156 EEUEE I ENT.

INVENTOR.

Q) WAG RUDOLF GRAEF has 825 N ouIuEzu o United States Patent hansen, Rhineland, Germany, a corporation of Ger- Filed Apr. 19, 1963, Ser. No. 274,116 Claims priority, application Germany, May 12, 1962,

8 Claims. (c1. 75 59 The present invention relates to a method of operating an iron-refining basic converter and for refining iron into steel.

While the Thomas-Gilchrist method of producing steel and refining high-phosphorous pig iron has not been fully exploited in the steel plants as a consequence of economic factors, it has long been desired to produce high-quality steels, such as those conventionally manufactured by Siemens-Martin or open-hearth methods, from such basic pig irons and/ or by employing converters including the general type widely known as a Bessemer converter. The latter is generally lined with an acid refractory (such as a silicious material) in contradistinction to the basic lining (of a magnesia limestone) characterizing the Thomas- Gilchrist method. Henceforth the term basic Bessemer converter shall be used interchangeably with the term Thomas converter.

To obtain high-quality steela Siemens-Martin furnace is conventionally employed. In the latter and in more recently developed processes, wherein gases enriched with oxygen are used, the refining gas stream is passed over the surface of the pig iron melt or directed thereagainst with the aid of water-cooled nozzles and the like. For significant degrees of refining, gases containing approxi mately 99.5% by volume, oxygen must be used.

In my copending application Ser. No. 274,288, entitled Method of Producing High-Quality Steel From Pig Iron, filed concurrently herewith, I disclose and claim a process wherein pig irons or so-called raw steels having substantially the following composition: 3.5-4.5% by weight carbon, 0.740% by weight manganese, 03-13% by weight silicon, 0.08% (and up to 1.5%) by weight phosphorus and from trace amounts to 0.06% sulphur, the balance being substantially entirely iron, are converted to steel. This copending application points out that a refined steel having minimal quantities of nitrogen, phosphorus and oxygen can surprisingly be produced in a basic converter when certain essential steps are followed, in spite of earlier beliefs within the art that an oxide or oxygen-free, high-quality steel could not be produced in a basic converter. Earlier eiforts along these lines are characterized by unfavorable heat balances and loss of thermal energy. Endeavors to improve the heat balance by enriching the gas stream with up to 50% oxygen also failed. Whereas the heat balance, although generally unfavorable, improved to some extent, other factors enter into the rejection of this technique as a practical means of producing high-quality steels (Steel and Iron, 1952, page 992). It should be noted that the use of oxygen jets directed at the surface of steel baths was capable of producing steels having the quality of Siemens-Martin or open-hearth steels only if the nitrogen content of the blast was sufficiently low (at most 1.5% in the gas stream) to attain steels having nitrogen contents between 0.005 and 0.008%. Oxygen contents in excess of 98.5% and preferably greater than 99.5% were required. Gases of this type were, of course, highly expensive and prior art attempts at producing high-quality steels from a pig iron of the foregoing composition necessitated a compromise between a Patented June 7, 1966 loss of thermal economy, the use of expensive refining gases and the proportion of nitrogen in the final steel.

In the aforementioned copending application, I have described and claimed a method of carrying out the refining of such pig iron into steel in a converter lined with a basic refractory. This technique enables the reduction in the oxygen content of the refining gas to one which is highly economical, permits the attainment of an optimum heat balance and results in a steel whose nitrogen, phosphorus and oxygen content is on the order of that of openhearth steels. The process essentially comprises the steps of initially reducing the silicon content of-the pig-iron melt (which silicon content can even approach that of conventional acid steels, i.e. about 1.5 by subjecting it to a preliminary refining process (prerefining) with an oxygenrich gas stream, whose oxygen content is nevertheless sufficiently low to ensure relatively low costs, and thereafter refining the pig-iron melt of reduced silicon content in a basic converter. The silicon content should, optimally, be reduced to a level below about 0.2% by treatment of the pig-iron melt with agas stream containing between substantially 40 and oxygen by volume. It has been found that so-called lances can effectively inject the oxygen below the surface of the bath to eifect the necessary reduction in silicon content. Advantageously, the prerefining process is 'halted when analysis indicates that the melt has a silicon content of or below 0.2%. The lances themselves can be of the water-cooled type to prevent damage thereto. An oxygen content of 70% is preferred for the preliminary step of prerefining.

