CA1048064A - Pre-reacted magnesia chrome ore grain and method of making same - Google Patents
Pre-reacted magnesia chrome ore grain and method of making sameInfo
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- CA1048064A CA1048064A CA213,638A CA213638A CA1048064A CA 1048064 A CA1048064 A CA 1048064A CA 213638 A CA213638 A CA 213638A CA 1048064 A CA1048064 A CA 1048064A
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Abstract
ABSTRACT OF THE DISCLOSURE
Synthetic magnesium hydroxide or magnesia yielding salt or finely divided MgO is admixed with finely divided chrome ore in which at least 50% of the particles are -325.mesh. The admixture is calcined in a single fire process to yield a dense homogeneous magnesia-chrome grain. The grain can then be processed in a conventional manner into brick shapes which sinter to a very high density. The resulting brick is very dense, has a low porosity. and exhibits a continuous microstructure with a high degree of integrity.
Synthetic magnesium hydroxide or magnesia yielding salt or finely divided MgO is admixed with finely divided chrome ore in which at least 50% of the particles are -325.mesh. The admixture is calcined in a single fire process to yield a dense homogeneous magnesia-chrome grain. The grain can then be processed in a conventional manner into brick shapes which sinter to a very high density. The resulting brick is very dense, has a low porosity. and exhibits a continuous microstructure with a high degree of integrity.
Description
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This invention relates to pre-reacted magnesia-chrome ore grain and a method for its manufacture. More specifically, this invention relates to pre-reacted grain which can be processed in a conventional manner into brick shapes which sinter to a very high density in conventional tunnel kilns.
Direct bonded refractory bricks or shapes are prepared from re-fractory compositions comprising predominantly chrome ore and magnesia.
The chrome ore consists essentially of the chromite spinel with minor accessory silicate gangue minerals. The magnesia consists essentially of magnesium oxide with minor amounts of silicates and other impurities.
Magnesium oxide in its pure form is often referred to as periclase.
Specifically, refractory chrome ores like most other ores are obtained from natural deposits. Refractory chrome ore consists of a solid solution of minerals containing Cr2O3, MgO, ~12O3 and iron oxides with a siliceous mineral gangue. On an oxide basis, refractory chrome ore usually analyzes from about 30 to 50 percent Cr2O3 and 2 to 9 percent of SiO2.
Refractory magnesia is made by "dead burning" the mineral magne-site (MgCO3), or such magnesium compounds as the hydrate or the chloride, to obtain a residual dense grain of magnesium oxide of stable character.
The term "dead burning" as used in relation to magnesite denotes a procedure in which magnesite is heated to from about 2900 to 4000F. Conventional processing for producing dense magnesia grain requires two burning or firing stages. First, the raw material such as magnesite or the hydrate is calcined to MgO and pelleted. The pelleted material is then dead burned to dens e Mg O, In recent years, materials of greater purity have become available.
For example, by beneficiation chrome ores with a silica content as low as 0. 2 percent can be obtainedO ~n equally important change has occurred in commercially available refractory magnesia which now commonly analyzes 97 to 99+ percent MgO~ In these relatively pure refractory magnesias, the silica usually constitutes less than 1 percent by weight on an oxide basisO
This invention relates to pre-reacted magnesia-chrome ore grain and a method for its manufacture. More specifically, this invention relates to pre-reacted grain which can be processed in a conventional manner into brick shapes which sinter to a very high density in conventional tunnel kilns.
Direct bonded refractory bricks or shapes are prepared from re-fractory compositions comprising predominantly chrome ore and magnesia.
The chrome ore consists essentially of the chromite spinel with minor accessory silicate gangue minerals. The magnesia consists essentially of magnesium oxide with minor amounts of silicates and other impurities.
Magnesium oxide in its pure form is often referred to as periclase.
Specifically, refractory chrome ores like most other ores are obtained from natural deposits. Refractory chrome ore consists of a solid solution of minerals containing Cr2O3, MgO, ~12O3 and iron oxides with a siliceous mineral gangue. On an oxide basis, refractory chrome ore usually analyzes from about 30 to 50 percent Cr2O3 and 2 to 9 percent of SiO2.
Refractory magnesia is made by "dead burning" the mineral magne-site (MgCO3), or such magnesium compounds as the hydrate or the chloride, to obtain a residual dense grain of magnesium oxide of stable character.
The term "dead burning" as used in relation to magnesite denotes a procedure in which magnesite is heated to from about 2900 to 4000F. Conventional processing for producing dense magnesia grain requires two burning or firing stages. First, the raw material such as magnesite or the hydrate is calcined to MgO and pelleted. The pelleted material is then dead burned to dens e Mg O, In recent years, materials of greater purity have become available.
For example, by beneficiation chrome ores with a silica content as low as 0. 2 percent can be obtainedO ~n equally important change has occurred in commercially available refractory magnesia which now commonly analyzes 97 to 99+ percent MgO~ In these relatively pure refractory magnesias, the silica usually constitutes less than 1 percent by weight on an oxide basisO
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In conventional magnesia-chrome and/or chrome-magnesia refrac-tories, the magnesia phase is bonded to the chromite phase by silicates.
