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By Bastian Hoffmann, Christian Moormann

The paper presents results of an experimental study on the axial load bearing behavior of vibratory driven full displacement piles in comparison with impact driven and cyclic quasi-static installed piles in dense sand. The test setup and the measurement devices used to this end are explained. Subsequently, the results of the axial loading like load settlement curves are shown and the impact of the installation method is interpreted. The results presented in this work are mainly based on three installation methods: test series one investigates axial behavior of vibratory driven piles, test series two describe the same cyclic pile installation process but without dynamic influence wherein a special test setup developed to install the piles cyclic quasi-static in the soil was used. A third test series carried out using the conventional impact pile driving method served the purpose of not only comparing the results against vibratory driving, but also aided in isolating the effects of dynamics inherent to the vibratory driving process.

May 1, 2022

By F. J. Sotillo

The objective of this investigation was to determine the feasibility of using a high-pressure roll mill (HPRM) to improve the separation efficiency of dolomite from apatite in downstream operations when treating high-dolomitic pebbles. High-dolomite samples from four phosphate mines were used for the test program. The research work plan included sample characterization studies; INSTRON tests to develop preliminary crushing data; high-pressure roll mill evaluations to determine crushing parameters, such as feed rates, rolls speed and gap, and applied torque, pressure and energy; and screen, microscopy, and flotation studies to assess dolomitelapatite separation efficiency. The results of the HPRM tests show that commercial products for all four mines can be obtained. Recoveries of 74-82% of the P2O5 and rejections of 46-83% of the MgO were reported with an specific energy consumption of 0.2-0.5 of that of a laboratory rod mill.

Jan 1, 1998

By Peter R. Drummond

THE paper consists of four sections: (1) a discussion of early work; (2) presentation of the results of a chemical investigation of Falcon-bridge nickel-copper blast-furnace matte; (3) application of the results of this investigation to mattes in general; and ( 4) a summary of conclusions. All general discussion will be confined to the fundamental system of ferrous-cuprous sulphides (FeS-Cu2S). Nickel-, lead-, and zinc-bearing mattes will be treated as minor varieties of the parent system. DISCUSSION OF EARLY INVESTIGATIONS Matte investigations may be divided into three classes: first, those treating the sulphides as alloys-theoretical studies of the system FeS-Cu2S, such as those of Hofman (ll)t or Carpenter and Hayward (2); second, chemical research on mattes made under smelter conditions, such as that carried out by Hawley (3) or Avetision ( 4); third, combinations of the . above, such as the investigations of Gibb and Philp (5) or Fulton and Goodner (6). Much research in the first of these classes, including determinations of . freezing points and of data for structure-composition diagrams of the system FeS-Cu2S, was carried out in the period 1890-1910. LeChatelier and Ziegler (1, 13) proved that FeS, when heated alone above its melting point, dissociated in part, and Friedrich ( quoted by Fulton, 6) showed the same for Cu2S. The reactions may be expressed by the following equations: [xFeS yFe + yS (volatilized)+ (x -y)FeS xCu2S )'Cu+ yS ( vohtilized) + (x -)')Cu,S ] Sulphur is lost by volatilization, leaving a melt of metallic iron plus ferrous sulphide in the one case, and of metallic copper plus cuprous sulphide in the other. If the mixed sulphides are heated far above their melting points, there results either an iron-rich matte containing Fe-FeS-Cu2S or a copper-rich matte containing FeS-Cu2S3-Cu, depending on the relative amounts of each sulphide in the mixture.