The resulting raw steel, whose carbon content is about the same, but whose manganese content may be reduced by about half, whose phosphorus content is lowered approximately by one-fifth and whose sulphur content is substantially unchanged, is introduced into a converter lined with a basic refractory (e.g. magnesia-limestone) and refined by the passage of an oxygen-content greater than that normally present in the atmosphere (about 25%), but less than approximately 45% with the range of 30-40% preferred. The bottom-blown melt can be tapped when its nitrogen content falls below 0.005%, its phosphorus content falls below 0.026% and its sulphur content falls below 0.004%. The converter gas can, .ad vantageously, be preheated to eliminate any sudden cooling of the melt which can be introduced to the converter in a liquid state. The refining process is found to have an exceptionally high thermal efliciency so that frequently it is necessary to add a cooling medium (eg about 25-30 kg. of iron ore per ton of the melt or quantities of steel scrap having a corresponding heat capacity). When pre- 'heated oxygen-enriched air blasts are used, the quantities of cooling medium will be increased according. It is possible to divide'the final-refining stage into at least two steps, with the raw steel being refined with the customary amount of oxygen in a gas stream during an initial stage (approximately 6'minutes) in the course of which the bath is cooled by additions of ore or scrap and is at a relatively low temperature somewhat above the melting point of the raw steel. Subsequently, the oxygen content of the refining gas can be raised to a maximum of about 45% and preferably ranges between 34-38%, this refining gas being supplied until only several seconds prior to the end of the refining process.

In another copending application Ser. No. 274,150 filed concurrently herewith and entitled Method of Refining an Iron Melt, I point out that the basic pig irons conventionally used in the Thomas process can be refined more advantageously by the addition thereto of a metallurgical slag advantageously derived from the steel making-process previously described. This slag, whose composition is reproducible although to a large extent indeterminate, can be added to the basic pig iron in a Thomas converter and has been found to be a marked improvement over earlier metallurgical slags used to improve the quality of pig-iron melts. When this slag or, to a lesser extent, slag derived from the open-hearth refining of acid steels is added to the melt, for example at the transition point between termination of decarbonization and commence? ment of the dephosphorization, the basic pig iron can be refined to a steel having the low-nitrogen content (e.g. less than 0.004%) previously characterizing only steels made by the more expensive open-hearth or electric-furnace techniques. The use of such slags eliminates the need for any special dephosphorization media and is compatible with conventional denitrogenation methods for the further lowering of the nitrogen con-tent. As previously mentioned, cooling means such as iron or scrap and even additional lime can be introduced into the converter. The cooling medium may be added during the initial or decarbonization stage with great advantage.

It is an object of the present invention to extend the principles advanced in these copending applications to the economical utilization of a steel plant and a basic converter.

Another object ofthis invention is to proivde a method of operating an iron-refining plant with a relatively high capacity and enabling them to produce high-grade steels.

These objects are attained, in accordance with the present invention, through the discovery that raw-steel melts having basic or even somewhat acid properties, as distinct from Thomas pig irons can be advantageously refined in basic converters used hitherto only for the refining of basic pig irons of the latter type and that the identical basic converter can be used for both types of refining with consequent savings in apparatus costs, transportation economies and high output.

In general, it may be noted that the usual steel plant required, for the refining of basic pig irons and raw steels of the aforedescrib'ed type, two independent refining aggregates. conventionally, a Siemens-Martin furnace was necessary for the treatment of the raw steel. In place of a Siemens-Martin furnace a surface-refining apparatus wherein the bath is subjected to an oxygen blast along its surface could also be employed. In addition to this, the conventional Thomas-refining apparatus was required to handle basic pigs. Moreover, even steel-making plants having surface-refining furnaces cannot elfectively refine all types of irons since pigs having high phosphorus contents (between substantially 1.5% and about 2%) cannot be treated in such furnaces unless extensive precautions are taken to insure the addition of large amounts of lime (e.g. in the form of fine particles blown on to the bath together with the oxygen stream). Slag disposal is also a problem in these conventional plants.

