Analysis and assessment of nickel and chromium pollution in soils around Baghejar Chromite Mine of Sabzevar Ophiolite Belt,Northeastern Iran

The key objective of this research was to estimate the Ni and Cr contents of soil around the Baghjar Chromite Mine（BCM）of Sabzevar Ophiolite Belt,Northeastern Iran,and assess the degree of soil pollution using the pollution indices.Soil samples（0-20 cm depth） were collected at various distances from the BCM.In the present research,heavy metals（Cr and Ni） in soil samples were analyzed by atomic absorption spectrometry to detect their concentrations and contour maps were produced to explain the metal spatial distribution.Also,the degree of metal pollution was quantified.The results indicate that the soils in the studied area are contaminated by Cr and Ni.The corresponding concentrations for Cr and Ni are（156.19±24.45） and（321.7±133.27） mg/kg,respectively,which exceed the corresponding maximum allowable concentrations in soils.The different indices demonstrate that soils around chromite mine are significantly contaminated with Cr and Ni,suggesting several times higher levels of toxic metals than normal ranges.The above results revealed that the heavy metal concentrations increase with increasing the distance from the mine and mining pollutants can be transported to long distances from their sources.

Comparison of Different-sized Chromite Mineralizations in the Yarlung-Zangbo Ophiolite Belt, Southern Tibet

Podiform chromitites are characteristically occurred in ophiolites(e.g.,Thayer,1964;Dickey,1975).However,the metallogenic processes for podiform chromitites are still unclear.Early models involved fractional crystallization and crystal settling from picritic or basaltic melts in magma chambers(Dickey,1975;Boudier and Coleman,1981),but it was also proposed that podiform chromitites formed from partial melting and melt extraction in host mantle peridotites(Dick,1977;Dick and Bullen,1984).Recent studies by the majority of authors have suggested that melt-rock interaction at the Moho transition zone may have played a key role in the formation of podiform chromitites(Zhou and Robinson,1994;Zhou et al.,1996,2005,2014;Robinson,2008;Page and Barnes,2009;Uysal et al.,2009,2012;González-Jiménez et al.,2011,2015).Based on the occurrence of some ultrahigh pressure minerals(e.g.diamond and coesite)in chromitites,it has been proposed recently that the formation of podiform chromitite is likely related to multiple processes inclusing mantle recycling(Yang et al.,2007;Yamamoto et al.,2013).Although geat progresses have been made towards understanding the genesis of podiform chromitites,some fundamental issues in remain unanswered.For examples,what are the major controls on the size of chromitites?And why some ophiolites contain large podiform chromitite bodies,whereas most ophiolitic massifs are essentially chromitite-barren? The Yarlung-Zangbo Ophiolite belt is one of the most famous ophiolite zone in the world.It contains fresh peridotites as well as different-sided podiform chromitites.The Luobusha ophiolite in the eastern segment of the belt hosts the largest chromite deposit in China.In the central and western segments of belt the Dazhuqu and Dongbo ophiolitic massifs contain some small-scale chromitite bodies.Such characteristics make the Yarlung-Zangbo Ophiolites an ideal subject to investigate the major controls on the metallogenesis of podiform chromitites. The Luobusha chromitites are large lens and enclosed in dunite.In contrast,the Dazhuqu and Dongbo chromitites display generally as narrow dykes or irregular seams with dunite envelopes.The closely spatial association of the chromitites and dunite envelopes,together with their textural features,support a petrogenetic model that the chromitites from the Luobusha,Dazhuqu and Dongbo massifs form from reaction of melt with host peridotite.In terms of chemical composition of chromite,there are distinctive differences between those from the Luobusha and the Dazhuqu or the Dongbo.Chromite from the Luobusha chromitites has high Cr#(71-82),whereas Chromite in the Dazhuqu chromitites show relatively low Cr#(16-63),and chromite in the Dongbo chromitites includes low Cr#(11-47)and high Cr#(70-81)types.For the Dongbo and Dazhuqu massifs,linear trends of Cr#with Mg O,Fe Ot,Ni,Ga,V and Sc in chromite from the chromitites and dunites of are similar to those of the host peridotites,suggesting that the melt-rock reaction may provide major budget of Cr for the chromitites.The similar compositions at a given Cr#in chromite from these rocks also demonstrate that the chromitites may have been formed by in-situ crystallization of chromite under low melt/rock ratio.In contrast,the Luobusha chromitites have different trends of compositions in chromite from that of the host peridotites,implying that the formation of the chromitite bodies requires a continual replenishment of Cr-rich melts from deeper mantle.Fractionation and accumulation of chromite from a large volume of Cr-rich melt may play an important role on the formation of the Luobusha chromitites.MORB-normalized trace element patterns of chromite from the Luobusha chromitites suggest that it has been formed from Cr-rich boninitic melt at surpra-subduction zone(SSZ)setting.However,the Dongbo and Dazhuqu chromitites have formed originally from a MORB-affinity melt at a mid-ocean ridge(MOR)environment. In summary,the Luobusha chromitites crystallized from a Cr-rich melt in a dynamic conduit,where fractional crystallization and crystal settling play a key role in formation of the large chromitites.In contrast,the small-scale mineralizations of the Dongbo and Dazhuqu chromitite pods are formed from in situ produced melts.Podiform chromitites can be formed in MOR environment,whereas the higher Cr content in boninitic melt and assimilation of subducted slab materials at SSZ setting may benefit the formation of large chromite deposit.

