r V # S » *^ r O ,v u ^ ^ t> < ^M^>^^ ** M ,$ ^ '•« s * A <, *o,** ,g v ^o 15 K^ ^ ? .»i^L'* ^ <"" ° '' ^ oY %/ ■ •* .G^ V> *'V;s 4 A <-, -ow *bV" 4 o %: /K Ifff!- J\ ijgg) }\ l^.-/\-. _, o ./\v^/^ /,^%A >*\.^i:/V l V . o » « A^*V ^5 ^ -, ^^ \~ V . r, **"S. A > * • • "' ^ f :Mk\ \f -i <, *^v g* *o '*.»* O <* * o o, * "of ip-n^ ^ S X ^ X °o r »' 6 V ^ -^ -^ °o A jP^ c5°^ a* 5 *" t ,*Z' *■> v v ,»?-^L% "<^ • A V -^ ^.^ * A V -^ : ^°^ ^0^ *» < ^0^ /Jfe*-. S.J' :Mfc. \^ .•»'•. \/ ;^ . . » • «G tt A Vy ^ - •• **x .6- v : — : x^" ^'• : 5W f V V- ** t^a-X^*^ ^A J£J 8913 Bureau of Mines Information Circular/1983 A/ ^ 2^1983 Dolomite Refractories, and Their Potential as Substitutes for Imported Chromite By Timothy A. Clancy UNITED STATES DEPARTMENT OF THE INTERIOR Information Circular 8913 Dolomite Refractories, and Their Potential as Substitutes for Imported Chromite By Timothy A. Clancy UNITED STATES DEPARTMENT OF THE INTERIOR James G. Watt, Secretary BUREAU OF MINES Robert C. Horton, Director Research at the Tuscaloosa Research Center is carried out under a cooperative agreement between the U.S. Department of the Interior, Bureau of Mines, and the University of Alabama. This publication has been cataloged as follows: Clancy, T. A. (Timothy A.) Dolomite refractories, and their potential as substitutes for im- ported chromite. (Bureau of Mines information circular ; 8913) Bibliography: p. 17-18. Supt. of Docs, no.: I 28.27:8913. 1. Dolomite. 2. Refractory materials. 3. Chromium ores. I. Ti- tle. II. Series: Information circular (United States. Bureau of Mines) ; 8913. TN295.U4 [TN957] 622s [666'. 72] 82-600333 CONTENTS Page Abstract 1 Introduction 1 Dolomite ore 2 Origin and mineralogy • 2 Refractory grade dolomite. 2 U.S. dolomite deposits. 4 Dolomite refractories 4 Aggregate processing 4 Brick processing 7 Brick usage 8 United Kingdom 8 Europe 8 Japan 8 United States 9 Properties of 14 U.S. dolomites 10 Materials and test procedures 10 Results and discussion . . 11 Summary 15 References 17 ILLUSTRATIONS 1. Photomicrograph of Ohio dolomite No. 1 presently used to produce refractory products 13 2. Photomicrograph of Missouri dolomite presently used to produce refractory products 13 3. Photomicrograph of Alabama dolomite No. 3 14 4. Photomicrograph of Pennsylvania dolomite No. 3 14 5. DTA curves for six dolomites.... 16 6 . DTA curves for five dolomites 16 7 . DTA curves for three dolomites 16 8. TGA curve for a sample of Pennsylvania dolomite No. 2 16 9. TGA curve for a sample of Michigan dolomite No. 1 17 TABLES 1 . Mineralogical properties of dolomite 2 2. Classification of granular refractory dolomite 3 3. Composition and properties of refractory grade dolomites..... 3 4 . Typical properties of dead-burned dolomite grains 7 5. Distribution of steel production by process 9 6. Approximate distribution of BOF brick usage in the United States 9 7. Dolomite brick properties 10 8. Properties of raw domestic dolomites 11 9. Petrographic analysis data for raw domestic dolomites 12 LIST OF UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT Btu/f t3 British thermal unit per cubic foot pm mm micrometer millimeter ° C degree Celsius pet percent g/cm 3 gram per cubic centimeter pct/min percent per minute hr hour psi pound per square inch lb pound wt-pct weight-percent tain minute DOLOMITE REFRACTORIES, AND THEIR POTENTIAL AS SUBSTITUTES FOR IMPORTED CHROMITE By Timothy A, Clancy 1 ABSTRACT To help reduce the Nation's dependence on imported chromite, the Bu- reau of Mines is conducting research on the use of dolomites as an alternate material. Dolomite is a plentiful domestic resource and of- fers certain advantages as a refractory raw material. A review of the literature has indicated that there are many sources of high-purity dolomite in this country and that European nations use a greater pro- portion of dolomite refractories, primarily in steelmaking, than the United States. The Bureau of Mines characterized 14 domestic dolomites as to chemistry, density, mineralogy, microstructure, and thermal behavior, to develop baseline data on their suitability as refractory raw materials. INTRODUCTION To help ensure a dependable domestic supply of essential minerals, the Bureau of Mines initiated an evaluation of domestic dolomites as a refractory raw material. Increased use of domestic dolomite as a re- fractory material would lessen the Nation's dependence on imported chromite and high energy consuming materials, such as seawater peri- clase. Historically, (10) 2 the United Kingdom, West Germany, Austria, and Japan have developed greater use of dolomite refractories than the United States particularly in secondary refining processes for steel- making. In 1979, Western Europe used 28.6 lb of dolomite refractories per ton of steel produced versus 14.8 lb of dolomite refractories for the United States. This paper reviews the properties and uses of dolomite refractories. Some preliminary data on the chemical and physical properties for 14 different raw domestic dolomite ores are included. These data will be used in future studies for comparison with the refractory properties of calcined grain produced from these ores. - ■ — 'Supervisory ceramic engineer, Tuscaloosa Research Center, Bureau of Mines, Univer- sity, AL. •^Underlined numbers in parentheses refer to items in the list of references at the end of this report. DOLOMITE ORE ORIGIN AND MINERALOGY Dolomite (2) (CaC0 3 •MgC0 3 ) , identified by Dolomieu in 1791, occurs as sedimen- tary deposits similar in nature to lime- stone. Geologically some dolomites are precipitated directly from seawater but most dolomites are a result of the alter- ation of calcium carbonate sediments or rocks by hypersaline brines. Good exam- ples are the almost-pure dolomite Silu- rian reefs in northern Illinois, Indiana, and Ohio, and in southern Michigan. Oth- er carbonate minerals are found associ- ated with dolomite, but usually not in great quantities. Because of their similar physical prop- erties, it is not easy to distinguish one carbonate mineral from another. The rate of solubility of the different minerals in dilute hydrochloric acid is the best technique to identify them in the field. Calcite is much more soluble in dilute acid than dolomite, so if a fresh rock surface is etched, the amount of dolomite left in