These silicates, such as merwinite, forsterite, and monticellite, are developed by reaction of the magnesia with the gangue silicates of the chrome ore to orthosilicatesO The bonding structure is essentially a bridge-work of silicate connecting and joining the predominant magnesia and chromite spinel phases. In direct bonded refractories, the periclase and chromite spinel phases are, as the same implies, directly joined together without intervention of a silicate phase. Thus, a direct bonded, fired 10 refractory shape typically has a microstructure of magnesia grain bonded to magnesia grain, magnesia grain bonded to primary chrome ore particles, magnesia grains bonded to secondary spinel crystals, which usually pre-cipitate from a liquid phase on cooling, magnesia, and chrome ore grain, and exolved euhedral spinels each bonded to silicate phases, and exolved spinel crystals within the periclase grains which form during the cooling part of the firingD
In the manufacture of direct bonded refractory bricks and shapes, chrome ore and magnesia of optimum grain sizing are mixed along with appropriate temporary binders in predetermined proportionate quantities.
20 Such binder cornpositions will usually consist of small amounts of water and a binder material or materials. Some typical binder materials would include lignosulfonates, pitch, magnesium, salts, chromic and sulfuric acids, and the likeO
The mixture of chrome ore, magnesia and binder is blended and pressed in a mold under a pressure in excess of 5,000 psi and preferably about 10,000 to 20,000 psi~ This pressed or molded shape is then dried in a suitable manner, such as for example, in an oven at a temperature in the range of about 90 to 180C. and preferably about 100 to 125C. After mixing, pressing and drying, the refractory shapes are fired in a kiln at 30 maturing temperatures usually in excess of at least about 1650C. Generally and preferably, such firing will be conducted at a maturing temperature in the range of about 1700 to 1900C.
~0~ 64 Conventional direct bonded fired bricks, however, usually do not exhibit any, and at the most very little, densification on firing when they are prepared from conventionally si~;ed grain material. In fact, these conventional refractories often exhibit a slight expansion when fired in brick shapes. Also, in such direct bonded refractory shapes, there usually exists some cracks, voids, or spaces between adjacent mineralogically dissimilar particles, The existence of voids between particles undesirably lessens the overall strength of the refractory shapeO
Refractory shapes have also been prepared by a melt-solidification or fused cast process where a molten refractory is cast into molds and carefully cooled and annealed, The microstructure of such a refractory i9 generally without gross voids and is monolithic. Refractory shapes produced by melt-solidification techniques have many desirable properties, but they are extremely difficult to manufacture.
In the past, refractory shapes have also been made from prefired magnesia-chrome ore grains. Typically, these prefired grains are prepared by mixing fairly coarse chrome ore of 1 mm or greater with either MgO, or MgCO3, briquetting, and firing to temperatures in excess of 3200F, and probably of the order of 3500F.
The prior art pre-fired grains usually have a characteristic micro-structure very similar to that of the direct bonded, fired refractories. Thus, the prior art pre-fired grains exhibit periclase to periclase, periclase to primary chrome ore, periclase to silicate and the like type of bonding. The prior art pre-reacted grains are generally fired at higher temperatures than direct bonded refractories, and these higher firing conditions usually result in a grain having a somewhat better knit grain structure than the direct bonded refractories. The prior art pre-reacted grains, however, resemble the direct bonded refractories in exhibiting no, or at the most only very little, densification on firing when made from conventionally grain-si~ed material. The prior art pre-reacted grains also often exhibit a slight expansion reaction when fired in brick shapesO
~4~69i~
SUMMARY OF THE INVENTION
The present invention provides a process for producing pre-reacted magnesia-chrome ore grain which comprises admixing a magnesium com-pound which will yield magnesia upon calcining with fine particle chrome ore in which at least 50% by weight of the chrome ore particles are -325 mesh, calcining the resulting mixture at a temperature of between about 2900 to 3300F, and crushing the resulting grain to a conventional grain size to produce a grain that can be shaped and fired to a final shaped pro-duct without further processing.
Preferably, the magnesium compound is synthetic magnesium hydroxide or MgCO3. It is also preferred that 90% by weight of the chrome ore particles are -325 mesh. The average grain size of the chrome ore particles preferably is about 5~um. The mixture of chrome ore and mag-nesium compound, such as magnesium hydroxide, preferably is dried after being nodulized and then calcined.
The crushed pre-reacted grain can be formed into a refractory shape and fired at conventional sintering temperatures for magnesia-chrome ore refractories to produce a fired refractory shape. The fired shape has a density and porosity comparable to refractory shapes produced by the melt 20 solidification process. The pre-fired grain typically has a bulk density of
In conventional magnesia-chrome and/or chrome-magnesia refrac-tories, the magnesia phase is bonded to the chromite phase by silicates.
These silicates, such as merwinite, forsterite, and monticellite, are developed by reaction of the magnesia with the gangue silicates of the chrome ore to orthosilicatesO The bonding structure is essentially a bridge-work of silicate connecting and joining the predominant magnesia and chromite spinel phases. In direct bonded refractories, the periclase and chromite spinel phases are, as the same implies, directly joined together without intervention of a silicate phase. Thus, a direct bonded, fired 10 refractory shape typically has a microstructure of magnesia grain bonded to magnesia grain, magnesia grain bonded to primary chrome ore particles, magnesia grains bonded to secondary spinel crystals, which usually pre-cipitate from a liquid phase on cooling, magnesia, and chrome ore grain, and exolved euhedral spinels each bonded to silicate phases, and exolved spinel crystals within the periclase grains which form during the cooling part of the firingD
In the manufacture of direct bonded refractory bricks and shapes, chrome ore and magnesia of optimum grain sizing are mixed along with appropriate temporary binders in predetermined proportionate quantities.