Occurrence and Evaluation of Chromium Reducing Bacteria in Seepage Water from Chromite Mine Quarries of Orissa, India

Chromium pollution due to leaching and weathering of chromite mine overburden in mine seepage water is a serious environmental problem. In an attempt to explore the indigenous chromate resistant and reducing bacteria from chromite mine quarry seepage, 145 aerobic, heterotrophic bacteria were isolated from 14 mine seepage samples derived from chromite mines of Sukinda valley and Baula-Nuasahi belt in Orissa, India. A total of 109 isolates which were tolerant to 2 mM Cr(VI) showed different degrees of Cr(VI) reducing activ-ity in complex KSC medium and synthetic Vogel Bonner (VB) broth. Ten isolates belonging to genera like Pseudomonas, Acinetobacter, Alcaligenes, Cupriavidus and Corynebacterium were selected on the basis of their chromate reducing efficiency and it was found that they could reduce more than 50 and 75% of Cr(VI) in VB broth and KSC medium respectively. The sole Gram-positive isolate, Corynebacterium paurometabo-lum SKPD 1204 (MTCC 8730) showed maximum chromate reducing capacity in both VB broth (63.7%) and KSC medium (92%) and was capable of reducing nearly 95% of the total Cr(VI) in the mine seepage when it was grown in the mine seepage supplemented with 2% VB concentrate.

Optimization of Chromate Reduction by Whole Cells of <i>Arthrobacter</i>sp. SUK 1205 Isolated from Metalliferous Chromite Mine Environment

Arthrobacter sp. SUK 1205 isolated from metalliferous chromite mine environment of Orissa, India showed wide degree of tolerance to heavy metals including Cr(VI), variety of antibiotics and was also capable of reducing Cr(VI) during growth. Freshly grown whole cells of this bacterium were evaluated for chromate reduction under batch culture using Vogel Bonner (V. B.) broth as the base. Cells of SUK 1205 were capable of completely reducing 100 μM Cr(VI) in V. B. broth within 48 h of incubation. Reduction of chromate increased with increase in cell density which attained maximum at 1010 cells/ml, however, reverse was the phenomenon when the concentration of Cr(VI) increased gradually. Glycerol, glycine and glucose promoted chromate reduction efficiency of the cells when used as electron donors. Optimum pH and temperature were found to be 7.0 and 35°C respectively. The process of reduction was inhibited by Ni(II), Mn(II), Zn(II) and Co(II), but Cu(II) and Fe(III) was promotive in nature. On the other hand, 2, 4-dinitrophenol was found to be neither promotive nor inhibitory for the reduction process, but carbonyl cyanide-m-chloro phenyl hydrazone, sodium azide, sodium fluoride and N,N,-dicyclohexyl carboiimide were inhibitory. Cells of SUK 1205 when permeabilized with toluene, triton X-100 and tween 80 showed an enhancement of the process and thereby indicated that reduction of Cr(VI) was mainly associated with soluble component of the cells. Arthrobacter sp. SUK 1205, therefore, showed great promise for use in Cr(VI) detoxification under a wide range of environmental conditions.

