IL46095A - Pre-reacted magnesia-chrome ore grain and method of making same - Google Patents

Description

PRE-Rf]ACTED MAGNESIA-CHROME ORE GRAIN AND METHOD OF MAKING SAME SPECIFICATION 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 refractory 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 Cr O , MgO, 2 Al O and iron oxides with a siliceous mineral gangue. On an oxide 2 3 basis, refractory chrome ore usually analyzes from about 30 to 50 percent Cr O and 2 to 9 percent of SiO . 2 3 2 Refractory magnesia is made by "dead burning" the mineral magnesite (MgCO ), or such magnesium compounds as the 3 hydrate or the chloride, to obtain a residual dense grain of magnesium • » oxide of stable character. The term "dead burning" as used in relation " material such as magnesite or the hydrate is calcined to MgO and pelleted. The pelleted material is then dead burneB to dense MgO.

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 obtained. An 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 w eight on an oxide basis.

In conventional magnesia- chrome and/or chrome-magnesia refractories, the magnesia phase is bonded to the chromite phase by silicates. These silicates, such as merwinite, forsterite, and monti- cellite, are developed by reaction of the magnesia with the gangue silicates of the chrome ore to orthosilicates. The bonding structure is essentially a bridgework 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 name implies, directly joined together without intervention of a silicate phase. Thus, a direct bonded, fired refractory shape typically has a mxcrostructure of magnesia grain bonded to magnesia grain, magnesia grain bonded to primary chrome ore particles, magnesia grains bonded to secondary spinel crystals, which usually precipitate from a liquid phase on cooling, magnesia, and chrome ore grain, and exolved euhedral spinels each bonded to silicate phases, and exolved spinel crystals wi thin the periclase grains which form during the cooling part of the firing. shapes, chrome ore and magnesia Of optimum grain sizing are mixed along with appropriate temporary binders in predetermined proportionate quantities. Such binder compositions will usuall consist of small amounts of water and a binder material or materials. Some typical binder materials would include lignosulf onates, pitch, magnesium, salts, chromic and sulfuric acids, and the like.

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 180 °C. and preferab ly about 100° to 125 °C. After mixing, pressing and drying, the refractory shapes are fired in a kiln at .maturing temperatures usually in excess of at' least about 1650°C. Generally and preferably, such firing will be conducted at a maturing temperature in the range of about 1700 ° to 1900°C.

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 sized 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 spages between adjacent mineralogically dissimilar particles.

The existence of voids between particles undesirabl lessens the ' 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 is 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 pre-fired grains are prepared by mixing fairly coarse chrome ore of 1 mm or greater with either sO, or MgCO^, briquetting, arid firing to temperatures in gr-een of 3200 °F, and probably of the order of 3500 °F.

. The prior art pre-fi red grains usually have a characteristic microstructure 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, densifica-tion on firing when made from conventionally grain-sized material.

The prior art pre-reacted grains also often exhibit a slight expansion • . ■· . ' · ·' ' " ' SUMMARY OF THE INVENTION The present invention provides a process for producing pre - reacted magnesia -chrome ore grain which comprises admixing a magnesiu compound 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 3300°FJ and crushing the resulting grain to a conventional grain size to produce a grain that can be shaped and fired to a final shaped product without further processing.

Preferably, the magnesium compound is synthetic magnesiu hydroxide or MgCO . It is also preferred that 90% by weight of the 3 chrome ore particles are -325 mesh. The average grain size of the chrome ore particles preferably is about 5 /*m. The mixture of chrome chrome ore compositions. The process of the present invention produces a dense magnesia- chrome ore grain which can be subsequently shaped and fired to a final shaped product by usin only a single burning stage rather than the two 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 comparable 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 restrictive of the invention.

' DETAILED DESCRIPTIO OF THE INVENTION The pre^ reacted grain in accordance with the present invention is desirably prepared from a mixture of synthetic magnesium hydroxide and chrome ore, although other magnesia yielding compounds such as natural MgCOg, fine particle MgO, or a magnesia yielding salt such as MgCLj* and the like can be used if they are of sufficient purity. 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 MgCOg or MgO of a purity of 85% or greater.

The magnesium hydroxide from seawater or brine is preferably utilized in the form of a filter cake of magnesium hydroxide which contains about 50% water. The particle size of the magnesia 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. R. , Transvaal, Rhodesian, and Turkish, can also be used in the present invention. Before the chrome ore is mixed with the synthetic magnesium hydroxide it is ground by conven¬ tional equipment such as by ball milling to provide a particle size dis¬ tribution containing at least 50% by weight of - 325 mesh (equivalent to less than 44 j. m) particles, and preferably about 90% to 100% by weight of -325 mesh par ticles. Best results are usually obtained when the average particle size of the chrome ore is between 5 \m and 10 m, with 5 μρη. presently considered to be optimum. 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 ut imate processing temperatures used to calcine the chrome ore-magnesium compound mixture and obtain a satisfactory density pre- reacted grain decreases.

