Showing papers on "Magnesite published in 1999"

Low-temperature nucleation of magnesite and dolomite

Abstract: By way of duplicating an experiment described by L iebermann (1967) nucleation of magnesite, huntite and/or dolomite has been attaine d at temperatures between 313 K and 333 K and under atmospheric pressure. Essential to these experiments are fluctuations in pH value. After interrupting an experiment after 1, 3, 5, or 8 of such fluctuations, the change from one or more metastable phases into the stable phase (magne site or dolomite) could be followed. A theoretical explanation for these low-temperature s yntheses can be found in stability relations. Ostwald's Rule stipulates the nucleation of a metas table phase before that of the stable phase. However fluctuations of sufficient amplitude and du ration are capable of crossing the border between the metastable and the stable fields. As a re ult the stable phase will nucleate together with the metastable phase. Conditions opposing the subsequent growth of the metastable phase (such as the slightly acidic conditions resulting f rom periodically introducing CO 2 into the solution) will favour the continued growth of the s table phase. K e y w o r d s : Magnesite, dolomite, huntite, l ow-temperature syntheses.

Carbonation and decarbonation of eclogites: the role of garnet

Abstract: Carbonates are potentially significant hosts for primordial and subducted carbon in the Earth's mantle. In addition, the coexistence of carbonate with silicates and reduced carbon (diamond or graphite), allows constraints to be placed on the oxidation state of the mantle. Carbonate-silicate-vapor reactions control how carbonate + silicate assemblages may form from carbon-bearing vapor + silicate assemblages with increasing pressure. In olivine-bearing rocks such as peridotite, considered the dominant rock type in the upper mantle, the lowest-pressure carbonate-forming reactions involve olivine (±clinopyroxene) reacting with CO2 (e.g., Wyllie et al. 1983). In eclogitic rocks, the essential mineral assemblage is omphacitic clinopyroxene + garnet, without olivine. Therefore, alternative carbonate-forming reactions must be sought. The carbonation of clinopyroxene via the reaction dolomite + 2 coesite = diopside + 2 CO2 was studied experimentally by Luth (1995). The alternative possibility that garnet reacts with CO2 is explored here by determining the location of the reaction 3 magnesite + kyanite + 2 coesite = pyrope + 3 CO2 between 5 and 11 GPa in multi-anvil apparatus. At the temperatures ≥1200 °C, carbonation of eclogitic rocks with increasing pressure will proceed initially by reaction with clinopyroxene, because the pyrope-carbonation reaction lies at higher pressures for a given temperature than does the diopside-carbonation reaction. Diluting the pyrope component of garnet and the diopside component of clinopyroxene to levels appropriate for mantle eclogites does not change this conclusion. At lower temperatures, appropriate for “cold” slabs, it is possible that the converse situation will hold, with initial carbonation proceeding via reaction with garnet, but this possibility awaits experimental confirmation. Decarbonation of an eclogite under “normal mantle” geothermal conditions by a decrease in pressure, as in an ascending limb of a mantle convection cell, would be governed by the formation of clinopyroxene + CO2. At higher pressure than this reaction, any CO2 produced by the breakdown of magnesite reacting with kyanite and coesite would react with clinopyroxene to produce dolomite + coesite. Release of CO2 from eclogite into mantle peridotite would form carbonate at sub-solidus conditions and produce a dolomitic carbonate melt if temperatures are above the peridotite-CO2 solidus.

Expansion mechanism of alkali–dolomite and alkali–magnesite reaction

Abstract: The aim of this paper is to give a better understanding of the mechanism of alkali–carbonate reaction, which is essential to solve this durability problem in concrete structures. After studying the microstructural and compositional changes of alkali–dolomite and magnesite reactions both in KOH solution and in cement paste under 150°C autoclaving condition, the authors proposed that the formation and growth of reaction products in a confined space generated the expansion force, which was responsible for alkali–carbonate reaction expansion.