According to the present invention, a converter lined with a basic refractory serves initially to refine a first melt of raw steel or low-phosphorus (open-hearth) pig iron by bottom-blowing a gas stream containing up to 45% oxygen and advantageously between and 40% oxygen through the melt. The resulting liquid steel is then removed with at least some of the slag concurrently formed in the converter permitted to remain for subsequent combination with the high-phosphorus basic-pig-iron charge to be added thereafter. I have found that best results are achieved when the slag has a highly viscous consistency and is relatively doughy or even, to a certain extent, semisolid. The lime content of this slag, in combination with any additional lime that may be added prior to or together with the basic pig iron, is at least sufiicient to refine the later to the desired degree and, advantageously, completely to steel.

To ensure that the slag is in a doughy or semisolid state, several techniques may be used according to the teachings of the present method; thus it is possible to introduce an eXcess of lime into the first or raw-steel melt so that the resulting slag is rich in lime, thereby imparting to it a flakey or doughy consistency. With a raw-steel melt of the aforedescribed composition, which requires for its refining to the finished steel about kg. of lime per ton of the melt, it is possible to add 80 to 100 kg./ ton to the melt to provide the desired excess. It will be clear, therefore, that the bottom-blowing of the melt in the converter will-result only in a partial liquification of the slag with the remainder being in subsequently semisolid form. Upon discharge of the raw steel by, say, tilting the converter, the nonliquid or viscous fraction of the slag remains for admixture with the basic-pig-iron melt. No special precautions are required to ensure the retention of the slag in the converter.

It is also possible, in conjunction with the addition of excess lime or in place thereof, to continue the passage of a gas stream (e.g. with a reduced pressure of about 0.5 atmosphere gauge) through the converter after emptying of the steel melt to maintain the residual slag in a relatively loose and mixable state and, at least partly, to foam the slag. When the additional lime is then fed into the converter, it is thoroughly admixed with the slag and again guarantees a doughy state. The basic or Thomas pig iron can then be poured into the converter from the usual ladle.

Another method of increasing the viscosity of the slag involves the introduction into the low-phosphorus pig iron melt, after it has been substantially completely refined, of a carbonaceous material which, preferably, also contains a significant proportion of iron. To this end, briquettes, granules, or the like of sintered carbon and iron, or even crude iron having a relatively high carbon content can be used effectively. The carbon released into the melt by the additive serves to reduce the quantity of iron oxides therein which otherwise increase the fluidity of the slag. The addition of carbon at this point also affords another advantage in that overblowing of the melt by the oxygen-containing refining gas or the excessive heating of the melt by the passage of the gas therethrough, both of which occurrences are common, normally results in an increase in the amount of oxygen retained by the steel. Conventionally, such steels required expensive deoxidation eiforts which are eliminated by the addition of carbon, which combines with the oxygen to form carmove slag from the raw-steel upon the refining thereof thus resulting in a further saving of about 3 minutes per smelting cycle. As a consequence of the high temperature of the slag remaining in the converter after pouring of the refined steel therefrom, the reactions of the basic pig-iron melt proceed with greater rapidity and the blow time can be about one minute less for a Thomas melt, according to the present invention, as compared with the blow time in the conventional basic process. Furthermore, the higher quantities of available heat can be used to reduce an ore additive and to melt scrap to be introduced into the converter.

The present method also affords considerable savings in material used for the refining of steel. For example, it may be noted that the high degree of lime utilization is such that approximately 25% less time is required for refining the two melts. If approximately 180 kg. of lime per ton of iron or steel is necessary for refining the lowphosphorus and the high-phophorus basic-pig-iron melt in separate aggregates, the use of a single converter as now proposed Will permit the use of about to kg. of lime per ton. Additionally, the iron contained in the slag byproduct of the first refining stage is, at lea-st in part, recovered by using this slag directly as an additive for the high-phosphorus basic-pig-iron melt. Even the iron oxide normally present in the slag is conserved since it is converted into iron by reduction. The reducing agents include the usual impurities (i.e. silicon, carbon, phosphorus, etc.) present in the Thomas melt and are themselves removed upon reaction with the slag.

The slag resulting from the refining of the high-phosphorus basic-pig-iron melt contains a substantial proportion of the phosphorus previously retained in the rawsteel slag produced earlier; this phosphorus is converted by conventional techniques into phosphates for fertilizing purposes and the like. The silicic acid of the raw-steel slag is also found in the Thomas slag and increases the acidity of the latter to ensure a higher citric-acid solubility.