20 Such binder cornpositions will usually consist of small amounts of water and a binder material or materials. Some typical binder materials would include lignosulfonates, pitch, magnesium, salts, chromic and sulfuric acids, and the likeO
The mixture of chrome ore, magnesia and binder is blended and pressed in a mold under a pressure in excess of 5,000 psi and preferably about 10,000 to 20,000 psi~ This pressed or molded shape is then dried in a suitable manner, such as for example, in an oven at a temperature in the range of about 90 to 180C. and preferably about 100 to 125C. After mixing, pressing and drying, the refractory shapes are fired in a kiln at 30 maturing temperatures usually in excess of at least about 1650C. Generally and preferably, such firing will be conducted at a maturing temperature in the range of about 1700 to 1900C.
~0~ 64 Conventional direct bonded fired bricks, however, usually do not exhibit any, and at the most very little, densification on firing when they are prepared from conventionally si~;ed grain material. In fact, these conventional refractories often exhibit a slight expansion when fired in brick shapes. Also, in such direct bonded refractory shapes, there usually exists some cracks, voids, or spaces between adjacent mineralogically dissimilar particles, The existence of voids between particles undesirably lessens the overall strength of the refractory shapeO
Refractory shapes have also been prepared by a melt-solidification or fused cast process where a molten refractory is cast into molds and carefully cooled and annealed, The microstructure of such a refractory i9 generally without gross voids and is monolithic. Refractory shapes produced by melt-solidification techniques have many desirable properties, but they are extremely difficult to manufacture.
In the past, refractory shapes have also been made from prefired magnesia-chrome ore grains. Typically, these prefired grains are prepared by mixing fairly coarse chrome ore of 1 mm or greater with either MgO, or MgCO3, briquetting, and firing to temperatures in excess of 3200F, and probably of the order of 3500F.
The prior art pre-fired grains usually have a characteristic micro-structure very similar to that of the direct bonded, fired refractories. Thus, the prior art pre-fired grains exhibit periclase to periclase, periclase to primary chrome ore, periclase to silicate and the like type of bonding. The prior art pre-reacted grains are generally fired at higher temperatures than direct bonded refractories, and these higher firing conditions usually result in a grain having a somewhat better knit grain structure than the direct bonded refractories. The prior art pre-reacted grains, however, resemble the direct bonded refractories in exhibiting no, or at the most only very little, densification on firing when made from conventionally grain-si~ed material. The prior art pre-reacted grains also often exhibit a slight expansion reaction when fired in brick shapesO
~4~69i~
SUMMARY OF THE INVENTION
The present invention provides a process for producing pre-reacted magnesia-chrome ore grain which comprises admixing a magnesium com-pound which will yield magnesia upon calcining with fine particle chrome ore in which at least 50% by weight of the chrome ore particles are -325 mesh, calcining the resulting mixture at a temperature of between about 2900 to 3300F, and crushing the resulting grain to a conventional grain size to produce a grain that can be shaped and fired to a final shaped pro-duct without further processing.
Preferably, the magnesium compound is synthetic magnesium hydroxide or MgCO3. It is also preferred that 90% by weight of the chrome ore particles are -325 mesh. The average grain size of the chrome ore particles preferably is about 5~um. The mixture of chrome ore and mag-nesium compound, such as magnesium hydroxide, preferably is dried after being nodulized and then calcined.
The crushed pre-reacted grain can be formed into a refractory shape and fired at conventional sintering temperatures for magnesia-chrome ore refractories to produce a fired refractory shape. The fired shape has a density and porosity comparable to refractory shapes produced by the melt 20 solidification process. The pre-fired grain typically has a bulk density of
3. 53 g/cc prior to crushing and the fired refractory shape typically has a final density of 3O 49 g/cc and an open porosity of 5O 6%.
The present invention thus provides a pre-reacted grain which can be processed into brick shapes which sinter to a very high density when fired in a conventional manner at a conventional grain size. The final refractory shapes of the present invention have a very dense, low porosity, continuous microstructure with a high degree of integrity that is stronger and more slag resistant than conventional magnesia-chrome ore composi-tions. The process of the present invention produces a dense magnesia-30 chrome ore grain which can be subsequently shaped and fired to a finalshaped product by using only a single burning stage rather than the two ~0~16~
burning stages conventionally used in producing dead burned or other types of magnesia containing grain. The present invention also provides a process which produces a final refractory shape having a density and porosity com-parable to brick produced by the melt solidification technique.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory, but are not r e str ictive of the invention .