Distinctive melt activity and chromite mineralization in Luobusa and Purang ophiolites, southern Tibet: constraints from trace element compositions of chromite and olivine

To investigate the factors controlling the mineralization in ophiolites we systematically compared the petrology and mineral compositions of the harzburgites/lherzolites, dunites and chromitites in the Luobusa and Purang ophiolites. Generally, the petrological features and trace element compositions of chromite and olivine in peridotite and chromitite are distinctly different between the two ophiolites.In Luobusa, boninitic melts are inferred to have interacted with the harzburgites and modified the distributions of some trace elements(e.g., Ni, Mn and V) in chromite and olivine. The subsequently formed dunites and chromitites experienced significant elemental exchange. In contrast, the Purang ophiolite contains a wider range of chromitite compositions and records diverse melt activities, such as the growth of relatively abundant secondary clinopyroxene. The metasomatic melts were enriched in Al and depleted in Si, Na and highly incompatible trace elements(e.g., Nb, Zr). Such melts resemble MORBlike melts proposed in the literature but are assumed to be more hydrous than typical MORB because of presence of hydrous minerals. The parental magmas of the Purang dunites and intermediate chromitites are inferred to be compositionally intermediate between boninitic and MORB-like melts. In addition,the more refractory nature of the Luobusa harzburgites facilitated a high Cr concentration gradient with the interacting melts, making it easier to increase Cr in the melts. Crystallization of clinopyroxene and amphibole in the Purang ophiolite accommodated significant amounts of Cr and water, respectively,and negatively affected Cr concentration and chromite crystallization. The concentration of chromite to form chromitites requires the presence of focused melt channels.

Distribution Characteristics of Minerals and Elements in Chromite Ore Processing Residue

In this paper, environmental scanning electron microscopy （ESEM） is applied to characterizing the mineral and element distribution of chromite ore processing residue （COPR）. The test results show that Cr-bearing brownmillerite occurs in the rim of COPR particle, while hydroandradite with Cr （Ⅵ） in its structure presents inside the COPR particle. Periclase and calcite occur in the interstitial area. Element analyses show that Ca, Fe and Al are distributed throughout the COPR particle, and Mg exists mostly in the interstitial area or on the particle surface. A lower content of Cr is evenly distributed in the COPR particle, while slightly higher concentration of Cr occurs inside the particle. It is suggested that it will take a relatively longer time for Cr to migrate out of COPR, especially fbr hexavalent chromium, so the leaching time and the particle size may be two important factors to affect the release of Cr （Ⅵ）.

New Data on the Mineralogy of Chromite from the Nuggihalli Schist Belt,Western Dharwar Craton,Karnataka,India:Petrogenetic Implications

The occurrence of rhythmic layering of chromite and host serpentinites in the deformed layered igneous complexes has been noticed in the Nuggihalli schist belt （NSB） in the western Dharwar craton, Karnataka, South India. For this study, the chromitite rock samples were collected from Jambur, Tagadur, Bhakatarhalli, Ranganbetta and Byrapur in the NSB. Petrography and ore microscopic studies on chromite show intense cataclasis and alteration to ferritchromite. The ferritchromite compositions are characterized by higher Cr number （Cr/[Cr＋AI]） （0.68-0.98） and lower Mg number （Mg/[Mg＋Fe]） （0.33-0.82） ratios in ferritchromite compared to that of parent chromite. The formation process for the ferritchromite is thought to be related to the exchange of Mg, AI, Cr, and Fe between the chromite, surrounding silicates （serpentines, chlorites）, and fluid during serpentinization.