Jan 1, 1940

By Charles M. Ryder

IN presenting this paper to the Institute I beg to mention, first, the results which I have obtained and the apparatus which I have employed, and to follow this with a brief description of the steps by which I have perfected this process. My present mode of testing is as follows : I obtain, either from the furnace or from the finished product in any stage in which it may chance to be, a small test-piece, usually about 4 of an inch square by 3 1/2 inches in length. I place this test-piece upon the poles of an ordinary electro-magnet, for about one minute, and thus magnetize it to its extreme limit. I then place it upon an indicator (which I will shortly describe), on which indicator there is already placed a piece of steel of similar form which has been magnetized in the same manner, the percentage of carbon of this second piece being definitely known. I then, by means of a scale, which forms part of the indicator, marked in degrees of carbonization, read the exact percentage of carbon contained in the piece under examination. My apparatus consists of an ordinary electro-magnet, connected with a suitable battery of moderate power, by which I magnetize the pieces as before described, and of an indicator which consists of a magnetic needle suitably mounted or suspended, so that it is free to deviate a few degrees only from the zero point, and having a rest, which is used to check the vibrations when in use, and to support its weight when not in use. At a right angle to the natural position of the needle I have a scale about 3 feet long, the zero point of which coincides with the natural position of the needle. This scale is marked in each direction from the zero point in degrees of carbonization. The marking of the scale is in accordance with the following rule: Taking the carbonization of the steel as a basis, I maintain the same proportion between the squares of the distances on the scale as exists between the percentages of carbon in the steel, for instance, if .08 of

Jan 1, 1877

By L. A. Anestiev

A method based on use of calorimetric data for the determination of solidification kinetic parameters is proposed. The method incorporates recent theories of nucleation and growth and allows modeling heat fluxes observed in calorimetric experiment. Statistically optimal estimations of the kinetic parameters, which include the activation's energies of nucleation and growth, are derived from experimental data by means of nonlinear regression analysis. The method is applicable to pure substances and alloys. The evaluation of the kinetic parameters characterizing solidification of pure Al is used for clarifying and exemplifying the method.

Jan 1, 2004

By A. Kapito

The ferrite-pearlite microstructure is the most popular microstructure for alloys used in structural applications, including railway wagon wheels. These alloys have been designed through alloying and thermo-mechanical processing to have a refined microstructure. Ferrite-pearlite alloys are low cost, weldable, have good fabricability, and are reliable under extreme conditions. Given these performance attributes, it seems unlikely then that their dominant position as structural steels would ever be challenged by alternative microstructures. One major achievement in the development of ferrite-pearlite steels has been in the refinement of their interlamellar spacing to very fine distances of the order of < 0.3 µm. A refined microstructure increases the hardness of the alloy, thus increasing its life under wear conditions. The interlamellar spacing in pearlitic steels has, however, been refined almost to its theoretical limit. The increasing demand for speed and increased axle loading on railway wagons requires the use of stronger, tougher, and more durable materials. This has opened the window for the development of novel bainitic steels. Bainitic alloys have a higher level of microstructural refinement than pearlitic ones. They have shown to have good wear resistance and rolling-contact fatigue resistance, and high toughness. This paper will discuss the progress to date on the development of bainitic railway wheel alloys. Four alloy chemistries have been chosen for possible further development.

Jan 1, 2012

By J. Laskowski, J. Lekki

"The mechanism of joint action of a frother (alpha-terpineol) and a collector (ethyl xanthate) in the flotation of chalcocite has been examined. In order to study the dynamic effects of frother and collector in bubble-particle collisions, the following aspects were investigated : determination of zeta-potential. Measurements of the thickness of fluid film on the surface of chalcocite and adsorption of xanthate, as influenced by alpha-terpineol, induction time and flotation versus concentration of alpha-terpineol and ethyl xanthate.The mechanism of the adsorption-desorption phenomenon on chalcocite with regard to non-ionic molecules, e.g. a-terpineol, has been considered, and the influence of this on the stage and thickness of the intervening wet layer between a bubble and a particle, changing the probability of collision and attachment, is suggested."

Dec 1, 1971

By Youn-Bae Kang

Evaporation mechanism of S from liquid Fe–C–S alloys at 1873 K was proposed by analyzing available experimental data. It has been known that increasing C content in liquid alloy increases activity coefficient of S ðfSÞ, and it could raise driving force for the evaporation reaction S = S(g). However, experimental data of the evaporation of S in the Fe–C–S alloys could not be accounted for only by considering the increases of fS. In the present study, formation of carbosulfides, CS(g) and CS2(g), was additionally taken into account in order to explain role of C for the accelerated S evaporation. Surface adsorption of S was also taken into account, which retards the evaporation rate of S. An evaporation model equation was formulated. It can be applied to calculate the evaporation rate of S over wider C content (from zero to its saturation to liquid alloy).