relief can be estimated visually. X-ray diffraction is commonly used in the laboratory for determining carbonate min- eralogy of bulk samples. Thin section petrographic analysis may be helpful, al- though it is difficult to distinguish carbonates in thin section unless stain- ing techniques are used. Impurities in dolomites vary consider- ably, but are economically important only if they affect the end uses of the rock. Impurities are tolerable for some uses if disseminated uniformly throughout the rock. Probably the most common impur- ity in dolomites is clay. The clay min- erals, mainly kaolinite, illite, chlo- rite, smectite, and mixed lattice types, may be either evenly distributed or con- centrated in laminae or thin partings. Chert, another common impurity, may be disseminated, or concentrated in nodules, lenses, or beds. It is composed mainly of very fine quartz (Si02) that easily incorporates impurities into its struc- ture so it may be found in almost all colors. Silica is also found in dolomites as discrete silt or sand-size grains of quartz. Dolomite and other carbonates are nor- mally classified as to composition. High-calcium limestone is more than 95 pet CaC03 , high-purity carbonate rock is more than 95 pet combined CaC03 and MgC03 , and high-magnesium dolomite is more than 43 pet MgC03 (theoretically pure dolomite is 45.7 pet MgC03). The mineralogical properties of dolomite are given in table 1. TABLE 1. - Mineralogical properties of dolomite Crystal system Hexagonal. Moh's hardness 3.5 to 4.0. Specific gravity.... 2.87 Color White or pink. Refractive indices: e 1.500 a) 1.679 Birefringence, 6.... 0.179 Solubility Slightly soluble in cold dilute HC1 . REFRACTORY GRADE DOLOMITE The American Society for Testing and Materials (1_) classifies dolomite refrac- tory raw materials as (1) raw refractory dolomite, (2) calcined refractory dolo- mite, and (3) dead-burned refractory do- lomite. This classification is based primarily on MgO content, loss on igni- tion, and impurity contents. Table 2 lists the requirements for each of these classes of refractory dolomite. TABLE 2. - Classification of granular refractory dolomite (ASTM C468-70) , weight-percent Classes MgO content , minimum Loss on ignition, maximum Impurities, maximum SiO- A1 2 3 + Ti0 2 Fe 2 3 range Sulfur 16 33 32 Raw refractory dolomite, "as received" basis Calcined refractory dolomite, ignition-free basis Dead-burned refractory dolomite (rotary kiln-fired), ignition-free basis NAp Not applicable. The use of dolomite as a refractory ap- pears to have started in about 1878 when S. G. Thomas experimented with tar-bonded dolomite linings in a Bessemer converter. Much of present-day dolomite refractory technology was developed in England dur- ing World War II. Since then, England has made great use of dolomite raw mate- rials for refractories. Chesters (4), in a chapter devoted to dolomite, presents NAp 2.0 2.0 1.75 2.00 2.25 1.50 2.50 2.50 NAp NAp 4-10 0.08 .16 NAp the compositions and properties of raw refractory grade dolomite. Some of the data are included in table 3. The compo- sitions are similar to those for dolo- mites described by others (_5, 26-27) . However, besides compositional require- ments the physical properties, such as grain density, refractoriness, strength, and microstructure, of a refractory dolo- mite material are important. TABLE 3. - Composition and properties of refractory grade dolomites Chemica 1 analy sis, wt -pet Physical properties Origin and formation Loss on Specific Bulk Poros- Si0 2 1 R 2 3 CaO MgO ignition gravity density, g/cm 3 ity, pet GREAT BRITAIN Dolomite (theoretical) NAp NAp 30.41 21.87 47.72 NAp NAp NAp Lower Permian systems: 0.33 0.52 30.63 21.50 47.37 2.84 2.47 13.0 .74 .72 30.25 21.28 47.00 2.84 2.41 15.1 .87 .60 30.32 21.23 47.13 2.85 2.39 16.3 Durham: Permian .89 .96 30.6 20.6 46.95 2.85 2.53 11.2 South Wales: Carbon- iferous limestone 4 ... 1.28 .81 32.48 19.41 45.15 2.82 2.77 1.8 UNITED STATES Ohio: Niagara system. .40 .80 30.1 21.0 47.20 2.87 2.66 7.9 .02 .17 30.6 21.2 47.50 2.87 2.55 12.5 Pennsylvania: Ledger .30 .65 30.8 21.1 47.10 2.84 2.78 2.1 Missouri: Bonne Terre .31 3.73 31.16 19.2 45.44 2.84 2.68 6.0 NAp Not applicable. 1 R 2 3 = A1 2 3 + Fe 2 3 . 2 Soft. ^Medium to soft. 4 Very hard. The quantity of dolomite produced for refractory uses is not a large portion of the total U.S. dolomite production. More than three-fourths of the dolomite quar- ried in the United States is used as an aggregate or a soil conditioner. In 1980 ( 19) , the production of refractory dolo- mite amounted to a total of 494,000 tons which was only 2.6 pet of the total lime and dolomite volume. The only States mentioned by Colby (_5) as producing re- fractory grade dolomite were Alabama, California, Colorado, Illinois, Michigan, Nevada, Ohio, Pennsylvania, Utah, and West Virginia. Ohio produces more dolomite than any other State and, in fact, produced ap- proximately 55 pet of the dead-burned dolomite consumed in the United States in 1979 ( 9_) . While most of the major re- fractory companies in the United States produce their own raw materials for fire clay, high-alumina, and magnesia prod- ucts, they have not developed the facil- ities to handle refractory dolomite. Do- lomite materials are produced by five or six of the smaller companies. Unpublished information by a leading dolomite producer (10) indicates that the use of dolomite as a refractory was not popular in the United States until re- cently. Since about 1912, dolomite in the United States has only been produced with iron added to make fettling grain. Although used extensively as a refractory in Europe from the time of the Bessemer converter (1860's), high-purity dolomite was essentially unavailable in the United States as a refractory raw material until the early 1960's when both Basic, Incor- porated, and the J. E. Baker Co. began to produce a high-purity, high-density dolo- mitic grain. In Europe, on the other hand, very little fettling grain was pro- duced and nearly all the dolomite was produced as a high-density, high-purity refractory raw material. The greater shortage and higher price of high-quality magnesite in Europe, as compared with the United States, probably contributed to the earlier development of refractory do- lomite materials in Europe. U.S. DOLOMITE