Geochemistry and mineralogy of platinum-group elements(PGE)in chromites from CentralnoyeⅠ,Polar Urals,Russia

The Polar Urals region of northern Russia is well known for large chromium （Cr）-bearing massifs with major chromite orebodies, including the Centralnoye I deposit in the Ray-Iz ultramafic massif of the Ural ophiolite belt. New data on platinum （Pt）-group elements （PGE）, geochemistry and mineralogy of the host dunite shows that the deposit has anomalous iridium （Ir） values. These values indicate the predominance of ruthenium--osmium--iridium （Ru--Os--Ir）-bearing phases among the platinum-group mineral （PGM） assemblage that is typical of mantle-hosted chromite ores. Low Pt values in chromites and increased Pt values in host dunites might reflect the presence of cumulus PGM grains. The most abundant PGM found in the chromite is erlichmanite （up to 15 μm）. Less common are cuproiridsite （up to 5 μm）, irarsite （up to 4--5 μm）, and laurite （up to 4 μm）. The predominant sulfide is heazlewoodite, in intergrowth with Ni--Fe alloys, sporadically with pentlandite, and rarely with pure nickel. Based on the average PGE values and esti- mated Cr-ore resources, the Centralnoye I deposit can be considered as an important resource of PGE.

Microbial leaching of chromite overburden from Sukinda mines,Orissa,India using Aspergillus niger

Leaching of nickel and cobalt from two physical grades （S1, 125-190 μm, coarser and S3, 53-75 μm, finer） of chromite overburden was achieved by treating the overburden （2% pulp density） with 21-d culture filtrate of an Aspergillus niger strain grown in sucrose medium. Metal dissolution increases with ore roasting at 600℃ and decreasing particle size due to the alteration of microstructural properties involving the conversion of goethite to hematite and the increase in surface area and porosity as evident from X-ray diffraction （XRD）, thermogravimetry-differential thermal analysis （DT- TGA）, and field emission scanning electron microscopy （FESEM）. About 65% Ni and 59% Co were recovered from the roasted S3 ore employing bioleaching against 26.87% Ni and 31.3% Co using an equivalent amount of synthetic oxalic acid under identical conditions. The results suggest that other fungal metabolites in the culture filtrate played a positive role in the bioleaching process, making it an efficient green approach in Ni and Co recovery from lateritic chromite overburden.

KT Boundary Chromites Determined to be Terrestrial:Cr Isotopic Evidence for Excavation and Ejection of Mafic/Ultramafic Rocks by the KT Boundary Impact

Evidence for a mantle and/or basaltic component in KT boundary distal ejecta is apparently inconsistent with ejection from Chicxulub Crater since it is located on;5km thick continental crust（De Paolo et al.,1983;Montanari et al.,1983;Hildebrand and Boynton,1988,1990）.Evidence for mafic/ultramafic target rocks was reinforced by discovery of chromites,some with shock planar deformation features（PDF）,in impact layer samples from sites in southern Colorado and eastern Wyoming（Bohor et al.,1990）.However,until now it was unclear whether the chromites originated with an impactor or with terrestrial target rocks.To this end,high-precision 54Cr/52Cr isotope ratios were measured on KT boundary chromites along with known terrestrial chromites.We find a terrestrial 54Cr/52Cr ratio in KT boundary chromites from impact layer samples collected at the above sites over the last several years（Fig.1）.Ejected terrestrial chromites suggest the impact sampled terrestrial mafic and/or ultramafic target rocks not known to exist in the Chicxulub target area.