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 size of the chrome ore is an important aspect of the present invention because it enables the chrome ore to become uniformly 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 ore1- magnesia ratio in the final refractory • * shape can vary widely. Generally, a composition suitable . for forming .a prc-reactcd grain that can be processed into a refractory shape that exhibits densification 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 ing on an oxide basis 30 to 70 percent MgO and 70 to 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 o-re is predominant.

The preferred processing technique comprises ihoroiighly and uniformly mixing the proper amount of predominantly -325 mesh chrome ore with magnesium hydroxi de filter cake containing about 50% water to form a pasty admixture.

The mixture of magnesium hydroxide and fine chrome ore is preferably dried in conventional equipment, such as a chain dryer, to produce 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 3300 "F Preferably, the calcining step includes holding the pellets at a peak temperature of about 3200°F for about one hour. During calcining, the magnesium compound 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 MgCr O , Mgl¾ and MgA 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 Enables complete . reaction of the chrome ore and magnesia which does not occur at temperatures substantially lower than those above.

To achieve 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 2. 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 step of the present invention typically has a bulk density range of 3. 3.0 to 3. 60 g/ cc, preferably 3. 40 tion temperature of about 3200°F if about 90% 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 percent.

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, 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 2800°F 'and preferably in the range of 3000 to 3300°F. At :, present, it is preferred to fire the refractory shape at. about 3200°F. burning or calcination step to form a dense MgO grain whereas the production of conventional 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 magnesia- chrome grain structure and can be processed in a conventional manner into brick shapes which sinter to a very high density in conventional tunnel kilns. The resulting brick has a density and porosity comparable to brick produced by the melt solidification method. The brick has a monolithic structure and is wi thout gross voids. The brick has a continuous microstructure 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 V #, the art without departing 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 A high lime-to-siliea ratio pre-reacted grain is prepared in this example. Magnesium hydroxide filter cake and Philippine chrome * ratio of 55 parts of filter cake solids on an MgO basis and 45 parts of chrome ore. The composition of the chrome ore and* the filter cake are I set forth in Table I. The overall lime-to-silica ratio of the filter cake i . ' and chrome ore initially is adjusted to 1. 5 by the addition of 0 . 55% calcined dolomite.

TABLE I SiO Fe Q„ . Al O CaO M O B?0„ Cr Q MgO (Calcined basis) 0. 60 0. 23 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 3200 °F with a one hour hold at peak temperature in a rotary furnace.

;The resulting grain has a 3. 53 g/ cc bulk density. This, grain is crushed : to: 40% -4 + 1.0 mesh 10% -10 + 20 mesh 8% -20 + 48 mesh 7% -48 mesh 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/ cc. The brick is conventionally fired to 3200 °F. The fired brick has a final density of 3. 49 g/ cc and 5. 6% porosity. This result is totally unexpect6d for this type of refractory i' synthetic magnesium hydroxide in filter cake form is prepared. The ! composition of the magnesium hydroxide on an oxide basis is 0. 62% \ SiO , 0. 20% Fe O3, 0. 29% Al O , 0. 60% CaO, 98. 17% MgO, and 0. 12% ■ 2 2 Λ : BgOg. The base batch is wet mixed and 1. 67% talc is added to produce a composition having a low lime to silica ratio of 0. 2. . The mixed slurry is dried and then pelletized and fired to 3200 °F in a rotary furnace. The !! resulting grain densit is 3. 53 g/ cc. The grain is crushed, and then 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 3200°F. The resulting grain has a density of only 3. 23 g/ cc.

This example thus illustrates the importance of haying very f inely divided chrome ore to produce a dense grain under the above temperature and pressure conditions.

The invention in its broader aspects is not limited to the specific details shown and described and departures may be made from such details without departing from the -principles of the invention and without sacrificing its chief advantages. ·

Claims (1)

1. · . · · ■ ; ■ · ■ ' WHAT IS CLAIMED IS: 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 grain that can be shaped and fired to a final shaped product without further processing. ) · ■ * . · ' • ■ ' . , · 8. The process of claim 7 wherein the chrome ore has an average particle size of about 5 /mi. 9. "the process of claim 1 in which the magnesium compound is magnesium hydroxide. 10. The process of claim 9 wherein the magnesium hydroxide and chrome ore are mixed in a weight ratio of about 30 to 80 parts magnesium hydroxide, on an oxide basis to about 70 to 20 parts of chrome ore. 11. The process of claim 9 wherein the calcining temper ature is maintained at about 3200°F and at least 90% by weight of the chro ore particles are -325 mesh. 12. - The process of claim 1 1 wherein "the average particle size of the chrome £>re particles are between about 5 and 10 jmi. t ( t 17. The process of claim 1 wherein the magnesium compound is MgCl^. 18. The process of claim 1 wherein the mixture is dried and pelletized before being calcined. · 19. 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. • 20. A refractory shape produced in accordance v/ith the process of claim 19. 21. A pre-reacted refractory grain produced in accordance with the process of claim

1. Tel-Aviv , B0vem er 20, 1974