DEPOSITS Colby (_5) in 1941 and Weitz (26^) in 1942 published extensive surveys on the dolomite resources of the United States and described the quantity, quality, and uses of the deposits of each State. The States with the most plentiful deposits of dolomite are Ohio, Indiana, Illinois, Wisconsin, Michigan, and Pennsylvania. Individual deposits are reported with re- serves varying from 10 to 350 million tons. Chemical analysis data are pre- sented for 212 deposits in Ohio, 18 in Indiana, 27 in Illinois, 111 in Wiscon- sin, 76 in Michigan, and 102 in Pennsyl- vania. All of these deposits are consid- ered to be high-grade dolomite materials that are defined by Weitz as material containing at least 98 pet total carbon- ates and less than 2 pet impurities in- cluding iron oxide, alumina, and silica. Additional information concerning dolo- mite resources is available in State Geological Survey publications of Cali- fornia, Illinois, Indiana, Michigan, Vir- ginia, West Virginia, and Wisconsin. The Alabama Geological Survey ( 25 ) reported that 11 dolomite quarries are in opera- tion primarily in the Birmingham area. Total reserves of these quarries are es- timated in billions of tons. Chemical analyses of these products indicated that most would be considered to be high-grade dolomites. The West Virginia Survey (27) provided information on limestone and dolomite quarries in that State. Only two of the quarries presently produce dolomite. DOLOMITE REFRACTORIES AGGREGATE PROCESSING The original refractory use of dolomite was in the uncalcined condition in open hearths or Thomas converters. As steel processing became better controlled, the need for calcined dolomite grain in- creased. Chesters (4) provides a good summary of the processing of dead-burned dolomite or "doloma," as it is called in England. The production of doloma fol- lows the reaction MgCa(C0 3 ) 2 + CaC0 3 + MgO + C0 2 (1) CaC0 3 + MgO + C0 2 + CaO + MgO + 2C0 2 . (2) This decomposition process is common- ly called calcination. Dolomite can be lightly calcined, as low temperature decomposition is called, or high fired to produce the dead-burned material. Production of higher purity dolomites, or low flux dolomites, has necessitated higher firing temperatures to produce dead-burned grain of satisfactory density. In the temperature range of 600° to 900° C, the dissociation of dolomite re- sults in the intermediate formation of calcium carbonate and magnesia, but heat- ing above 900° C leaves only magnesia and lime as the products. On further heating, these oxides undergo crystal growth, the eventual size being very small in both cases. If reaction 2 is stopped immediately after the C0 2 is driven off, around 900° to 1,000° C, the product is too reactive and porous for use as a refractory raw material. There- fore, the calcination must be carried out at temperature of about 1,700° C in order to reduce the amount of porosity. Very tight control is needed in the manufacture of calcined dolomite, as re- fractories produced from it can suffer from one or the other of the following: 1. Tendency to hydrate owing to reac- tion of free lime with moisture in the air. 2. Tendency for "dusting" or disinte- gration owing to an inversion and volume change on cooling of dicalcium silicate formed in the material. The term "stabilization," as associated with dolomite refractories, has been used to cover the following three procedures: 1. The coating of calcined dolomite with pitch to reduce the rate of hydration. 2. The conversion of free lime to a silicate or ferrite to reduce hydration. 3. The addition of boric acid, phos- phates, or other "stabilizers" to prevent the inversion of dicalcium silicate. It would appear better to use the word "stabilization" solely for the last two procedures. In a Bureau of Mines publication in 1942, Schallis (20) presented a survey on the calcination of raw dolomite. Partic- ular mention is made of the hydration problem of dead-burned dolomite. Methods such as coating with tar or covering with treated paper have been successful in permitting storage for a few weeks or even a few months. To aid calcination, help stabilize the calcium oxide, and im- prove its ability to sinter, iron oxide was added to dead-burned dolomite before the charge went to the kiln. Also, it has been found that the conversion of the lime into dicalcium and tricalcium sili- cates by the addition of silica will re- duce the hydration tendency of the lime. Unfortunately, tricalcium silicate will break down into lime and beta dicalcium silicate, which is not volume stable and tends to disintegrate. In order to sta- bilize the dicalcium silicate, iron oxide can be added to the dolomite. Seil (21- 23 ) received several patents directed toward stabilization of dead-burned dolo- mite grains by incorporating specified additions of Si0 2 and A1 2 3 as a means of reducing hydration tendencies. Similar- ly, Lee ( 13 ) patented processes for the formation of low melting liquid phases in dolomite refractories in order to improve hydration resistance. While the use of so-called "stabilized" dead-burned dolomite was extensive in the past, this practice is not in widespread use today. One reason for this fact is that the silica and iron oxide additions reduce the refractoriness of dolomite; another reason is that producers and con- sumers of dolomite materials have devel- oped better handling methods for reducing hydration. Most of the dolomite used for refrac- tories is produced in either shaft kilns or rotary kilns. Both types are normally fired with gas or oil. Some European shaft kilns have been operated like blast furnaces, using alternating layers of raw dolomite and coal. Prior to firing, the dolomite is crushed and screened to a size suitable for feeding to the kiln. Material for feeding to shaft kilns is usually between 50 and 150 mm while mate- rials for feeding to a rotary kiln is usually between 3 and 40 mm. In both cases, the crushed feed is washed with water to remove fine particles, particu- larly clay contaminations. The thermal processes can be considered as divided into the following four stages: drying; calcination, yielding a porous mixture of lime and magnesia; burning, in which po- rosity is greatly reduced; and cooling, which mainly serves to preheat incoming air. Lee ( 14) , in 