Kinetics of solid-state reduction of chromite overburden

The demand for alternative low-grade iron ores is on the rise due to the rapid depletion of high-grade natural iron ore resources and the increased need for steel usage in daily life.However,the use of low-grade iron ores is a constant clinical task for industry metallurgists.Direct smelting of low-grade ores consumes a substantial amount of energy due to the large volume of slag generated.This condition can be avoided by direct reduction followed by magnetic separation(to separate the high amount of gangue or refractory and metal parts)and smelting.Chromite overburden(COB)is a mine waste generated in chromite ore processing,and it mainly consists of iron,chromium,and nickel(<1wt%).In the present work,the isothermal and non-isothermal kinetics of the solid-state reduction of self-reduced pellets prepared using low-grade iron ore(COB)were thoroughly investigated via thermal analysis.The results showed that the reduction of pellets followed a firstorder autocatalytic reaction control mechanism in the temperature range of 900-1100℃.The autocatalytic nature of the reduction reaction was due to the presence of nickel in the COB.The apparent activation energy obtained from the kinetics results showed that the solid-state reactions between COB and carbon were the rate-determining step in iron oxide reduction.

Petrogenesis of the Neoarchean zincian chromite within ultramafic xenoliths,Bastar Craton,India

The present study reports and discusses the genesis of zincian chromite in the ultramafic xenoliths from the Dongripali area,Bastar craton,Central India.The zincian chromite is in the ultramafic xenoliths of Bengpal supracrustal rock hosted by Neoarchaean Bundeli gneisses.Compositionally zincian chromite shows a range of Cr_(2)O_(3)(39.69 to 51.66 wt%),Al_(2)O_(3)(05.30 wt%to 08.71 wt%),FeO(21.74 wt%to 27.51 wt%),Fe_(2)O_(3)(10.19 wt%to 19.36wt%)with higher ZnO content ranging from 1.73 wt%to 4.08 wt%.Accordingly,their Cr#[Cr/(Cr+Al)]varies in a narrow range from 0.83 to 0.85.Its calculated melt composition supports metamorphic or post-magmatic nature rather than common occurrences such as inclusion in diamonds,meteorites,and association with any sulfide-rich mineralised belt.This reveals that the post-magmatic processes play a vital role in transforming chromite to zincian chromite.The empirical thermometric calculation from chromite,amphibole,and pyroxene support their metamorphic origin and formed during low-P and high-T amphibolite grade facies of metamorphism(~700℃).The Neoarchaean granitic magmatism has a significant role in generating and transferring the heat during contact metamorphism with hydration of ultramafic xenoliths and further alteration,i.e.,serpentinisation.The olivine is a major repository for Mn,Zn,and Co in peridotite/ultramafic;these elements get mobilised during the metamorphism and serpentinisation.This is a possible reason for the mobilisation of zinc and incorporation in the chromite within altered ultramafic.As a result,chromiterich ultramafic xenolith subjected to metamorphic process gets enrichment of Zn and Fe due to elemental exchange.It converts common chromite into zincian chromite,as reported in altered ultramafics elsewhere.

Origin of Listwanite in the Luobusa Ophiolite,Tibet,Implications for Chromite Stability in Hydrothermal Systems

Listwanite from the Luobusa ophiolite, Tibet, forms a narrow, discontinuous band along the eastern part of the southern boundary fault. We undertook a detailed petrographic and geochemical study to understand the mineral transformation processes and the behaviour of major and trace elements during listwanite formation. Three alteration zones characterized by distinct mineral components and texture are recognized and, in order of increasing degree of alteration, these are： zonem is rich in serpentine minerals; zonen is rich in talc and carbonates; and zone_Ⅰ is mainly composed of carbonates and quartz. Geochemical data for the three alteration zones show significant modification of some major and trace elements in the protolith, although some oxides show linear correlations with MgO. Gold mineralization is recognized in the Luobusa listwanite and may signify an important target for future mineral exploration. Gold enrichment occurs in both zone_Ⅰ and zone_Ⅱ and is up to 0.91 g/t in one sample from zonei. We show that CO_2-rich hydrothermal fluids can modify both the occurrence and composition of chromite grains, indicating some degree of chromite mobility. Low-Cr anhedral grains are more easily altered than high-Cr varieties. The compositions of chromite and olivine grains in the listwanite suggest a dunite protolith.