From low-phosphorus pig iron melts of the foregoing composition, it is possible to produce, by the steps previously indicated, high-purity refined steels having from trace amounts to a maximum of 0.02% phosphorus and sulphur. The nitrogen content of the finished steel can be held below 0.003% when, prior to the refining of the low-phosphorus pig iron in the basic converter, the melt is subjected to the prerefining or preliminary steps mentioned above and constituting the subject matter of my application of even date entitled Method of Producing High-Quality Steel From Pig Iron (Ser. No. 274,288).

The above and other objects, features and advantages of this invention will become more readily apparent from the following description and example, reference being made to the appended drawing, the sole figure of which is a diagrammatic illustration of a steel-making plant employing a basic converter.

Essentially the plant comprises a converter whose steel'casing 11 is lined with a basic refractory material 12 of magnesia-limestone. The bottom of this converter is provided with inlet apertures 13 through which air or an oxygen-enriched gas stream can be introduced from a conduit 14 having a preheating device 15 provided therealong. The converter 10 is carried by the usual trunnions 16, only one of which is shown, for tilting movement adapted to discharge the contents of the converter. The steel plant also includes a pan or heated hearth in which a raw-steel or low-phosphorus pig-iron melt 21 can be refined preliminarily by the injection of an oxygenrich gas stream below the level of the melt by means of lances 22. While only one lance is shown, it should be noted that a plurality of them may be provided as required. Each lance has a central tube 23 through which the gas stream passes to the nozzle 24 and is surrounded by a water jacket 25 having inlet and outlet connections 26, 27. While this pan is shown to be disposed above the converter 10 for convenience in illustrating the present method, it should be noted that it is also possible to pro vide the usual ladles and. the like for conveying the rawsteel melt to the converter.

A basic pig iron having substantially the composition described in my copending application entitled Method of Refining an Iron Melt (Ser. No. 274,150) of even date can be introduced into the converter from a ladle 30 which is supported by the usual crane or cradle tongs 31 not shown in detail. Another ladle 32 serves to carry the refined steel away from the converter. melt is treated in pan 20 with a gas stream containing between substantially 40 and 80% oxygen until its silicon content falls below 0.2%. The plug 33 is then removed to tap the pan 20 and drain the raw steel melt into the converter 10. A gas stream containing between 25 and oxygen, preheated if desired, is then bottomb'lown through the melt to purify the steel and produce a slag 34. To insure that at least part of the slag will have a doughy consistency, an excess of lime is thrown into the melt during the refining process. Upon conclusion of the refining step, the converter can be tapped via an outlet 35 or, preferably, tilted clockwise to empty the steel into the ladle 32. The slag 34 remains in the converter and falls to the bottom thereof. A gas stream is flown into the converter through the apertures 13 during return of the converter to its upright position to A raw-steel agitate the slag, thoroughly mix it and to foam it to a greater or lesser extent. The gas stream also prevents blocking of the air-inlet apertures by'the slag. Additional lime is then put into the converter and thoroughly mixed with the slag by the gas stream passing therethrough, whereupon ladle 30 introduces a basic-pig-iron melt into the converter for subsequently refining therein in contact with the remaining slag. The refined Thomas melt can be discharged into another ladle 32 while the Thomas slag, rich in silicic acid and phosphorus, is emptied into a slag pot 40 which can be kept heated in a chamber 41 if desired. Tongs 42 carry the slag pot away from the converter.

Example- A raw-steel pig iron having somewhat acidic characteristics and substantially the following composition:

3.5% by weight carbon, 0.5% by weight silicon, 1.75% by weight manganese, 0.23% by weight phosphorus, 0.04% by weight sulphur,

the balance being substantially entirely iron, is treated in the pan 20 via water-cooled injection lances with a gas stream containing 70% oxygen. When the silicon content of this melt falls to 0.2%, the pan is tapped and the liquid raw steel introduced into the converter.

The raw steel fed into the converter has substantially the following composition:

3.5% by weight carbon, 0.2% by weight silicon, 0.9% by weight manganese, 0.19% by weight phosphorus, 0.04% by weight sulphur.