1962, described a means to achieve higher firing temperatures and higher heating efficiencies by the use of insulating brick as a backup lining and the use of oxygen additions to the com- bustion air. An addition of high-purity oxygen comprising 3 to 10 pet of the total oxygen in the enriched combustion air is adequate to give the firing condi- tions necessary to produce dense grain. A. recent departure from this conven- tional single-stage firing process has been the introduction of a two-stage fir- ing process involving a pelletization or high-pressure briquetting stage. This process is particularly useful for pro- ducing high-density grain from dolomites that are difficult to dead-burn to a high density in a single-stage process. The dolomite is first decomposed to produce a reactive oxide that is then pelletized and dead-burned in a rotary or shaft kiln to densities of 3.20 to 3.30 g/cm 3 . As described by Obst ( 17) , it is also possible to produce magnesia-dolomite clinkers (coclinkers) by mixing the reac- tive oxide with reactive magnesium oxide before pelletization. These clinkers can have MgO contents from 50 to 80 pet. The amount of direct bonding between the per- iclase grains increases in proportion to the MgO content. Coclinkers has all the advantages and disadvantages of dolomite, but has a higher MgO content. It is pre- ferred to achieve the MgO enrichment by addition of calcined MgO grains, espe- cially in the fine fraction. Chesters (40 compared the chemical com- positions of British dolomites and those of other dead-burned materials. Results of this comparison are given in table 4. Present-day commercial dead-burned dolo- mite contains small amounts of silica, alumina, and iron oxide as accessory ox- ides. The iron oxide is usually present as the ferric form and will combine with lime to form dicalcium ferrite. Usually a small amount of iron will exist as FeO. The ratio of ferric to ferrous oxide will depend on the firing temperature and com- bustion conditions in the kiln. Alumina is not reduced under ordinary conditions and forms mineral phases that have low melting temperatures. Therefore, it is desirable to keep the alumina contents of dolomite refractories relatively low. The overall chemistry as well as the ratio of accessory oxides to the combined MgO and CaO content affect both physical and chemical resistance of dead-burned dolomite grains. Since the majority of dolomite grains are used in the form, of organically bonded brick or specialty mixes, this is the logical form in which to measure hydration resistance. Hubble (8) devised a hydration test that led to the establishment of a standard test, ASTM C492-66 (2^). Dolomite material of a plus 35-mesh size was placed in a cabinet at a temperature of 71° C and relative TABLE 4. - Typical properties of dead-burned dolomite grains Chemi cal ana lysis , wt-pct MgO, by difference, Bulk Origin Si0 2 A1 2 3 Fe 2 3 CaO density, wt-pct g/cm 3 England: 2.50 1.21 1.64 57.60 37.05 3.00 2.35 2.58 1.42 1.30 1.40 1.67 58.20 56.06 36.12 38.39 2.85 South Wales 3.00 .88 .45 1.30 56.80 40.57 3.10 .83 .44 1.14 56.70 40.89 3.25 1.00 .30 1.50 NAp ] 36.0 3.15 1.05 .92 .28 56.10 41.50 3.20 .70 .60 .45 .60 .60 3.00 57.20 62.50 41.05 33.30 3.10 3.00 United States (low flux) .40 .30 .30 56.90 40.40 3.25 United States (standard) 1.10 .60 1.20 51.80 38.0 3.20 NAp Not applicable. 1 Minimum. humidity of 85 pet. After 24 hr, the material was removed, dried, and screened at 35 mesh to determine the amount of material passing through. The rate of hydration was found to be dependent on the heat treatment the dolomite had re- ceived, on the amount of iron oxide in the dolomite, on the dolomite grain siz- ing, and on the number of broken grains present. BRICK PROCESSING The majority of dolomite brick is used in the form of either pitch-bonded or tempered, although others are of the burned-impregnated type. A limited num- ber of fired dolomite brick containing no carbon are used in rotary cement kiln linings and electric furnace linings, although direct-bonded magnesia-chrome bricks have generally been the accepted refractory products for both these appli- cations. Kappmeyer (11) presented a sur- vey of the carbon-containing types of bricks. The processing of the unburned types consists of preheating the sized refractory grain and the pitch material separately, mixing these two materials in a heated mixer, and pressing brick shapes at 4,000 to 10,000 psi on mechanical presses. the brick. Generally, to obtain the de- sired combination of maximum brick den- sity and maximum residual carbon, the amount of pitch will be 5.0 to 6.75 wt- pct. The type of pitch has an important influence on the strength of the brick at the low temperatures associated with part of the burn-in cycle. Brick with exces- sive pitch has low strength for a short time at low temperatures and has col- lapsed under its own weight. After the brick is pressed, it is cooled for storage or taken directly to ovens for tempering. Tempering of the pitch-bonded brick improves several char- acteristics. The low-temperature hot strength of the brick is markedly in- creased, eliminating concern about possi- ble failure of the lining during burn-in. Also, tempering results in a significant improvement in the resistance of the brick to hydration. By tempering, the safe storage period for dolomite brick can be extended from only a few days to several weeks. The temperatures involved in tempering generally range from about 90° to 650° C, but are more commonly 230° to 315° C. Exposure times range from 30 min to 48 hr, with the shorter time being associated with the higher temperatures. The amount of pitch varies and is an important influence on the properties of Pitch-impregnated brick is produced by forcing pitch into the open pores of a presintered (burned) brick made from do- lomite grain aggregates. The properties of the burned brick may vary widely ac- cording to composition and degree of heat treatment before impregnation. The burned brick may be impregnated with pitch to some extent simply by dipping the brick into liquid pitch at 120° to 315° C, but more commonly it is impreg- nated by using a vacuum pressure system to accelerate the rate at which the pitch is forced into the brick pores. The quantity of pitch picked up by a brick