By Stephen L. Chryssoulis, Daniela Venter, Stamen Dimov

"Comparative surface analysis of floated and rejected free gold grains from a number of full scale benefication plants and from pilot plant testwork, has shown silver (the most common impurity in gold) and sulphur invariably in greater concentration on floated gold grains, implying that they assisted gold flotation (gold activators). Collector loading measurements on individual gold grains from concentrates of staged rougher flotation tests on a cyclone U/F product of a pyritic gold ore, revealed first progressively lower concentrations of the collector monomer on gold from each follow-up concentrate and secondly a direct proportionality between the surface concentration of silver and the collector monomer. These two observations leave no doubt that silver activates gold and that collector monomers play a significant, previously not fully recognized role in gold flotation.INTRODUCTIONGold to some degree is naturally floatable in industrial systems, which means that it can be recovered without collector addition (Marsden and House, 1992). This is due to the adsorption of hydrocarbons and deposition of sulphur, which are dependent on the metallic properties of gold, particularly its high electrical conductivity, which allows surface electrochemical reactions to occur catalytically and selectively. Cyclic voltammetry has shown that gold can be rendered hydrophobic by deposition of a surface layer of sulphur (Sº) from S2- and HS- solutions (Walker et al., 1984). Twenty monolayers of sulphur were required to produce a hydrophobic gold surface.However, no sulphide flotation plant relies solely on the natural floatability of gold for its recovery. Gold hydrophobicity is enhanced by the addition of flotation collectors such as xanthates (X), dithiophosphates (DTP), dithiophosphinates (DTPI), mercaptobenzothiolates (MBC) etc., as used in sulphide mineral flotation (Dekok, 1973: Adams et al., 1986). The mechanism invoked in enhancing the hydrophobicity of pure gold with the aforementioned four groups of collectors is similar to that of pyrite, whereby collector anions are oxidized at the gold surface to form dimers (Groot, 1987; Marsden and House, 1992; Basilio et al., 1992) As an example, xanthate ions are oxidized at the gold surface to form the neutral dimer dixanthogen (X2):"

Jan 1, 2003

By N. S. Dalal, X. Shi

"It has been recently suggested that the exceptionally high antitumor and antibacterial activity of natural fredericamycin A (FMA) is related to its ability to spontaneously generate the superoxide anion (02-) and hydroxyl (""OH) radicals in aerobic solutions [Hilton, B. D., Misra, R., & Zweier, J. L. (1986) Biochemistry 25, 5533]. With a view to understand the mechanistic details, attempts were made to reproduce earlier electron spin resonance (ESR) evidence for the oxygenated free radical formation in well-aerated solutions of natural FMA in dimethyl sulfoxide and dilute H 20 2• Little or no evidence was obtained for the formation of the 0 2- and methoxy (""OCH3) radicals, while the detected formation of the 'OH and methyl ('CH3) radicals was attributable largely to mechanisms not involving FMA. These results thus reopen the T question regarding the mechanism of its exceptionally high tumoricidal-bacteriocidal activity. This paper reports on an electron spin resonance (ESR) investigation of the formation of oxygenated free radicals in solutions of natural fredericamycin A (abbreviated as FMA) which has recently been reported to be a highly potent anti¬tumor antibiotic (Pandey et al., 1981; Warnick-Pickle et al., 1981). The impetus for this undertaking was provided by recent reports (Hilton et al., 1986; Stinson, 1986) suggesting that the strong tumoricidal activity with high therapeutic—toxic index of FMA was related to its ability for the spontaneous generation of superoxide anion (02-) and hydroxyl (OH) radicals. For example, the suggested mechanism for the formation of 02- by FMA was (Hilton et al., 1986)FMA-OH + O2 FMA-O + H+ + 02-Here FMA—OH is simply the FMA molecule emphasizing the —OH moiety which becomes FMA-O, the free radical rep¬resenting the oxidized form of the drug. While some other highly effective anticancer drugs in current clinical application, such as bleomycin, are also thought to exert their antitumor effects via the formation of reactive drug-based or oxygenated free radicals (Carmichael et al., 1985; Gianni et al., 1983), FMA was considered unique in that only in this case the free radical formation was thought to be spontaneous, rather than through a metal- or an enzyme-activated process (Hilton et al., 1986; Stinson, 1986). Thus it was suggested (Hilton et al., 1986; Stinson, 1986) that FMA could be the forerunner of a new class of anticancer drugs. However, we noted that while the hypothesis seemed plausible, the reported ESR spectral evidence appeared to be ambiguous. Because of the high significance of the earlier conclusions (Stinson, 1986), we undertook a systematic ESR study of the radical formation in FMA solutions, and as discussed below, our conclusions disagree with those reported earlier."