DETAILED DESCRIPTION OF THE INVENTION
The pre-reacted grain in accordance with the present invention is 10 desirably prepared from a mixture of synthetic magnesium hydroxide and chrome ore, although other magnesia yielding compounds such as natural MgCO3, fine particle MgO, or a magnesia yielding salt such as MgC12, and the like can be used if they are of sufficient purityO The synthetic magnesium hydroxide for use in the present invention can be conveniently obtained as an aqueous slurry from either a seawater or brine source. Alternatively, the magnesium source can be natural MgCO3 or MgO of a purity of 85% or greater .
The magnesium hydroxide from seawater or brine is preferably utili7ed in the form of a filter cake of magnesium hydroxide which contains 20 about 50% water. The particle size of the magnesia yielding compound preferably is -150 meshO
The chrome ore that is used in the present invention is preferably the Philippine variety of chrome ore, but other varieties of chrome ore such as U.S.S.Ro, Transvaal, Rhodesian, and Turkish, can also be used in the present invention. Before the chrome ore is mixed with the synthetic mag-nesium hydroxide it is ground by conventional equipment such as by ball milling to provide a particle size distribution containing at least 50% by weight of -325 mesh (equivalent to less than 44~Um) particles, and preferably about 90% to 100% by weight of -325 mesh particlesO Best results are usually 30 obtained when the average particle size of the chrome ore is between 5~m and 10~m, with 5,~m presently considered to be optimumO The chrome ore used in the present invention is thus an extremely finely ground ore.
Generally, as the degree of fineness of the chrome ore increases, the ultimate processing temperatures used to calcine the chrome ore-magnesium compound mixture and obtain a satisfactory density pre-reacted grain de-creases .
After the chrome ore is ground to the desired fine particle size, it is mixed with the synthetic magnesium hydroxide in appropriate amounts, The final particle si2;e of the chrome ore is an important aspect of the present invention because it enables the chrome ore to become uniformly 10 distributed throughout the mixture and dissolve in the magnesia during the single firing step used to produce the pre-reacted grain of this invention.
The chrome ore-magnesia ratio in the final refractory shape can vary widely. Generally, a composition suitable for forming a pre-reacted grain that can be processed into a refractory shape that exhibits densifïca-tion upon sintering comprises by weight on an oxide basis about 20 to 90 percent of the magnesia yielding compound such as magnesium hydroxide and about 80 to 10 percent chrome ore. Preferably, such a composition will upon calcining produce a product containing on an oxide basis 30 to 70 per-cent MgO and 70 to 30 percent Cr2O3.
The refractory shapes produced in accordance with the present invention include both magnesia-chrome ore and chrome-ore magnesia bricks.
Magnesia-chrome ore bricks are those prepared from a batch comprising magnesia and chrome ore in which the magnesia is predominant. Chrome ore-magnesia bricks are prepared from batches in which the chrome ore is predominantO
The preferred processing technique comprises thoroughly and uni-formly mixing the proper amount of predominantly -325 mesh chrome ore with magnesium h~rdroxide filter cake containing about 50% water to form a pasty admixture.
The mixture of magnesium hydroxide and fine chrome ore is pre-ferably dried in conventional equipment, such as a chain dryer, to produce ~48~64 predensified nodules. The predensified nodules can be further compacted by pelletizing or fed to a calcination kiln as dried nodules. Generally, as the density of the kiln feed increases, there is a lower loss of material as dust fines and the density of the grain after calcination increases. The magnesium hydroxide and chrome ore can also be mixed together by co-ball milling the magnesium compound and chrome ore either by a wet or dry process.
In accordance with the invention, the magnesium compound chrome ore mixture is calcined at a temperature of between 2900 to 3300F. Pre-10 ferably, the calcining step includes holding the pellets at a peak temperatureof about 3200F for about one hour. During calcining, the magnesium com-pound such as magnesium hydroxide is converted to high purity magnesia and the chrome ore is dissolved in the magnesia to produce a monolithic grain structure as opposed to a direct bonded magnesia to chrome ore structure. The chromite spinel present in the chrome ore is completely altered by the formation of magnesia spinels of MgCr2O4, MgFe2O4, and MgA12 O4 O
The calcination temperature used in the single burn to produce the pre-reacted grain is an important aspect of the present invention because it 20 enables complete reaction of the chrome ore and magnesia which does not occur at temperatures substantially lower than those above~
To achie~re the dissolution of the chrome ore in the magnesia, the overall silica content of the magnesium compound-chrome ore mixture used to prepare the pre-reacted grain should be maintained at about 4 percent or less, preferably 1 to 3 percent, based on the weight of the grain mixture, and the overall lime to silica ratio should be kept at less than 20 A silica content of higher than 4% will result in the production of silicate bonded magnesia-chrome ore grain rather than the monolithic structure obtained by the present invention. The pre-reacted grain produced from the calcination 30 step of the present invention typically has a bulk density range of 3. 30 to 3.60 g/cc, preferably 3.40 to 3. 50 g/cc. Densities within this range can be achieved by a calcination temperature of about 3200F if about 90y~ by weight of the chrome ore particles are -325 mesh, Higher calcination temperatures are required to achieve this density if the percent of chrome ore particles of -325 mesh is substantially less than 90 percentO
After the pre-reacted grain is produced, it is crushed to conventional grain sizing.