is directly related to the initial porosity of the brick. The residual car- bon content in the brick naturally in- creases with greater pitch content and/or increased pitch softening point. How- ever, because brick porosity is confined to a narrow range to achieve other desir- able properties, the quantity of pitch that can be introduced is limited. With this limitation, it is desirable to use pitch with the highest softening point compatible with the operating character- istics of the vacuum impregnating system. It is interesting to compare the amount of energy required to produce tempered brick with that required for impregnated brick. Production of impregnated brick requires 1.64 million Btu/ft 3 which is 20 to 30 pet more energy than for the same volume of tempered brick (1.35 million Btu). Also, experience indicates that properly made tempered brick can give service life equivalent to that of the impregnated, burned brick. BRICK USAGE United Kingdom Leonard (15) reviewed BOF lining prac- tices in the United Kingdom. Both dolo- mite and magnesite were used. There has been a trend towards magnesia enrichment of dolomite refractories by additions of magnesite. Improvements in the quality of dead-burned dolomite and bricks made from it were achieved by more selective quarrying and blending of deposits and the greater use of rotary kilns and shaft kilns with higher firing temperatures. It became possible to produce grain of such consistent chemistry and density that silica content was restricted to 1 pet and densities of over 3.0 g/cm- 5 were achievable. Spencer (24) reported that in 1970, pelletized dead-burned dolomite grain was introduced in England. This arose be- cause most of the highest purity dolomite in the United Kingdom is difficult to sinter to high densities in a single fir- ing process. The decomposition of high- purity dolomite to an active oxide fol- lowed by pelletizing under high pressures and sintering results in densities in the 3.25- to 3.30-g/cm- 5 range. Of course, this two-stage firing process has the disadvantage of increased costs. With the introduction of this pelletized dead- burned dolomite, linings gave improved furnace performances of approximately 10 to 15 pet. Europe Hardy (6.~Z_) discussed BOF linings and lining wear from the standpoint of a steelmaking consumer. In Europe, a long history of basic Bessemer steelmaking re- sulted in the establishment of raw dolo- mite as standard lining materials. The first major change in usage patterns came with the advent of big capacity furnaces. The danger of slumping during burn-in is greater with big vessels and, therefore, almost all vessels in Europe of 200-ton capacity or more used tempered blocks. Magnesia-enriched tempered dolomite and, in some cases, tempered magnesite have been used in selected zones to combat slag attack. Japan Hardy (6^) and Leonard (15) both de- scribed the improvements in Japanese steel refractories. In Japan, which lacks suitable reserves of most raw mate- rials, the practice has been to use syn- thetic magnesia-dolomite clinkers and seawater periclase in BOF refractories. From the late 1950's until about 1970, the average MgO content of BOF linings increased from 50 to 60 pet up to 80 to 90 pet. This is indictive of increased usage of high magnesia coclinkers and of seawater periclase. In 1976, refractory consumption in BOF vessels of enriched dolomite was about one-half that of peri- clase. Dolomite bricks were initially pitch-bonded, but fired mixtures with periclase were introduced in the late 1960*s. By the early 1970* s, the use of MgO-enriched dolomite was well advanced, first as pitch-bonded, then as fired brick. In Japan, both slag testing and thermal shock resistance testing have been used for evaluating refractories for BOF linings. Both authors stress the ex- tremely long lining lives, over 1,000 heats, being achieved in Japanese steel plants resulting, in part, from strict control of slag chemistry and gunning maintenance. United St ates It is logical to discuss dolomite brick usage in terms of iron and steelmaking since this usage constitutes between 50 and 70 pet of the total output of the re- fractories industry. Kappmeyer (12) es- timated that of refractories used in the steel industry, about 3 pet are consumed in coke ov Q ns , 10 pet in blast furnaces, 60 pet in BOF's, 12 pet in pouring pits, and 15 pet in continuous casting, rol- ling, and other forming operations. World steel production, broken down by process, is shown in table 5. It is interesting to observe the change in types of refractories used in the BOF steelmaking process. Table 6 presents the approximate distribution of BOF brick used in the United States. TABLE 5. - Distribution of steel production by process, million tons (12) TABLE 6. - Approximate distribution of BOF brick usage in the United States, percent (12) Process 1960 1971 1985 47 261 13 39 21 230 272 94 10 90 B0F/0-B0F 850 240 379 627 1,200 Brick 1967 1970 1980 Burned impregnated 10 32 41 17 30 29 24 17 40 20 Magnesite, all types... Dolomite, all types.... 25 15 Some other processes are included in the total. Kappmeyer (11) compared the properties of dolomite-containing steel plant re- fractories of both the tempered and burned-impregnated types. These property comparisons are shown in table 7. While the burned brick has lower levels of re- sidual carbon, this type shows higher re- sistance to slag erosion. Although few dolomite-containing brick are used in the burned condition, substantial amounts are used as pitch-bonded or tempered. In 1980, Marr ( 16) surveyed the appli- cations of dolomite materials as refrac- tories. Marr stated that dead-burned dolomite is used in the form of both monolithic products and brick products. Dolomite gunning mixes have been used extensively, especially in electric arc furnaces. Hearths of both open hearth and electric furnaces have been made of rammed dolomite. Tar-bonded dolomite bricks have been found to be satisfactory for BOF linings particularly when used in combination with magnesite bricks. The combining of continuous casting and ladle refining processes in steelmaking is com- mon now and results in higher ladle oper- ating temperatures and basic slags. Therefore, traditional clay and alumina bricks are being replaced by basic prod- ucts, quite often, dolomite. Other applications in which fired dolo- mite brick has performed well are argon- oxygen-decarburization (AOD) furnaces, cement and lime rotary kilns, and nickel or copper refining smelters. The swing to low-cost dolomite brick in the United States never reached the level predicted around 1965. Peatfield and Spencer (18), in 1979, in discussing 10 TABLE 7. - Dolomite brick properties Chemical compc jsition, Bulk Hot modulus Residua! . carbon Slag Brick, type wt-pct den- sity, g/cm 3 of rupture, psi content, wt-pct, after coking to — ero- and sample MgO CaO Fe 2 3 A1 2 3 Si0 2 sion 1 1,200° C 1,980° C 1,090° C 1,650° C Tempered dolomite: TD-1 40. A 56.9 0.3 0.3 0.4 2.84 NAp NAp 3.8 2.6 2.4 TD-2 40.0 55.9 .0 .2 .8 2.84 NAp NAp 3.4 2.7 2.5 TD-3 40.2 55.6 .9 .2 .6 2.84 NAp NAp 3.4 2.7 3.2 TD-4 40.8 56.5 .2 .1 .3 2.84 NAp Nap 3.7 2.5 3.6 Tempered dolomite peri- clase: DPT-1... 