Secondary phases in natural magnesite sintered with addition of titania, ilmenite and zirconia

Abstract: The detail microstructures of sintered natural magnesite with addition of titania, zirconia and ilmenite mineralisers were studied using scanning electron microscopy (SEM) with special emphasis on energy dispersive X-ray analyses (EDXA) The study was directed towards the determination of secondary phase composition in sintered magnesite with different mineraliser addition In order to evaluate the secondary phases with addition of transition metal oxides, keeping titania (TiO2) as a reference mineraliser, a natural analog of titania containing iron ie, Ilmenite (Fe2O3 · TiO2) and pure Zirconia (ZrO2) were added to magnesite which was then sintered up to 1700 °C The relative behaviour of these pure and mixed oxide mineralisers during the sintering of magnesite has been evaluated with respect to their microstructure and secondary phase composition The present paper showed that titania addition to natural magnesites resulted in the formation of calcium titanate in the secondary phase upon sintering Zirconia, on the other hand, did not show any reaction product in the secondary phase The addition of titania in the form of mixed oxide, ilmenite, also gave rise to calcium titanate formation, providing a similar microstructure, specially with reference to spatial distribution of phases in the secondary phase

Origin of giant stratabound deposits of magnesite and siderite in Riphean carbonate rocks of the Bashkir mega-anticline, western Urals

Abstract: In the Bashkir mega-anticline (western Urals) stratabound magnesite, siderite, fluorite and base-metal deposits are hosted by a sequence of Riphean sediments with a thickness of more than 12 km. The giant deposits of siderite (Bakal) and sparry magnesite (Satka) belong to the largest known mineral deposits of this type on Earth but are still disputed with respect to their origin. Both the Fe- and Mg-carbonate ores are clearly characterized by mimetic preservation of sedimentary and diagenetic textures of the host carbonate sediments, giving evidence of epigenetic metasomatic replacement. In the stratiform magnesite deposits of Satka, O- and C-isotopes, REE pattern and the lithostratigraphic position of the ore, point to the reflux of early diagenetic Mg-rich brines being responsible for the selective replacement of brecciated dolomite. The Bakal siderite deposits are hosted by Lower Riphean carbonate rocks and are controlled by a Lower to Middle Riphean unconformity marked by deep erosion and subsequent transgression-related sedimentation of coarse clastics. Their independence of carbonate lithofacies and their trace element distribution are indicative of metasomatic processes. Fe-bearing fluids have probably been generated by low-grade metamorphic (catagenetic) devolatization from underlying argillites, causing the metasomatic formation of large siderite ore bodies in the Bakal carbonates due to the focusing of the fluid flow by the overlying Mid-Riphean coarse clastics.

Genesis of diamonds in the lower mantle

Abstract: The “forbidden” assemblage (ferropericlase + enstatite) as inclusions in diamonds has been taken as evidence to imply that these inclusions and their host diamonds formed initially in the lower mantle. Magnesite is probably the only stable carbonate at depths greater than ∼220 km. Like dehydration reactions, the reaction boundary for the decarbonation of magnesite has a positive dT/dP slope at lower pressures, which becomes negative at higher pressures, if no other phase intervenes. This reaction boundary probably intersects the geotherm between ∼900 and ∼1100 km, below which magnesite decomposes into an assemblage periclase + diamond + oxygen. Thus, ferropericlase is the most likely inclusion in diamond formed in the lower mantle. The high frequency of sole occurrence of ferropericlase in diamonds from Sao Luiz, Brazil seems to substantiate the present speculation.

Method for the production of a high-purity concentrated magnesium chloride solution by acid leaching and neutralization

Abstract: The present invention is concerned with a method for the extraction of magnesium from magnesium-containing materials such as magnesite, dolomite and silicates materials like serpentine residues, products from the amphibole or pyroxene group. The magnesium-containing material is first leached with hydrochloric acid in at least lone leaching reactor, and subsequently submitted to a series of at least three neutralizing steps in separate reactors under controlled pH and oxydo-reduction potentials. The slurry produced is filtered to produce a substantially pure magnesium chloride solution containing less than 1 ppm of each contaminant (Fe, Ni, Mn etc.) present therein.

Features and Genesis of Vein-Type Magnesite Deposit in the Doddakanya Area of Karnataka, India

Abstract: Vein-type magnesite deposit of Doddakanya area is situated in the southern part of the Archaean Dharwar craton. Magnesite is confined to a c.3.3 Ga old ultramafic body which constitutes a tectonically dismembered part of a layered ultramafic-mafic complex emplaced into supracrustal rocks of the Sargur Group. The magnesite-bearing ultramafic body is composed of dunite with minor harzburgite and exhibits varying degrees of serpentinization and weathering. About 50 vol.% of the ultramafic body is serpentinized and the intensity of serpentinization decreases with depth. The serpentine minerals are represented essentially by lizardite/chrysotile exhibiting pseudomorphic mesh-texture. Serpentinization of the ultramafic body took place within 6 km of the Earth's surface at low temperatures (<200°C) and essentially involved volume expansion (isochemical) process. During weathering, the relict olivine of the serpentinized ultramafic body was pseudomorphously replaced by a mixture of amorphous material (with variable amounts of Fe, Mg, Si and volatiles) and minor ferric oxyhydroxides and secondary silica. The Mg mobilized during weathering participated in the formation of magnesite in tensional fractures developed during the late- to post-serpentinization event. Oxygen isotope data (δ 18 O = av. 30.6‰ SMOW) indicate the precipitation of magnesite at ambient temperatures and carbon isotope values (δ 13 C = av. -2.6‰ PDB) suggest the derivation of carbon from a biogenic soil source dominated by C4 plants. The latter constrains the timing of the magnesite formation to a period not older than 8 - 7 Ma.