The crushed, pre-reacted grain can then be processed in accordance with conventional refractory shaping procedures. Thus, the crushed pre-reacted grain can be pressed or molded into a desired shape, such as brick, 10 under a pressure in excess of 5000 psi and preferably about 10,000 to about 20,000 psi. This pressed or molded shape is then fired in a kiln at maturing temperatures, usually in excess of at least about 2800F and preferably in the range of 3000 to 3300F. At present, it is preferred to fire the re-fractory shape at about 3200F.
It should be noted that the term "firing" as employed herein shall embrace all three stages of the total cycle, namely, "heating", ~'holding", and "cooling". By "heating stage" is meant--that portion of the firing cycle wherein the temperature of the pressed refractory composition is elevated from room temperature to the desired maturing temperature. The "holding 20 stage" is that portion of the firing cycle wherein the maturing temperature is maintainecl for a predetermined amount of time. And, of course, the "cooling stage" comprises lowering the temperature of the brick from maturing temperature to room temperature or thereabout~
The pre-reacted grains of the present invention have a number of desirable properties. The grains are produced in a single burning or calcination step to form a dense MgO grain whereas the production of con-ventional magnesia grain requires two burning steps. The use of only a single burning step to produce dense MgO grain results in a substantial cost savings. Further, the pre-reacted grains exhibit a dense homogeneous 30 magnesia-chrome grain structure and can be processed in a conventional manner into brick shapes which sinter to a very high density in conventional ~D4~64 tunnel kilns. The resulting brick has a density and porosity comparable to brick produced by ~he melt solidification rnethod. The brick has a monolithic structure and is without gross voids. The brick has a continuous micro-structure with a high degree of integrity and is stronger and more slag resistant than conventional direct bonded magnesia-chrome ore compositions.
The many facets of this invention are further illustrated by the following examples which are not to be construed as limitations thereof.
Various other embodiments, modifications and equivalents of these examples will readily suggest themselves to those skilled in the art without departing 10 from the spirit of the present invention or the scope of the appended claims.
All percentages and parts referred to herein are by weight unless otherwise specifically indicated. All screen sizes are U.S. Sieve Series - ASTM
E-11-61 unless otherwise noted.
EXAMPLE I
~ high lime-to-silica ratio pre-reacted grain is prepared in this example~ Magnesium hydroxide filter cake and Philippine chrome ore that is ball milled to produce a particle si~;e range in which 90% by weight of the ore passes a 325 mesh screen are uniforrr~ly mixed in the ratio of 55 parts of filter cake solids on an MgO basis and 45 parts of chrome oreO The 20 composition of the chrome ore and the filter cake are set forth in Table I.
The overall lime-to-silica ratio of the filter cake and chrome ore initially is adjusted to 1.5 by the addition of 0.55% calcined dolomite.
TABLE I
SiO2 Fe23 A12O3 CaO MgO B2O3Cr23 MgO (Calcinedbasis) 0.60 0.Z3 0.31 1.84 97.00 0.02 Chrome ore 2.5 14.6 29.7 0.35 16.8 36.0 The slurried mixture is dried, pelletized, and burned to 3200F with a one hour hold at peak temperature in a rotary furnace. The resulting grain has a 3.53 g/cc bulk densityu This grain is crushed to:
- 10 _ 8~
40% -4 + 10 mesh 10% -10 ~ 20 mesh 8% -20 + 48 mesh 7 % 48 me sh 10% 60% -325 mesh 25% 95% -325 mesh The crushed grain is pressed into brick shapes. The green bulk density of the brick is 3.34 g/cCD The brick is con~rentionally fired to 3200F.The fired brick has a final density of 3.49 g/cc and 5.6% porosity. This 10 result is totally unexpected for this type of refractory conventionally pro-cessed and fired in this manner.
Another batch of pre-reacted grain is prepared in a manner similar to the procedure of Example I. A mixture containing on an oxide basis about 45% chrome ore which has 90% by weight of -325 mesh particles and about 55% MgO derived from-a low lime to silica synthetic magnesium hydroxide in filter cake form is prepared. The composition of the magnesium hydroxide on an oxide basis is 0. 62(7'o SiO2, 0O20% Fe2O3, 0O 29% A1203, O, 60% CaO, 98.17% MgO, and 0.12% B2O32 The base batch is wet mixed and 1.67% talc 20 is added to produce a composition ha~ring a low lime to silica ratio of 0.2.
The mixed slurry is dried and then pelletized and fired to 3200F in a rotary furnace. The resulting grain density is 3.53 g/ccO The grain is crushed, and then formed into a refractory shape by pressing at 12,500 psi to a green bulk density of 3.13 g/cc and fired at a temperature of 3200F. The resulting product has a density of 3. 44 g/ccO
EXAMPLE III
Another batch of pre-reacted grain is prepared using 55% of 60%
-3Z5 mesh finely ground MgO with the same chemical analysis on an oxide basis as given in Example II. The finely ground MgO is co-ball milled with 30 45% chrome ore which has 90% by weight of -325 mesh particles, average particle size 5-10~1(m, to insure good mixing of the chrome ore and the magnesia. The mixture is compacted to a dobie shape at 15,000 psi and fired at 3200F. The resulting dobie is crushed to yield a grain with a density of 3.48 g/cc and an open porosity of 6%.
