A new look at pig iron.
While steel scrap remains the major source of iron units for ferrous foundries, the trend toward using pig iron as a charge supplement appears to be growing. This is due in large part to the increasing use of high-quality scrap by minimills as well as the buildup of residual elements in some steel scrap making it unsuitable in the production of high-quality iron and steel castings.
The history of iron making can be traced back to ancient Mesopotamia many centuries before Christ, but the Chinese were first to produce iron castings on what might be called a commercial basis. By creating a greater air draft and holding a mixture of iron ore and charcoal at high temperature longer, the Chinese produced a liquid iron suitable for pouring into sand molds.
For centuries, they led the field until the technology reached Europe in the 11th and 12th centuries. There, three centuries later, pig iron was introduced for remelting purposes, but the real revolution in iron making, using charcoal to reduce iron oxide, surfaced in the 18th century. The insatiable demand for charcoal decimated the great continental forests and seriously threatened the rapidly growing iron industry.
The successful and timely use of coke, instead of charcoal, was discovered by Abraham Darby in England and again revolutionized iron making technology. An improved blast furnace for the production of "direct furnace" castings and pig iron for remelting followed. From that point, iron castings became the common building block of the industrial revolution.
Although iron foundry charge makeup has evolved considerably to include steel scrap, the remelting of pig iron remains a current and essential practice in the production of iron castings. Long declining in foundry use because of cost and the ready availability of inexpensive common scrap, the technical and economic benefits of pig iron are resurfacing and playing a key role in the production of high-quality iron castings.
Pig Iron Manufacturing
Two basic types of pig iron are manufactured:
* primary pig irons produced from ore;
* secondary pig irons from remelting steel scrap in a hot blast cupola with a rich coke rate.
The variety of pig iron grades depends on the ore, reduction process, refining treatments and/or alloying practices. Classification of standard grades, based primarily on phosphorus content, is listed TABULAR DATA OMITTED TABULAR DATA OMITTED in Table 1. Although not indicated in the specification, carbon content in pig iron may vary between more than 4% to as low as 2.6%.
The Production Process
Reduction of iron oxides (hematite and/or magnetite) in a blast furnace and ilmenite smelting are the principal processes used to manufacture foundry grade pig iron.
The blast furnace is a counter-current exchanger of heat and mass in which descending iron oxide (|Fe.sub.2~|O.sub.3~ and/or |Fe.sub.3~|O.sub.4~), C (as coke or charcoal) and slag-making materials (such as dolomite) remove heat from an ascending stream of hot reducing gases, mainly nitrogen (|N.sub.2~), carbon monoxide (CO) and carbon dioxide (C|O.sub.2~).
A variety of chemical reactions takes place during the descent through the furnace shaft (an eight-hour journey). Because the gas lowest in the furnace is richer in CO, it has a stronger reducing effect on iron oxide. Thus, the ore pellets are gradually reduced as a result of mass transfer of the reducing gases (mainly CO) into the pellets. The liquid slag and metal collect in the furnace hearth, separate because of different densities, and are tapped.
The concentrations of residual elements such as manganese (Mn) and phosphorous (P) depend upon ore purity. After tapping, the hot metal is desulfurized with lime, calcium carbide and/or magnesium. Special treatments may be used to reduce concentrations of residuals before the metal is cast into ingots.
The purity level of blast furnace pig iron can vary, making some pig iron inadequate for certain demanding foundry applications, such as the production of ferritic ductile iron castings.
Smelting Ilmenite Ore
The ilmenite smelting process, developed by QIT-Fer et Titane, Inc. in the 1950s, uses a high purity ilmenite ore that is a mixture of iron and titanium oxide. The ore is crushed, upgraded and calcined to reduce its sulfur content. Mixed with dried coal, it is fed to electric arc furnaces in which temperatures typically exceed 3092F (1700C). Carbon, added as coal, reduces the iron oxide to high-purity metallic iron that sinks through the molten slag to the furnace bottom.
At timed intervals, the titanium oxide slag and high-purity iron are tapped from the furnace. The slag is cooled, crushed and used for the manufacturing of white pigment. The molten iron is desulfurized and deoxidized by reactive agents and, depending on the iron grade desired, undergoes further treatments, including recarburization, before casting into ingots.
The ilmenite smelting process allows the production of high-purity pig iron with a high degree of heat-to-heat chemical consistency, making it an ideal melting stock for ductile iron castings.
Pig Iron for Gray Iron
In the 1930s, a wide range of pig irons was available for the production of predictable quality castings. One or more of the pig irons was melted with cast iron scrap to produce castings. Improvements in melting technology facilitated the use of a higher percentage of steel scrap in charges, eventually becoming the main charge material for gray iron production.