Economic potential of the ultramafic rocks of Jamaica and Tobago: two contrasting geological settings in the Caribbean

Abstract: Ultramafic rocks in Jamaica are dunites with minor lherzolite, often serpentinised, and are part of a dismembered ophiolite complex. In Tobago, dunites, wehrlites, pyroxenites and hornblendites form the lower part of a plutonic complex of island arc affinity. The mineral assemblages and chemistry reflect these differences. Chromite in Jamaica is high in Al and Mg, whereas in Tobago it is rich in Fe, as in Alaskan-type intrusives. Ni-Cu-PGE assemblages in Jamaica are pentlandite, with later low temperature heazlewoodite, awaruite and native copper, the latter with Pt and Pd. In Tobago an assemblage of pentlandite, pyrrhotite, pyrite and chalcopyrite is much less affected by later alteration. PGE phases also occur. The dunites in Jamaica have sufficient MgO to be a potential source of olivine. The higher Fe in olivine from Tobago indicates that olivine cumulates in plutonics from island arc settings are a less suitable source of the mineral. Ni-laterites in Jamaica are unlikely because of high topographic relief. The prospect for Ni-laterites in Tobago is low as there is little Ni in the olivines. Chrysotile asbestos, talc and magnesite are absent in both islands. This is probably a consequence of the lack of secondary serpentine recrystallisation to form fibrous chrysotile veins, the deep tectonic level and lack of hydrothermal circulation for magnesite to form, and the absence of metamorphic/metasomatic events and/or late stage extension tectonics which might have yielded talc.

Technology for producing magnesium sulfate in treatment of titanium white waste sulfuric acid

Abstract: A process for preparing magnesium sulfate during the treatment of waste sulfuric acid solution in the production of powdered titanium oxide includes such technological steps as mixing said waste soltion with magnesite or magnesium oxide, stirring, ion exchange reaction, pressure filter to obtain supernatant of magnesium sulfate, evaporation and concentration, cooling crystallization and centrifugal filter to obtain solid magnesium sulfate Its advantages include simple process, low cost and reuse of filtered dregs to prepare compound S-Mg fertilizer

Analysis of limestones and dolomites by x-ray fluorescence*

Abstract: Sources of calcium are generally widespread and quite extensive. These sources are of limestone, d o lomite, marl, chalk, and oyster shell. Cement plants account for nearly half the demand while more than two hundr ed lime plants in the United States and Puerto Rico consume about twenty five percent. Since the chemical composition of the limestone or other sources of calcium is critical in the cement and the lime industry, particularly for the dele terious compounds such as Na 2O, MgO, P 2O 5, and K 2 O, accurate determinations are critical. Due to the tonnage per hour, these determinations must be made rapidly and accurately. X-ray fluorescence can thereby satisfy this need for accuracy and p recision. Production of lime is performed by calcining limestone and the industry is generally located and c o ncentrated in the States of Michigan, Pennsylvania, and Missouri. The resulting product is quicklime-CaO, or hydrate d lime-Ca(OH)2. Substantial amounts of quicklime is further processed into calcium carbide in order to produce acet ylene gas. In this case, the determination of P 2 O 5 is critical since minor amounts of phosphorous in the acetylene gas can cause premature explosions. Other uses for lime are well known in the treatment of water, the paper and pulp indust ry, and the steel industry for the production of slag to remove impurities. Dolomitic lime is heavily utilized in the manufacture of magnesite refractories by reacting the dolomitic lime with brines from the Michigan basin to produc e MgO and CaCl 2 . Sample preparation for these materials has been performed by grinding and pelletizing or fusion wit h Li 2B 4O 7. In addition to the chemistry, matrix and interelement effects will be discussed as related to the che m ical analysis by X-ray fluorescence.