EXAMPLE IV
A batch of pre-reacted grain is prepared as in Example III with 55%
MgO and 45% chrome ore ground only to 60% by weight -325 mesh. The mixture is co-ball milled and pressed at 15~000 psi and fired to 3Z00F.
The resulting grain has a density of only 3. 23 g /cc. This example thus illustrates the importance of having ~7ery finely divided chrome ore to pro-10 duce a dense grain under the abo~re temperature and pressure conditions.
The invention in its broader aspects is not limited to the specific details sho~rn and described and departures may be made from such details without departing from the principles of the in~rention and without sacrificing its chief advantagesO
The present invention thus provides a pre-reacted grain which can be processed into brick shapes which sinter to a very high density when fired in a conventional manner at a conventional grain size. The final refractory shapes of the present invention have a very dense, low porosity, continuous microstructure with a high degree of integrity that is stronger and more slag resistant than conventional magnesia-chrome ore composi-tions. The process of the present invention produces a dense magnesia-30 chrome ore grain which can be subsequently shaped and fired to a finalshaped product by using only a single burning stage rather than the two ~0~16~
burning stages conventionally used in producing dead burned or other types of magnesia containing grain. The present invention also provides a process which produces a final refractory shape having a density and porosity com-parable to brick produced by the melt solidification technique.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory, but are not r e str ictive of the invention .
DETAILED DESCRIPTION OF THE INVENTION
The pre-reacted grain in accordance with the present invention is 10 desirably prepared from a mixture of synthetic magnesium hydroxide and chrome ore, although other magnesia yielding compounds such as natural MgCO3, fine particle MgO, or a magnesia yielding salt such as MgC12, and the like can be used if they are of sufficient purityO The synthetic magnesium hydroxide for use in the present invention can be conveniently obtained as an aqueous slurry from either a seawater or brine source. Alternatively, the magnesium source can be natural MgCO3 or MgO of a purity of 85% or greater .
The magnesium hydroxide from seawater or brine is preferably utili7ed in the form of a filter cake of magnesium hydroxide which contains 20 about 50% water. The particle size of the magnesia yielding compound preferably is -150 meshO
The chrome ore that is used in the present invention is preferably the Philippine variety of chrome ore, but other varieties of chrome ore such as U.S.S.Ro, Transvaal, Rhodesian, and Turkish, can also be used in the present invention. Before the chrome ore is mixed with the synthetic mag-nesium hydroxide it is ground by conventional equipment such as by ball milling to provide a particle size distribution containing at least 50% by weight of -325 mesh (equivalent to less than 44~Um) particles, and preferably about 90% to 100% by weight of -325 mesh particlesO Best results are usually 30 obtained when the average particle size of the chrome ore is between 5~m and 10~m, with 5,~m presently considered to be optimumO The chrome ore used in the present invention is thus an extremely finely ground ore.
Generally, as the degree of fineness of the chrome ore increases, the ultimate processing temperatures used to calcine the chrome ore-magnesium compound mixture and obtain a satisfactory density pre-reacted grain de-creases .
After the chrome ore is ground to the desired fine particle size, it is mixed with the synthetic magnesium hydroxide in appropriate amounts, The final particle si2;e of the chrome ore is an important aspect of the present invention because it enables the chrome ore to become uniformly 10 distributed throughout the mixture and dissolve in the magnesia during the single firing step used to produce the pre-reacted grain of this invention.
The chrome ore-magnesia ratio in the final refractory shape can vary widely. Generally, a composition suitable for forming a pre-reacted grain that can be processed into a refractory shape that exhibits densifïca-tion upon sintering comprises by weight on an oxide basis about 20 to 90 percent of the magnesia yielding compound such as magnesium hydroxide and about 80 to 10 percent chrome ore. Preferably, such a composition will upon calcining produce a product containing on an oxide basis 30 to 70 per-cent MgO and 70 to 30 percent Cr2O3.
The refractory shapes produced in accordance with the present invention include both magnesia-chrome ore and chrome-ore magnesia bricks.
Magnesia-chrome ore bricks are those prepared from a batch comprising magnesia and chrome ore in which the magnesia is predominant. Chrome ore-magnesia bricks are prepared from batches in which the chrome ore is predominantO
The preferred processing technique comprises thoroughly and uni-formly mixing the proper amount of predominantly -325 mesh chrome ore with magnesium h~rdroxide filter cake containing about 50% water to form a pasty admixture.
The mixture of magnesium hydroxide and fine chrome ore is pre-ferably dried in conventional equipment, such as a chain dryer, to produce ~48~64 predensified nodules. The predensified nodules can be further compacted by pelletizing or fed to a calcination kiln as dried nodules. Generally, as the density of the kiln feed increases, there is a lower loss of material as dust fines and the density of the grain after calcination increases. The magnesium hydroxide and chrome ore can also be mixed together by co-ball milling the magnesium compound and chrome ore either by a wet or dry process.