The converter 10 has a capacity of about 25 tons;

a gas stream with progressively increasing oxygen content 0.07% by weight carbon, 0.43% by weight manganese, 0.016% by Weight phosphorus, 0.018% by weight sulphur, and 0.004% by weight nitrogen,

the balance being substantially entirely iron. Approximately 200 kg. of doughy slag per ton of steel is retained within the converter.

As the converter is returned to its upright position, an air blast of 0.5 atmosphere gauge continues to flow through apertures 13 to agitate the slag and thoroughly admix it with the 20-50 kg. per ton of additional lime now added thereto. The excess lime previously introduced into the melt and the additional lime together comprise the total lime necessary to refine a Thomas pig iron which is now introduced into the converter from the ladle 30. This pig iron has the following composition:

3.5-4% by weight carbon, 03-06% by weight silicon, 0.81.3% by weight manganese, 1.52% by weight phosphorus, 0.40.8% by weight sulphur,

the balance being substantially entirely iron. Using a blast gas somewhat enriched in oxygen (between 25 and 30%), the melt is decarbonized for, say, 6-10 minutes. Prior to the dephosphorization stage, additional cooling medium in the form of scrap steel is added. The

blast is continued for another 3 minutes and is followed by a one-minute afterblow. The resulting basic steel has a slightly higher carbon content than that of the earlier or raw-steel melt produced in the same converter although its nitrogen, phosphorus and silicon contents are low.

I claim:

1. A method of operating a bottom-blow iron-refining basic converter, comprising the steps of:

(a) passing a gas stream containing between substantially 25% and 45% by volume oxygen through a first melt of relatively low-phosphorus pig iron in said converter in the presence of a slag former to produce a slag and liquid steel;

(b) draining said liquid steel from said converter while leaving at least part of said slag therein;

(c) adding a relatively high-phosphorus basic pig iron consisting essentially of 3.5 to 4 Weight percent carbon, 0.3 to 0.6 weight percent silicon, 0.8 to 1.3 weight percent manganese, 1.5 to 2 weight percent phosphorus and 0.4 to 0.8 weight percent sulphur, the balance being substantially entirely iron, to said converter;

(d) passing an oxygen-containing gas stream through said high-phosphorus basic pig iron to effect a reaction of said slag therewith, thereby refining said high-phosphorus basic pig iron;

(e) adding a vfresh melt of said low-phosphorus pig ironto said converter; and

(f) repeating steps (a) through (e) to alternately refine said low-phosphorus and said high-phosphorus pig irons.

2. A method of operating a bottom-blow iron-refining basic converter, comprising the steps of passing a gas stream containing between substantially 25 and 45 by volume oxygen through a first melt of a relatively lowphosphorus pig iron consisting essentially of 3.5 to 4.5 Weight percent carbon, 0.7 to 4.0 weight percent manganese, 0.3 to 1.3 weight percent silicon, 0.08 to a maximum of 1.5 weight percent phosphorus and from trace amounts to 0.06we'ight percent sulfur, the balance being substantially entirely iron in said converter in the presence of an excess of lime above that necessary to refine said low-phosphorus pig iron to produce a lime-rich slag and liquid steel; draining said liquid steel from said converter while leaving at least part of said slag therein; adding a relatively high-phosphorus basic pig iron consisting essentially of 3.5 to 4 weight percent carbon, 0.3 to 0.6 weight percent silicon, 0.8 to 1.3 weight percent manganese, 1.5 to 2 weight percent phosphorus and 0.4 to 0.8 weight percent sulphur, the balance being substantially entirely iron, to said converter, and passing an oxygen-containing gas stream through said high-phosphorus basic pig iron.

3. A method of operating a bottom-blow iron-refining basic converter, comprising the steps of passing a gas stream containing between substantially 25 and 45% by volume oxygen through a first melt of relatively lowphosphorus pig iron in said converter in the presence of an excess of lime above that necessary to refine said lowphosphorus pig iron to produce a lime-rich slag and liquid steel; draining said liquid steel from said converter while leaving at least part of said slag therein in at least a highly viscous state; adding to said converter a relatively high-phosphorus basic pig iron consisting essentially of 3.5 to 4 weight percent carbon, 0.3 to 0.6 weight percent silicon, 0.8 to 1.3 weight percent manganese, 1.5 to 2 weight percent phosphorus and 0.4 to 0.8 weight percent sulphur, the balance being substantially entirely iron, and sufi'icient lime to refine said high-phosphorus basic pig iron to steel in combination with the lime of said slag; and passing an oxygen-containing gas stream through said high-phosphorus basic pig iron to effect a reaction of said slag therewith, thereby refining said high-phosphorus basic pig iron.