Jan 1, 1989

By Hans Lundberg

Introduction Today, geophysical exploration is recognized as a special science wedged between geology and mining. Universities and technical institutes have special professors teaching geophysics applied to oil and ore prospecting. The list of literature on geophysical exploration has grown from practically nothing at all some twenty-five years ago to ~ several thousands of books, some of them very imposing volumes. Most of the methods now used have been developed since the first World War, and very large systematic exploration campaigns have been carried out in many places, in many countries. It has been rather difficult to obtain enough information as to what is going on in Canada. However, since the Associate Committee on Geodesy and Geophysics of the National Research Council became more actively engaged in promoting geophysical research, it may be easier to obtain information and, in the future, the quarterly issued Canadian Geophysical Bulletin may throw considerable light on the current geophysical activities in Canada. Some twenty crews have been engaged the last few years in geophysical prospecting for ore. In the United States, between 500 and 600 crews are employed exclusively in geophysical prospecting. Most of these, however, are engaged in oil exploration. During the winter 1942-1943, the German Government employed some fifty crews on geophysical exploration, and some ten crews were working for private enterprises.

Jan 1, 1948

By N. S. Dalal, X. Shi

"Electron spin resonance (ESR) measurements on solutions and isolated powders provide direct evidence for the involvement of Cr(V) species in the reduction of Cr(VI) by NAD(P)H. ESR analysis of an isolated Cr(V)NAD( P)H solid yields 6 11 = 1.9831 and 6.i. = 1.9772, indicating that the unpaired electron occupies the d22 orbital of the Cr(V) ion, with square-pyramidal geometry. Addition of hydrogen peroxide (H2 0 2 ) to the N AD(P)HCr( VI) reaction mixtures suppresses the Cr(V) species and generates hydroxyl (""OH) radicals. The ""OH radicals were detected via ESR spin trapping, employing 5,5-dimethyl-1-pyrroline-N-oxide and a-(4-pyridyl-1- oxide)-N-tert-butylnitrone as spin traps. The dependence of Cr(V) and ""OH radical formation on the H20 2 and Cr(VI) concentrations indicates that the Cr(V) species react with H20 2 to generate the ""OH radicals. Similar results were obtained by using various diols (arabinose, cellobiose, FAD, fructose, glyceraldehyde, ribose, and tartaric acid), a-hydroxycarboxylic acids, and glutathione. Investigations with superoxide dismutase showed no significant participation of 02 in the generation of OH radicals. These results thus indicate that the Cr(V) complexes, produced in the reduction of Cr(VI) by cellular reductants, react with H20 2 to generate ""OH radicals, which might be the initiators of the primary events in the Cr(VI) cytotoxicity.In this paper we report an electron spin resonance (ESR)3 spectroscopic detection of Cr(V) species in the reduction of potassium dichromate, henceforth called Cr(VI), by NAD(P)H and other diols. Also reported here is the detection of the hydroxyl (""OH) radical in the reaction between hydrogen peroxide (H2O 2) and a variety of Cr(V) complexes. This work was undertaken because while the reduction of Cr(VI) to lower oxidation states has been suggested as a key step m the mechanism of Cr(VI) toxicity, mutagenicity, and DNA alteration, the details are not yet clear (1-5).Earlier studies on the mechanism of Cr(VI) reduction in cellular systems include those on the reduction of chromate by microsomes (6-8), rat liver DT-diaphorase (9), and aldehyde oxidase (10). All these studies imply that Cr(VI) can be reduced to Cr(Ill) by cellular constituents. Regarding the mechanism of Cr(Vl) carcinogenicity, Jennette postulated that Cr(V) is the ultimate carcinogenic form of chromium compounds (8). This conclusion has led to many further investigations of the formation of Cr(V) in the reduction of Cr(VI) under biologically relevant conditions (11-33). More recently, however, Kawanishi and co-workers (34) have investigated the mechanism of DNA cleavage induced by Cr(VI) in the presence of H20 2 and have argued that it is not Cr(V) itself that is carcinogenic, but it is the reactive oxygenated species (02, 10 2 , ""OH) (produced by the decomposition of the Crv(02) ¾ - ion) that cause the DNA alteration."