60.2 37.9 .3 .3 .6 2.96 NAp NAp Nap 2.8 Nap DPT-2. . . 57.5 37.4 3.3 .5 2.0 3.01 NAp NAp NAp 2.6 NAp DPT-3... 61.2 36.1 .6 .3 1.4 2.95 NAp NAp NAp 3.0 NAp Burned im- pregnated dolomite: TD-1 40.8 57.9 .2 .2 .6 3.14 1,865 610 NAp 1.5 1.3 ID-2 42.0 55.5 .3 .5 .6 3.06 1,080 550 NAp .9 1.2 ID-3. ... 40.2 55.6 .8 .2 .6 3.04 1,860 380 NAp .8 1.1 Burned im- pregnated dolomite peri- clase: IDP-1... 66.9 31.6 .1 .2 1.2 2.98 910 680 NAp 1.3 1.1 IDP-2. . . 60.0 38.3 .2 .2 .7 3.12 865 370 NAp 1.5 .8 Nap Not applicable. Relative depth of brick eroded away as compared with established standards. basic raw materials for steelmaking refractories, mentioned that dolomite- and magnesia-based materials are the only materials that are readily available and cost effective. The selection between magnesia- and dolomite-based products de- pends not only on the technical merits of the materials and lining life require- ments, but also on their relative econo- mies. For example, magnesia products in the United States are only 40 to 50 pet more expensive than dolomite products, whereas in Europe, they are 200 to 300 pet more expensive. This reason has been quoted for the greater development of dolomite in England. The absence in the United States of a strong basic Bes- semer tradition is probably another im- portant reason. PROPERTIES OF 14 U.S. DOLOMITES MATERIALS AND TEST PROCEDURES Samples of 14 different raw dolomite samples were obtained from sources in Alabama, Ohio, Pennsylvania, Missouri, Michigan, California, and Wisconsin. Eight of these materials were obtained from suppliers of refractory grade dolomites, while the other six were rep- resentative of dolomites that are used for nonref ractory applications. Approxi- mately 50 lb of each sample was received. Representative portions of each sample were used in the various characterization studies. Powdered samples were sent to an independent analytical laboratory for chemical analysis and loss on ignition (LOI) determinations according to the procedures of ASTM 0574-71. Mineralogi- cal analyses were conducted on minus 325-mesh material by X-ray diffraction. Differential thermal analysis (DTA) and thermogravimetric (TGA) curves were ob- tained on the materials using a commer- cially available thermal analyzer. Ap- parent specific gravities were measured using an air comparison pycnometer. Pe- trographic analyses and cathode- luminescent photographs were made on thin sections from each material. RESULTS AND DISCUSSION The results of the chemical analyses and loss on ignition, apparent specific gravity, and mineralogical determinations are shown in table 8; petrographic analy- sis data are given in table 9. All 14 of these samples meet the chemical require- ments for refractory grade dolomites as specified in table 2. Only three of the 11 samples had impurity contents totaling over 2.0 wt-pct with the major impurities being either Si02 or Fe 2 03. The theoret- ical LOI value for pure dolomite is 47.72 wt-pct. All of the samples had LOI val- ues over 45.0 wt-pct, and seven had LOI values greater than 47.0 wt-pct. The most predominant accessory minerals were quartz and calcite. The apparent spe- cific gravity values were all between 2.81 and 2.87. This property measure- ment, when greater than 2.80, is usually a good indication of dolomite that can be fired to high-grain density. Photomicrographs of four of the samples are shown in figures 1 through 4. These photomicrographs illustrate the wide range in grain sizes and microstructures of the various dolomites. The micros tructure of sample Ohio No. 1 (fig. 1) is characterized by small grains (average diameter of approximately 100 ym) having no twinning and with poorly TABLE 8. - Properties of raw domestic dolomites Source and sample Chemical analysis, wt-pct MgO CaO SiO- A1 2 2 Fe 2 3 Loss on ignition Apparent specific gravity, g/cm 3 Accessory mineral phases 1 Hydra- tion, 2 wt-pct Calculated liquid phase, wt-pct Alabama : 1 2 3 Ohio: 1 R 2 R 3 R Pennsyl- vania: 1 R 2 R 3 Michigan: 1 2 Missouri : 1 R Wisconsin: 1 Califor- nia: 1.. R Ref ractor Q, quartz 20.80 20.39 20.12 21.20 19.46 20.99 21.26 21.01 21.22 21.18 20.95 19.20 21.16 21.70 30.19 30.13 30.52 30.61 29.57 30.13 27.61 30.76 30.83 30.61 30.34 31.16 30.78 31.07 .12 .11 .48 .02 .69 .40 .16 .29 .15 .49 .52 .31 .27 .50 0.56 .39 .82 .11 .83 .68 .06 .22 .20 .08 .08 .12 .04 .07 0.22 .31 .27 .06 2.99 .12 .30 .39 .22 .10 .19 3.61 .18 46.68 47.30 46.47 47.54 45.05 47.26 46.41 47.08 47.06 47.19 47.42 45.44 46.95 .15 45.85 2.87 2.86 2.85 2.87 2.84 2.87 2.85 2.86 2.81 2.84 2.84 2.84 2.86 2.82 Q,C Q Q,C Q Q Q Q Q,C c Q,C Q,C Q,C Q,C Q,C 83.5 80.9 48.4 100.0 ND 100.0 72.6 95.1 58.3 98.6 ND 5.4 99.5 98.8 10.4 9.8 15.2 .5 8.9 2.3 4.2 2.3 4.6 ND 13.8 2.3 4.2 y-grade dolomi ; C, calcite. te. ND 2 As dete Not determined, rmined per ASTM C492-66 (1981). 12 TABLE 9. - Petrographic analysis data for raw domestic dolomites Source and Crystal- Grain Formation General description sample Unity 1 size, ram and age Alabama: 1 1 0.06 -0.18 'Patchy areas of coarse crys- tals, not equigranular , some dark organic material. 1 .25 -1.25 > Ketona, Upper ^Equigranular, curved grain Cambrian. boundaries, no trace of orig- inal texture. 3 1 .18 -1.25 Coarse crystals along frac- tures, not equigranular, no trace of original texture. Ohio: 1 R 1 .04 - .18 Not equigranular, patchy zones of coarse crystals, porous. 