In accordance with the invention, the magnesium compound chrome ore mixture is calcined at a temperature of between 2900 to 3300F. Pre-10 ferably, the calcining step includes holding the pellets at a peak temperatureof about 3200F for about one hour. During calcining, the magnesium com-pound such as magnesium hydroxide is converted to high purity magnesia and the chrome ore is dissolved in the magnesia to produce a monolithic grain structure as opposed to a direct bonded magnesia to chrome ore structure. The chromite spinel present in the chrome ore is completely altered by the formation of magnesia spinels of MgCr2O4, MgFe2O4, and MgA12 O4 O
The calcination temperature used in the single burn to produce the pre-reacted grain is an important aspect of the present invention because it 20 enables complete reaction of the chrome ore and magnesia which does not occur at temperatures substantially lower than those above~
To achie~re the dissolution of the chrome ore in the magnesia, the overall silica content of the magnesium compound-chrome ore mixture used to prepare the pre-reacted grain should be maintained at about 4 percent or less, preferably 1 to 3 percent, based on the weight of the grain mixture, and the overall lime to silica ratio should be kept at less than 20 A silica content of higher than 4% will result in the production of silicate bonded magnesia-chrome ore grain rather than the monolithic structure obtained by the present invention. The pre-reacted grain produced from the calcination 30 step of the present invention typically has a bulk density range of 3. 30 to 3.60 g/cc, preferably 3.40 to 3. 50 g/cc. Densities within this range can be achieved by a calcination temperature of about 3200F if about 90y~ by weight of the chrome ore particles are -325 mesh, Higher calcination temperatures are required to achieve this density if the percent of chrome ore particles of -325 mesh is substantially less than 90 percentO
After the pre-reacted grain is produced, it is crushed to conventional grain sizing.
The crushed, pre-reacted grain can then be processed in accordance with conventional refractory shaping procedures. Thus, the crushed pre-reacted grain can be pressed or molded into a desired shape, such as brick, 10 under a pressure in excess of 5000 psi and preferably about 10,000 to about 20,000 psi. This pressed or molded shape is then fired in a kiln at maturing temperatures, usually in excess of at least about 2800F and preferably in the range of 3000 to 3300F. At present, it is preferred to fire the re-fractory shape at about 3200F.
It should be noted that the term "firing" as employed herein shall embrace all three stages of the total cycle, namely, "heating", ~'holding", and "cooling". By "heating stage" is meant--that portion of the firing cycle wherein the temperature of the pressed refractory composition is elevated from room temperature to the desired maturing temperature. The "holding 20 stage" is that portion of the firing cycle wherein the maturing temperature is maintainecl for a predetermined amount of time. And, of course, the "cooling stage" comprises lowering the temperature of the brick from maturing temperature to room temperature or thereabout~
The pre-reacted grains of the present invention have a number of desirable properties. The grains are produced in a single burning or calcination step to form a dense MgO grain whereas the production of con-ventional magnesia grain requires two burning steps. The use of only a single burning step to produce dense MgO grain results in a substantial cost savings. Further, the pre-reacted grains exhibit a dense homogeneous 30 magnesia-chrome grain structure and can be processed in a conventional manner into brick shapes which sinter to a very high density in conventional ~D4~64 tunnel kilns. The resulting brick has a density and porosity comparable to brick produced by ~he melt solidification rnethod. The brick has a monolithic structure and is without gross voids. The brick has a continuous micro-structure with a high degree of integrity and is stronger and more slag resistant than conventional direct bonded magnesia-chrome ore compositions.
The many facets of this invention are further illustrated by the following examples which are not to be construed as limitations thereof.
Various other embodiments, modifications and equivalents of these examples will readily suggest themselves to those skilled in the art without departing 10 from the spirit of the present invention or the scope of the appended claims.
All percentages and parts referred to herein are by weight unless otherwise specifically indicated. All screen sizes are U.S. Sieve Series - ASTM
E-11-61 unless otherwise noted.
EXAMPLE I
~ high lime-to-silica ratio pre-reacted grain is prepared in this example~ Magnesium hydroxide filter cake and Philippine chrome ore that is ball milled to produce a particle si~;e range in which 90% by weight of the ore passes a 325 mesh screen are uniforrr~ly mixed in the ratio of 55 parts of filter cake solids on an MgO basis and 45 parts of chrome oreO The 20 composition of the chrome ore and the filter cake are set forth in Table I.
The overall lime-to-silica ratio of the filter cake and chrome ore initially is adjusted to 1.5 by the addition of 0.55% calcined dolomite.
TABLE I
SiO2 Fe23 A12O3 CaO MgO B2O3Cr23 MgO (Calcinedbasis) 0.60 0.Z3 0.31 1.84 97.00 0.02 Chrome ore 2.5 14.6 29.7 0.35 16.8 36.0 The slurried mixture is dried, pelletized, and burned to 3200F with a one hour hold at peak temperature in a rotary furnace. The resulting grain has a 3.53 g/cc bulk densityu This grain is crushed to:
- 10 _ 8~
40% -4 + 10 mesh 10% -10 ~ 20 mesh 8% -20 + 48 mesh 7 % 48 me sh 10% 60% -325 mesh 25% 95% -325 mesh The crushed grain is pressed into brick shapes. The green bulk density of the brick is 3.34 g/cCD The brick is con~rentionally fired to 3200F.The fired brick has a final density of 3.49 g/cc and 5.6% porosity. This 10 result is totally unexpected for this type of refractory conventionally pro-cessed and fired in this manner.