Thermodynamics of chromite ore oxidative roasting process

To explicate the thermodynamics of the chromite ore lime-free roasting process, the thermodynamics of reactions involved in this process was calculated and the phrases of sinter with different roasting times were studied. The thermodynamics calculation shows that all the standard Gibbs free energy changes of the reactions to form Na2CrO4, Na2O-Fe2O3, Na2O·Al2O3 and Na2O3 SiO2 via chromite ore and Na2CO3 are negative, and the standard Gibbs free energy changes of the reactions between MgO, Fe2O3 and SiO2 released from chromite spinel to form MgO-Fe2O3 and MgO·SiO2 are also negative at the oxidative roasting temperatures （1 173 1 473 K）. The phrase analysis of the sinter in lime-free roasting process shows that Na2O·Fe2O3, Na2O·Al2O3 and Na2O·SiO2 can be formed in the first 20 min, but they decrease in contents and finally disappear with the increase of roasting time. The final phase compositions of the sinter are Na2CrO4, MgO·Fe2O3, MgO·SiO2 and MgO. The results indicate that Na2CrO4 can be formed easily via the reaction ofNa2CO3 with chromite ore. Na2O·Fe2O3, Na2O-Al2O3 and Na2O·SiO2 can be formed as intermediate compounds in the roasting process and they can further react with chromite ore to form Na2CrO4. MgO released from chromite ore may react with iron oxides and silicon oxide to form stable compounds of MgO·Fe2O3 and MgO·SiO2, respectively.

Naquite, FeSi, a New Mineral Species from Luobusha, Tibet, Western China

A new mineral species, named naquite（FeSi）, is found in the podiform chromitites of the Luobusha ophiolite in Qusong County, Tibet, China. The detailed composition is Fe 65.65, Si 32.57 and Al 1.78 wt%. The mineral is cubic, space group P213. The irregular crystals range from 15 to 50 μm in diameter and form an intergrowth with luobusaite. Naquite is steel grey in color, opaque, with a metallic lustre and gives a grayish-black streak. The mineral is brittle, has a conchoidal fracture and no apparent cleavage. The estimated Mohs hardness is 6.5, and the calculated density is 6.128 g/cm3. Unit-cell parameters are a 4.486 （4） A, V 90.28 （6）A^3, Z=4. The five strongest powder diffraction lines [d inA（hkl） （I/I0）] are： 3.1742 （110） （40）, 2.5917（111） （43）, 2.0076 （210） （100）, 1.8307 （211） （65）, and 1.1990 （321） （36）. Originally called ＇fersilicite＇, the species and new name have now been approved by the CNMNC （IMA 2010-010）.

Platinum-group Mineral(PGM) and Base-metal Sulphide(BMS) Inclusions in Chromitites of the Zedang Ophiolite, Southern Tibet, China and their Petrogenetic Significance

Voluminous platinum-group mineral （PGM） inclusions including erlichmanite （Os,Ru）S2, laurite （Ru,Os）S2, and irarsite （Ir, Os,Ru,Rh）AsS, as well as native osmium Os（Ir） and inclusions of base metal sulphides （BMS）, including millerite （NiS）, heazlewoodite （NiaS2）, covellite （CuS） and digenite （Cu3S2）, accompanied by native iron, have been identified in chromitites of the Zedang ophiolite, Tibet. The PGMs occur as both inclusions in magnesiochromite grains and as small interstitial granules between them; most are less than 10 ~m in size and vary in shape from euhedral to anhedral. They occur either as single or composite （biphase or polyphase） grains composed solely of PGM, or PGM associated with silicate grains. Os-, Ir-, and Ru-rich PGMs are the common species and Pt-, Pd-, and Rh-rich varieties have not been identified. Sulfur fugacity and temperature appear to be the main factors that controlled the PGE mineralogy during crystallization of the host chromitite in the upper mantle. If the activity of chalcogenides （such as S, and As） is low, PGE clusters will remain suspended in the silicate melt until they can coalesce to form alloys. Under appropriate conditions of fS2 and fO2, PGE alloys might react with the melt to form sulfides-sulfarsenides. Thus, we suggest that the Os, Ir and Ru metallic clusters and alloys in the Zedang chromitites crystallized first under high temperature and low fS2, followed by crystallization of sulphides of the laurite-erlichmanite, solid-solution series as the magma cooled and fS2 increased. The abundance of primary BMS in the chromitites suggests that fS2 reached relatively high values during the final stages of magnesiochromite crystallization. The diversity of the PGE minerals, in combination with differences in the petrological characteristics of the magnesiochromites, suggest different degrees of partial melting, perhaps at different depths in the mantle. The estimated parental magma composition suggests formation in a suprasubduction zone environment, perhaps in a forearc.