4. A method as defined in claim 3 wherein said excess of lime is between substantially 60 and 100% of the lime requirement for refining said low-phosphorus pig iron based upon the weight of the low-phosphorus charge in said converter.

5. A method as defined in claim 3, further comprising the steps of passing an oxygen-containing gas stream through said slag in said converter subsequent to the removal of the liquid steel therefrom and prior to the addition of said high-phosphorus basic pig iron; and adding the further lime required to refine said high-phosphorus basic pig iron to said converter during passage of the gas stream through said slag to mix the lime so added with the slag.

6. A method as defined in claim 5 wherein the gas stream passed through said slag upon removal of said liquid steel has an entrance pressure of about 0.5 atmosphere gauge.

7. A method as defined in claim 3, further comprising' the step of adding a carbonaceous material in the form of briquettes containing a substantial proportion of iron to said first melt after substantially complete refining of said low-phosphorus pig iron.

8. A method as defined in claim 3, further comprising the step of reducing the silicon content of said =low-phosphorus pig iron below about 0.2% by subjecting the loW- phosphorus pig-iron melt to treatment with a gas stream containing between substantially 40 and oxygen, prior to its introduction into said converter.

References Cited by the Examiner UNITED STATES PATENTS 1,826,497 10/1931 Bicheroux 7552 2,668,759 2/ 1954 Tenenbaum 7552 2,671,018 3/1954 Graef 7552 2,781,256 2/1957 Richards 7552 2,804,3 8/1957 G-raef 75-52 3,004,847 10/1961 Lambert et al. 7552 3,076,703 2/1963 Metz 7552 FOREIGN PATENTS 504,119 4/ 1939 Great Britain.

521,701 5/1940 Great Britain.

718,001 11/1954 Great Britain.

DAVID L. RECK, Primary Examiner.

BENJAMIN HENKIN, Examiner.

H. W. TARRING, Assistant Examiner.

Claims (1)

1. A METHOD OF OPERATING A BOTTOM-BLOW IRON-REFINING BASIC CONVERTER, COMPRISING THE STEPS OF: (A) PASSING A GAS STREAM CONTAINING BETWEEN SUBSTANTIALLY 25% AND 45% BY VOLUME OXYGEN THROUGH A FIRST MELT OF RELATIVELY LOW-PHOSPHORUS PIG IRON IN SAID CONVERTER IN THE PRESENCE OF A SLAG FORMER TO PRODUCE A SLAG AND LIQUID STEEL; (B) DRAINING SAID LIQUID STEEL FROM SAID CONVERTER WHILE LEAVING AT LEAST PART OF SLAG THEREIN; (C) ADDING A RELATIVELY HIGH-PHOSPHORUS BASIC PIG IRON CONSISTING ESSENTIALLY OF 3.5 TO 4 WEIGHT PERCENT CARBON, 0.3 TO 0.6 WEIGHT PERCENT SILICON, 0.8 TO 1.3 WEIGHT PERCENT MANGANESE, 1.5 TO 2 WEIGHT PERCENT PHOSPHORUS AND 0.4 TO 0.8 WEIGHT PERCENT SULPHUR THE BALANCE BEING SUBSTANTIALLY ENTIRELY IRON, TO SAID CONVERTER; (D) PASSING AN OXYGEN-CONTAINING GAS STREAM THROUGH SAID HIGH-PHOSPHORUS BASIC PIG IRON TO EFFECT A REACTION OF SAID SLAG THEREWITH, THEREBY REFINING SAID HIGH-PHOSPHORUS BASIC PIG IRON; (E) ADDING A FRESH MELT OF SAID LOW-PHOSPHORUS PIG IRON TO SAID CONVERTER; AND (F) REPEATING STEPS (A) THROUGH (E) TO ALTERNATELY REFINE SAID LOW-PHORPHORUS AND SAID HIGH-PHOSPHORUS PIG IRONS.

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