Another batch of pre-reacted grain is prepared in a manner similar to the procedure of Example I. A mixture containing on an oxide basis about 45% chrome ore which has 90% by weight of -325 mesh particles and about 55% MgO derived from-a low lime to silica synthetic magnesium hydroxide in filter cake form is prepared. The composition of the magnesium hydroxide on an oxide basis is 0. 62(7'o SiO2, 0O20% Fe2O3, 0O 29% A1203, O, 60% CaO, 98.17% MgO, and 0.12% B2O32 The base batch is wet mixed and 1.67% talc 20 is added to produce a composition ha~ring a low lime to silica ratio of 0.2.
The mixed slurry is dried and then pelletized and fired to 3200F in a rotary furnace. The resulting grain density is 3.53 g/ccO The grain is crushed, and then formed into a refractory shape by pressing at 12,500 psi to a green bulk density of 3.13 g/cc and fired at a temperature of 3200F. The resulting product has a density of 3. 44 g/ccO
EXAMPLE III
Another batch of pre-reacted grain is prepared using 55% of 60%
-3Z5 mesh finely ground MgO with the same chemical analysis on an oxide basis as given in Example II. The finely ground MgO is co-ball milled with 30 45% chrome ore which has 90% by weight of -325 mesh particles, average particle size 5-10~1(m, to insure good mixing of the chrome ore and the magnesia. The mixture is compacted to a dobie shape at 15,000 psi and fired at 3200F. The resulting dobie is crushed to yield a grain with a density of 3.48 g/cc and an open porosity of 6%.
EXAMPLE IV
A batch of pre-reacted grain is prepared as in Example III with 55%
MgO and 45% chrome ore ground only to 60% by weight -325 mesh. The mixture is co-ball milled and pressed at 15~000 psi and fired to 3Z00F.
The resulting grain has a density of only 3. 23 g /cc. This example thus illustrates the importance of having ~7ery finely divided chrome ore to pro-10 duce a dense grain under the abo~re temperature and pressure conditions.
The invention in its broader aspects is not limited to the specific details sho~rn and described and departures may be made from such details without departing from the principles of the in~rention and without sacrificing its chief advantagesO
Claims (19)
1. A process for producing a pre-reacted magnesia chrome ore grain comprising mixing a magnesium compound which will yield magnesia upon calcining with finely divided chrome ore in which at least 50% by weight of the chrome ore particles are -325 mesh, calcining the resulting mixture at a temperature of between about 2900°F to 3300°F, and crushing the resulting grain to a conventional grain size to produce a product containing on an oxide basis from 30 to 70% MgO and from 70 to 30% Cr2O3 which is a grain that can be shaped and fired to a final shaped product without further processing.
2. The process of Claim 1 wherein the magnesium compound is magnesium hydroxide.
3. The process of Claim 1 wherein the mixture is dried and nodulized before being calcined.
4. The process of Claim 1 wherein the calcining temperature is maintained at about 3200°F.
5. The process of Claim 4 wherein at least 90% by weight of the chrome ore particles are -325 mesh.
6. The process of Claim 5 in which the chrome ore is ball milled to produce 90% by weight of -325 mesh particles with an average particle size of 10µm or less.
7. The process of Claim 5 wherein the chrome ore has an average particle size of between about 5 and 10µm.
8. The process of Claim 7 wherein the chrome ore has an average particle size of about 5µm.
9. The process of Claim 2 wherein the calcining temperature is maintained at about 3200°F and at least 90%
by weight of the chrome ore particles are -325 mesh.
by weight of the chrome ore particles are -325 mesh.
10. The process of Claim 9 wherein the average particle size of the chrome ore particles are between about 5 and 10µm.
11. The process of Claim 2 in which magnesium hydroxide in the form of a filter cake containing about 50%
water is mixed with the finely divided chrome ore to form a pasty mixture.
water is mixed with the finely divided chrome ore to form a pasty mixture.
12. The process of Claim 1 in which the magnesium compound is magnesium oxide.
13. The process of Claim 1 in which the magnesium compound is magnesium carbonate.
14. The process of Claim 1 in which the magnesium compound is a magnesia yielding salt.
15. The process of Claim 1 wherein the magnesium compound is MgCl2.
16. The process of Claim 1 wherein the mixture is dried and pelletized before being calcined.
17. A process for forming a refractory shape comprising shaping the crushed pre-reacted grains of claim 1 and firing said shaped grains at a temperature of about 2900°F.
to about 3300°F.
to about 3300°F.
18. A refractory shape produced in accordance with the process of Claim 17.
19. A pre-reacted refractory grain produced in accordance with the process of Claim 1.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA213,638A CA1048064A (en) | 1974-11-13 | 1974-11-13 | Pre-reacted magnesia chrome ore grain and method of making same |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA213,638A CA1048064A (en) | 1974-11-13 | 1974-11-13 | Pre-reacted magnesia chrome ore grain and method of making same |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1048064A true CA1048064A (en) | 1979-02-06 |
Family
ID=4101617
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA213,638A Expired CA1048064A (en) | 1974-11-13 | 1974-11-13 | Pre-reacted magnesia chrome ore grain and method of making same |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1048064A (en) |
-
1974
- 1974-11-13 CA CA213,638A patent/CA1048064A/en not_active Expired
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