Refractory Mg-Al-Cr material and its production process

Abstract: The refractory material is compounded with metal chromium sludge in 90 wt% and magnesite in 10-90 wt%, the metal chromium sludge is industrial sludge produced during high- temperature smelting of metal chromium at 2200 deg.c and the magnesite is metallurgical magnesite or electric smelting magnesite. It is prepared through the processes of crushing the material to the size of 0.042-20 mm, magnetic separation to eliminate ferromagnetic matter, sieving, mixing while adding mineralizing agent in the amount of 0.5-13% total weight of the mixture material.

Magnesium and sulphur mineral fertilizer and a procedure for its preparation

Abstract: The mineral fertilizer, based on magnesium sulphate, comprises 16-30% of total MgO, 12-30% of MgO soluble in water and 30-50% of sulphur (%SO3) and preferably, it is found in granulated form. The fertilizer may be obtained by solubilizing with sulphuric acid, a mixture which contains magnesium formed by products coming from the extraction and processing of magnesite, such as cyclone powder, magnesium hydroxide and magnesium calcium carbonate. The fertilizer is capable of supplying magnesium to soils, crops and fields which require the same.

Special features of the mineral composition of magnesite from deep horizons of the Satkinskoe deposit

Abstract: Some special features of the mineral composition of magnesite from deep horizons of the Satkinskoe group of deposits is considered. Minerals that are paragenetically connected with magnesite and affect substantially the production process and quality of refractory products are described. It is established that the concentration of SiO2 in the magnesites increases regularly with increase in the depth of the horizon. Ways to solve the problem of fabricating quality magnesite for roasting are suggested.

Thermodynamic and technico-economic analyses of the HCl-leach of magnesite and serpentine

Abstract: Commercial HCI-leaching of magnesite and serpentine yields highly concentrated aqueous solutions containing 25 mass % to 33 mass % MgCl 2 . The partial molar enthalpies and heat capacities, the ionic activities of Mg 2+ and Cl - , and the Raoultian activity of H 2 O have been assessed for the concentrated MgCl 2 -H 2 O solutions at temperatures of 348 to 373 K. These thermodynamic data are used to calculate the pH of aqueous MgCl 2 solutions saturated with a Mg-bearing mineral such as magnesite, serpentine, brucite, forsterite, enstatite or talc. On the basis of these thermodynamic data, technico-economic aspects are discussed as to neutralization with pulverized MgCO 3 , water evaporation from the MgCl 2 solutions, and ore composition including such minor constituents as CaCO 3 , FeCO 3 and Fe 3 O 4 .

Process for continuously smelting electrofused magnesia

Abstract: Using magnesite or powdered magnesium oxide as raw material and by adopting the phocess of continuously smelting electrocast magnesite clinker with the feature of kiln structure and operation regime timed descending casting column. The said process in contrast with existent technology may raise the yield of electrocast magnesite clinker by 10-20%, save the energy consumption by 15-30%, raise economic benefit by 2-3 times increase quality product by 35%.

Effects of Roasting Conditions of Magnesite in Northern Gansu on the Quality of Magnesium Cement

Abstract: Through roasting experiments discussions were made for the effects of roasting temperature on the decomposition ratio of magnesite and activity of magnesium oxide.Furthermore,the relation of the activity of magnesium oxide and the mechanical quality of magnesium cement were investigated.The optimum roasting process conditions were also decided for the elemination of the unfavorable influence of the Fe\-2O\-3 impurity on the activity of magnesium oxide.\;

Method for producing ferric-rich magnesite clinkers and shaped pieces on their base, by microwave firing

Abstract: Before microwave firing it is necessary the magnesite raw of breuneritic type containing ferric oxide over 4 % wt. of total weight to preheat to at least 700 C to obtain an activity for microwave heating. Microwave firing of shaped building materials from high iron magnesite sinter prepared by firing in conventional furnaces is possible also without previous preheating. Their absorben abilities are reached by creation of primary and secondary spinel by conventional firing the sinter.

Smelting method for high-grade magnesia sand by electric melting

Abstract: A method for smelting high-grade electrocast magnesite clinker features that the magnesite whose content of MgO is higher than 46.5% is used as raw material, and during smelting, rare earth-magnesiumalloy and magnesium fluoride are added, and the raw material and the additive are charged layer by layer, resulting in perfect crystallization, high volume density and high quality of its product, and low consumption of energy.

High-density periclase clinker: A promising material for refractory production

Abstract: Data on testing sintered periclase clinker obtained from flotation magnesite concentrate for the production of refractory parts are presented. It is shown that by organizing production of high-density periclase powders in the plant from flotation magnesite concentrate with the minimum possible content of impurity SiO2 and CaO oxides the quality of the products can be improved with a much longer service life provided for linings of heating installations.