2 R 1 .125- .375 Guelph, Silurian. Not equigranular, contacts wavy, slightly dirty, organic I material along stylolites, voidy . 3 R 1 .06 - .375 Equigranular, dirty, wavy con- tacts, porous, pores may con- „ tain organic material. Pennsylvania: 1 R P .06 - .375 'Not equigranular, irregular grain boundaries, wavy grain boundaries with cloudy Ledger, Lower centers. 2 R P .125-1.0 > Cambrian. s Some circular patches of fine grains, some pressure-induced twinning; no indication of original texture. 3 P .125-1.0 Michigan: 1 P .06 - .18 1 Engadine, I Middle f Equigranular , wavy contacts, I porous, dirty. 2 P .125- .75 Silurian. [ Do. Missouri: 1 R I .06 - .375 Bonne Terre, Upper Cambrian. Not equigranular, excellent zoning, could be areas of iron, very cloudy, an altered subtidal limestone, perhaps oolitic circular patterns. Wisconsin: 1 I .04 - .675 Niagara, Middle Silurian. Finely crystalline, poorly sorted crystals, not equi- granular, no trace of origi- nal structure. California: I P 1 -5 Sur, Jurassic. Coarsely crystalline, twinned, equigranular contacts straight, clear crystals. Rof raphnr \r—cr v a A a <^r>1 Ami t- a 1 T -i T-i ^ c ^ mmck i^i aha* T> haay 13 FIGURE 1. - Photomicrograph of Ohio dolomite No. 1 presently used to produce refractory products. FIGURE 2. = Photomicrograph of Missouri dolomite presently used to produce refractory products, K.4-'. JpiO 0.2,,;. '' "^ I I %$ >■ / ^il $wl», mm |jS FIGURE 3. - Photomicrograph of Alabama dolomite No. 3. FIGURE 4. - Photomicrograph of Pennsylvania dolomite No. 3, 15 defined grain boundaries. The micro- structure of sample Missouri No. 1 (fig. 2) consists of medium-sized grains (aver- age diameter of approximately 300 ym) having no twinning and with better de- fined grain boundaries. The microstruc- ture of sample Alabama No. 3 (fig. 3) consists of large, angular grains (aver- age diameter of approximately 600 ym) having no twinning and with well-defined boundaries. The microstructure of sample Pennsylvania No. 3 (fig. 4) consists of large, angular grains (average diameter of approximately 750 ym) having a large number of twinned grains or striations and with well-defined grain boundaries. Of the 14 raw dolomites characterized in this investigation, only two (Pennsyl- vania No. 2 and 3) are suitable for cal- cining to high-density, dead-burned grain in a single-step firing process. While these two samples did not exhibit any marked differences from the other dolo- mite samples with regard to chemistry, mineralogy, or thermal decomposition, they contain the largest grain sizes of all the materials observed. Besides hav- ing grains that are approximately twice the size of those of most of the other samples, these two samples also contain a large number of twinned grains , as can be seen in figure 4. While it cannot be assumed that either the larger grain size or the twinned grains have any influence upon the calcination and densif ication characteristics of these two dolomite samples, further investigations into the fired grain processing and properties may provide the answers. Examples of the thermal analysis data are shown in figures 5 through 7. With regard to DTA data, it is possible to group the dolomites by the similarities in the endothermic peak locations, as has been done with the curves in figure 6. Thus, it is evident in figure 6 that three of the Alabama materials behave similarly upon heating. Comparing the DTA curves in figure 5, it is seen that the two Michigan dolomites have large peaks around 880° C as do most of the other dolomites, but both of the Michigan materials have a small peak around 650° C, which none of the other materials exhibit. All the DTA curves for these materials indicate typical endothermic peaks exhibited by most dolomite mate- rials. The sharper, lower temperature peak ranging from 780° to 820° C corre- sponds to the decomposition of MgC0 3 , and the broader, higher temperature peak ranging from 860° to 920° C corresponds to the decomposition of CaC03 . Examples of typical TGA curves are shown in figures 8 and 9. While the DTA curves have separate peaks representing a two-step decomposition process , the TGA curves , which were run at half the heat- ing rate of the DTA scans , indicate only a single step decomposition. The total weight losses for these dolomites coin- cide well with the LOI values reported in table 8. The TGA weight loss for Michi- gan dolomite No. 1 was 47.62 wt-pct ver- sus 47.19 wt-pct LOI, and the TGA weight loss for Pennsylvania dolomite No. 2 was 47.78 wt-pct versus 47.08 wt-pct LOI. It is anticipated that when the refrac- tory properties of the calcined grain produced from the 14 different dolomites are determined that these properties can be related to differences in the chemical compositions, and especially the differences in microstructure and thermal decomposition of the raw dolomites. SUMMARY A review of the literature on dolomite resources showed that large quantities of high-purity dolomite materials exist in the United States. Most of these depos- its are located in the area around the Great Lakes as well as in Pennsylvania, Alabama, California, and West Virginia. Many of these resources have been used to provide dolomites suitable for various uses other than refractory products. A few of the deposits have proven useful as refractory grade dolomites. Besides meeting requirements for high purity lev- els, refractory grade dolomites also must 16 600 640 680 720 760 800 840 880 920 960 1,000 TEMPERATURE, ° C FIGURE 5. - DTA curves for six dolomites. 600 640 680 720 760 800 840 880 920 960 1,000 TEMPERATURE, ° C FIGURE 6. - DTA curves for five dolomites. 