Carbothermic reduction of chromite fluxed with aluminum spent potlining

Aluminum spent potlining (SPL) was employed as both the fluxing agent and a source of carbonaceous reductant for the carbothermic reduction of chromite, aiming to allow effective separation of alloy from the slag component. The experimental results show that the carbonaceous component of the SPL is more reactive towards chromite reduction compared to graphite. The formation of refractory spinel (MgAl2O4) on chromite particles hinders further reduction and alloy growth. The slag-making components of the SPL (e.g. nepheline and NaF) form molten slags at low temperatures (~1300℃) and partly dissolve the refractory spinel as well as the chromite. Destruction of the spinel layer with enhanced mass transfer greatly improves the alloy growth, which can be further promoted by reduction at a higher temperature (e.g. 1500℃). Ferrochrome alloy particles grow large enough at 1500℃ in the presence of SPL, allowing effective separation from the slag component using elutriation separation.

Leaching kinetics of acid-soluble Cr(Ⅵ) from chromite ore processing residue with hydrofluoric acid

Leaching kinetics of acid-soluble Cr（VI） in chromite ore processing residue （COPR） using hydrofluoric （HF） acid solution as a leaching agent was investigated for potential remediation of COPR with industrial waste water containing HF. The results show that HF can effectively destabilize the Cr（VI）-bearing minerals, resulting in the mobilization of Cr（VI） from COPR into the leachate. Particle size significantly influences the leaching of acid-soluble Cr（VI） from COPR, followed by leaching time, whereas the effects of HF concentration and leaching temperature are slight and the influence of stirring rate is negligible. The leaching process of acid-soluble Cr（VI） from COPR is controlled by the diffusion through the product layer. The apparent activation energy is 8.696 kJ/mol and the reaction orders with respect to HF concentration and particle size is 0.493 8 and -2.013 3, respectively.

Effect of mechanical activation on alkali leaching of chromite ore

Mechanical activation was used to improve the extraction of chromium in molten NaOH.It is observed that the extraction ratio reaches 97% after leaching for 200 min when chromite ore is mechanically activated for 10 min,but only 34% if not activated.Mechanical activation can decrease the particle size,increase the surface area,and enhance the lattice distortion.Further,the mechanisms for mechanical activation were exposed.The results show that the mechanical activation mainly focuses on chromite ore particle size decrease and the lattice distortion.The formation of aggregation weakens the strengthening effect of mechanical activation for releasing high surface energy.

Study on mechanisms of different sulfuric acid leaching technologies of chromite

The extraction of chromate from chromite via the sulfuric acid leaching process has strong potential for practical use because it is a simple and environmentally friendly process. This paper aims to study the sulfuric acid leaching process using chromite as a raw material via either microwave irradiation or in the presence of an oxidizing agent. The results show that the main phases in Pakistan chromite are ferrichromspinel, chrompicotite, hortonolite, and silicate embedded around the spinel phases. Compared with the process with an oxidizing agent, the process involving microwaves has a higher leaching efficiency. When the mass fraction of sulfuric acid was 80% and the leaching time was 20 min, the efficiency could exceed 85%. In addition, the mechanisms of these two technologies fundamentally differ. When the leaching was processed in the presence of an oxidizing agent, the silicate was leached first and then expanded. By contrast, in the case of leaching under microwave irradiation, the chromite was dissolved layer by layer and numerous cracks appeared at the particle surface because of thermal shock. In addition, the silicate phase shrunk instead of expanding.