Economics limit the use of pig iron as a charge component in the production of low-market-value gray iron castings. Computer modeling helps the foundry metallurgist determine the optimum charge of steel and pig iron required to produce a specific gray iron grade. Spectrographic and thermal analysis techniques permit close control of final melt chemistry.
Pig iron is often used as a gray iron charge material to dilute the harmful effects of lead, antimony, arsenic or excessive phosphorus--elements that may be present in the scrap.
A recent study by the International Pig Iron Secretariat (Dusseldorf, Germany) and the Centre Technique des Industries de la Fonderie (Sevres, France) showed that the use of pig iron in gray iron production is effective in minimizing casting defects, can reduce feeder volume and improves casting machinability.
Pig Iron for Ductile Iron
Unlike gray iron, the production of ductile iron requires a low sulfur-base iron with controlled levels of carbon and silicon. Through gray iron contains significant amounts of Mn and P, these elements must be closely monitored during the production of ductile iron.
Manganese promotes the formation of eutectic carbides and a pearlitic matrix structure that is undesirable for producing ferritic, as-cast ductile iron. Phosphorous, only slightly soluble in iron, combines with the iron to produce hard phosphides (|Fe.sub.3~P) that degrade the mechanical properties of ductile iron. Control of final microstructure and the base iron analysis is best achieved by melting a mix of good quality steel scrap and low Mn, P and S pig iron.
Table 2 lists elements detrimental to casting quality and shows how each influences the final matrix. The demand for better grades of cast iron to produce high-end castings has encouraged the development of pig iron with low levels of these adverse elements. To further support the importance of the furnace charge, a recent study|3~ states:
"The high degree of purity, the low percentages of pearlite and carbide stabilizing elements and low concentrations of trace elements promote a largely ferritic casting matrix that, in most cases, eliminates the need for annealing. Where annealing treatments were done, however, the result was an excellent level of casting toughness."
The price of melting stock is far from being the only cost associated with the production of ductile iron castings. A list of the principal processing variables that illustrate the cost-effectiveness of using some pig iron instead of a 100% steel scrap charge is given in Table 3.
TABULAR DATA OMITTED
Heat treatment is commonly used to improve the quality of poor as-cast ductile iron. Related costs include charges for castings transfer, equipment amortization, maintenance, fuel, labor, straightening, delivery delay and reheating or scraping to meet specifications. Overall heat treating costs illustrate that using pig iron, quality steel scrap and selected foundry returns is economically justifiable.
Pig Iron Advantages
A study by W.J. Duca|4~ concluded that certain low Mn, P and S pig irons require less energy to melt compared to other furnace charge materials such as ductile iron returns or steel scrap.
TABULAR DATA OMITTED
Foundry experience in Europe by W. Sommers (Austria), T. Skaland (Norway) and P. Platek (Poland) defined for the International Pig Iron Secretariat the benefits realized in using pig iron as a charge supplement.
According to Sommers, pig iron provides smooth, fast melting for maximum performance in medium-frequency induction furnaces. The least expensive melt combination (steel scrap, returns and SiO) required 768 kWh/ton of melting power while pig iron, returns and SiO dropped power consumption to 575 kWh/ton, an energy saving of 33%.
Skaland said the use of high-purity pig iron and low Mn (0.02%) scrap steel with low impurity levels allows the production of ferritic ductile iron with excellent as-cast mechanical properties, even when limiting silicon levels to 2.5%. This results in low sulfur and oxygen levels in castings (about 30 ppm) and eliminates the need for heat treating to get a desired microstructure.
Poland's P. Platek reported pig iron simplified the start-up of a new Japanese-designed foundry by stabilizing melt chemistry and yield. It results in fewer casting defects; simplified melting/molding conditions; eliminated the need for risers in some cases; reduced personnel for process control, storage and technical management; and increased furnace production by 10-15%.
The three were unanimous in noting that the addition of pig iron to a gray or ductile iron charge provided trace element analysis uniformity, reliable carbon recovery, lower energy consumption, high bulk density and low Mn content.
Steel scrap remains the major source of iron units for ferrous foundries but the trend toward using high-purity pig iron as a charge supplement is gaining strength.
As the availability of suitable scrap deteriorates, upgrading variable quality iron units is becoming a prime option for foundries as they strive to be competitive.
1. Standard Specification for Merchant Pig Iron, Annual Book of ASTM Standards, Vol. 01.02, ASTM, Philadelphia, PA (1985).
2. Iron Castings Handbook, 3rd Ed., Iron Castings Society (1981).
3. H. Haastert, "Special Type Pig Iron for Nodular-Graphite Cast Iron," Foundry Trade Journal, May 1969.
4. W. Duca, Heat Content of Cast Iron, AFS Transactions, Vol. 81, (1973).
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|Title Annotation:||upgrading of charge metals with pig iron by iron and steel foundries|
|Date:||Aug 1, 1993|
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