600 640 680 720 760 800 840 880 920 960 1,000 TEMPERATURE, ° C FIGURE 7. - DTA curves for three dolomites. meet requirements for high grain density and resistance to hydration. The ideal refractory grade dolomite material is one that can be calcined in a single pass through a kiln. Since very few such sources are available, some do- lomite producers have introduced a two- step firing process consisting of a low- temperature calcination followed by bri- quetting and a high-temperature firing. The double-firing process adds signifi- cantly to the price of the resultant grain. Another product that dolomite producers have developed is an MgO-enriched do- lomite coclinker. By adding periclase 120 no « 100 o. O 90 LU 2 80 a. s < " 70 60 KEY PENNSYLVANIA DOLOMITE N0.2 ( ) Sample weight { ) Derivative ' L. 100 200 300 400 500 600 TEMPERATURE, ° C 700 800 900 1,000 FIGURE 8. - TGA curve for a sample of Penn- sylvania dolomite No. 2. powder to the dolomite before the bri- quetting operation, a grain of higher MgO content and thus improved slag resistance can be produced. The European countries, especially Eng- land, have led in the increased usage of dolomite refractories. This fact has been attributed to the greater price dif- ferential between dolomite and periclase in Europe versus the United States and to a traditionally greater use of Bessemer converters for steelmaking in Europe. An investigation of 14 raw domestic do- lomites was conducted with the purpose of characterizing these materials and comparing their properties with the 17 120 110 100 5 90 UJ uj 80 s < 60 1 I " I ' I ' — KEY MICHIGAN DOLOMITE NO ( ) Sample weight (---\ Derivative so]— — ' ■ ' ■ ' ■ ' ■ ■ ' L ' ' ' ' ■ ' ' , '- 2 E 2 a 1 < > 100 200 300 400 500 600 700 800 900 1,000 TEMPERATURE, ° C FIGURE 9. TGA curve for a sample of Michi- gan dolomite No. 1 . refractory properties of calcined grain produced from them. The raw materials were characterized as to chemical, physi- cal, and thermal properties. All of the materials contained at least 49.0 wt-pct combined MgO and CaO. Raw apparent spe- cific gravities ranged from 2.81 to 2.87 and the raw bulk densities ranged from 2.55 to 2.80 g/cm 3 . The major accessory minerals associated with these dolomites were calcite and quartz. The thermal analyses of the materials were characterized by two endothermic peaks, one occurring between 780° and 820° C and the other occurring between 860° and 920° C. Examination of thin section photomicrographs of the raw dolo- mites indicated that the average crystal- lite grain size ranged from around 100 ym up to about 750 urn. The microstructures of two Pennsylvania dolomites that are suitable for calcining to high density dead-burned grain in a single firing were characterized by the largest average crystallite grain sizes and by a large number of twinned grains. It is possible that the large grain sizes and occurrence of twinned grains has some influence upon the calcination and densif ication of these dolomites. Further investigations into fired grain processing and proper- ties may resolve this question. With the large reserves of high purity dolomite in the United States and the price advantage that dolomite holds over seawater periclase, it appears that the U.S. refractory practice should move toward higher dolomite usage. REFERENCES 1. American Society for Testing and Materials. Standard Classification of Granular Refractory Dolomite. C468-70 in 1981 Annual Book of ASTM Standards: Part 17, Refractories, Glass, Ceramic Materials; Carbon and Graphite Products. Philadelphia, PA, 1981, pp. 383-384. 4. Chesters, J. H. Refractories: Pro- duction and Properties. The Iron and Steel Institute. London, 1973, 553 pp. 5. Colby, S. F. Occurrence and Uses of Dolomite in the United States. BuMines IC 7192, 1941, 21 pp. 2. Standard Test Method for Hydration of Granular Dead-Burned Refrac- tory Dolomite. C492-66 in 1981 Annual Book of ASTM Standards: Part 17, Refrac- tories, Glass, Ceramic Materials; Carbon and Graphite Products. Philadelphia, PA, 1981, pp. 404-405. 6. Hardy, C. W. , and A. J. Owen. De- velopment of Refractory Linings to Meet Operational Requirements in Oxygen Vessels. Proc. Conf. on Basic Oxygen Steelmaking: A New Technology. The Metals Society, London, May 1978, pp. 123-130. 3. Carr, D. D. , and L. F. Rooney. Limestone and Dolomite. Ch. in Ind. Miner, and Rocks, American Institute of Mining, Metallurgical, and Petroleum En- gineers. New York, 1975, 1,360 pp. 7. Hardy, C. W. B0F Refractories — A Question of Continuity. The Refractory J., November 1972, pp. 9-17. 18 8. Hubble, D. H. , and W. J. Lackey. Hydration Test for Dead-Burned Dolomite. Am. Ceram. Soc. Bull., v. 41, No. 7, 1962, pp. 442-446. 9. Industrial Minerals Consumer Sur- vey. Raw Materials for the Refractories Industry — Dolomite, High Performance at Low Cost. Metal Bull., Ltd., London, 1981, pp. 57-65. 10. J. E. Baker Co. Unpublished in- formation compiled by technical staff. September 1981, 71 pp.; available upon request from J. E. Baker Co., York, PA. 11. Kappmeyer, K. K. , and D. H. Hub- ble. Pitch-Bearing MgO-CaO Refractories for the BOP Process. Ch. in High Tem- perature Oxides, ed. by A. M. Alper. Academic Press, Inc., New York, 1970, 358 pp. 12. . Trends and Challenges in the Future of Steelplant Refractories. Ironmaking and Steelmaking, v. 3, No. 3, 1976, pp. 113-128. a Basic Refractory Material. Proc. Brit. Ceram. Soc, No. 28, June 1979, pp. 225-241. 18. Peatfield, M. , and D. R. F. Spen- cer. Developments in Refractory Materi- als for LD Linings. Ironmaking and Steel- making, v. 6, No. 5, 1979, pp. 221-234. 19. Pressler, J. W. Lime. BuMines Minerals Yearbook 1980, v. 1, pp. 507- 518. 20. Schallis, A. Dolomite-Base Re- fractories. BuMines IC 7227, 1942, 11 pp. 21. Seil, G. E. High Melting Point Silicate Refractory. U.S. Pat. 2,207,557, July 9, 1940. 22. _. Preparation of Refractory Material^ U.S. Pat. 2,207,072, July 9, 1940. 23. . Corrected Basic Refractory. U.S. Pat. 2,287,455, June 23, 1942. 13. Lee, H. C. Process for Making Re- fractory Materials. U.S. Pat. 2,272,324, Feb. 10, 1942. 24. Spencer, D. R. F. Developments in LD Refractories. Refractory J., November-December 1975, pp. 8-26. 14. Ceram. pp. 807-811. _. Dead-Burned Dolomite. Am. Soc. Bull., v. 41, No. 12, 1962, 15. Leonard, L. A. Dolomite and Sil- ica — Survival or Revival. The Refractory J., September-October 1978, pp. 12-27. 16. Marr, R. J. High Purity Doloma as a Refractory Material. Pres. at ILAFA/ ALAFAR Conf., Lima, Peru, Nov. 2-5, 1980, 22 pp.; available upon request from J. E. Baker Co. , York, PA. 25. Szabo, M. W. Private Communica- tion 1980. Available upon request from M. W. Szabo, Ala. Geol. Survey, Univer- sity, AL. 26. Weitz, J. H. High-Grade Dolomite Deposits in the United States. BuMines IC 7226, 1942, 86 pp. 27. West Virginia Geological and Eco- nomic Survey. West Virginia Mineral Pro- ducers Directory, MRS No. 1, 7th ed. , 1980, 110 pp. 17. Obst, K. H. , and W. Muenchberg. 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