Jan 1, 1990

By P. Klossek

It is estimated that China accounts for 94% of the world supply of rare earths (Roskill, 2011). This fact, together with the impact of China?s export policy, poses a substantial risk to the security of the rare earth supply as well as a rare earth price risk. This situation has motivated an increase in the number of rare earth projects under development, i.e., there are currently more than 440 worldwide (Technology Metals Research, 2012). However, economically and environmentally acceptable processing technology to produce tradable products from rare earths (e.g., separated rare earth oxides) is not freely available on the market at the moment. In this respect, a lot of research and development has been initiated on this topic, in academia as well as in industry. Using a proprietary model and scenario-based analysis, we studied the impact that the development of rare earth processing technologies would have on the rare earth market and the economic feasibility of rare earth projects mentioned before. The analysis showed that this impact varies depending on the technology type (e.g., technology can affect project production volume, cost structure or both) and the legal context of the invention (e.g., free or limited by patenting availability of a new technology). The effect of a technology which influences project production volume is not the same as of a technology which changes project cost structure. The impact of a patented processing technology differs from the impact of a processing technology which is freely available on the market. In the case of a new technology which is limited by patenting, its impact depends on the market position (?producing?, ?next-coming?, or ?not producing? project) of the patent holder. The paper contributes to existing research by studying the dependence of the expected micro- and macroeconomic impact of rare earth processing technology developments on the properties of a new technology (e.g., costs, legal framework etc.). By this means, the paper adds to existing knowledge and attempts to make the rare earths market more transparent and predictable from a perspective outside China.

Aug 1, 2013

By F. J. Sotillo

Mineral processing requires good classification of particles to attain an adequate separation of minerals. This is of utmost importance in the case of sulfide minerals. An ideal classification is obtained when particles are separated at a given mesh, which is fixed according to the characteristics of each sulfide ore. Under these ideal conditions, particles coarser than the mesh of separation are recovered in the underflow, whereas particles finer than the mesh of separation are recovered in the overflow. This paper presents the results of pilot plant tests that were performed on Ag-Pb-Zn and on Ag-Au Peruvian sulfide ores using a Density Separator, and the comparison of these results with those obtained using hydrocyclones, under similar operating conditions. The results show that the ratio of circulating load is reduced by 50% by using a Density Separator with respect to that obtained by using hydrocyclones. Moreover, the effect of the improvement in the separation efficiency obtained by using the Density Separator on the kinetics of flotation is also shown. Silver recovery of the Ag-Pb-Zn ore increased by 18% whereas gold grade in the concentrate of the Ag-Au ore is doubled.

Jan 1, 1998

By A Minchinton

With blasting, all far-field phenomena including, but not limited to, stress wave propagation, fracture, vibration and muck pile formation are intrinsically and irrevocably dependent upon near-field phenomena or mechanisms that we describe using physics-based mathematics typically transcribed into computer codes. The energetics involved in such near- to far-field transitions are not fundamentally relevant as they are simply progenitors of these processes.The notion of explosive energy as a descriptor for explosive or blasting performance is a case in point. These energies, which are generally used in blasting tables, are almost exclusively based on ideal detonation theory alone and do not include the influence of the rock, charge diameter, blasthole length and the late-time dynamic expansion that can be predicted using codes based on non-ideal detonation theory.With decoupled charges in presplits for example, the pressure drop between the charge and the blasthole wall can be simply described to first order using the energy relationship associated with adiabatic expansion. However, a more realistic mechanistic solution involves interacting fluids and turbulence leading to dynamic processes such as the channel effect.CITATION:Minchinton, A, 2015. On the influence of fundamental detonics on blasting practice, in Proceedings 11th International Symposium on Rock Fragmentation by Blasting, pp 41–54 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Aug 24, 2015

By Lina Lopez, Jose Sanchidrian, Manuel Lopez, Pablo Segarra

Previous work in which rock properties and blasting parameters (geometry, charging and timing) were reported together with truck filling times, is taken as a starting point for a thorough analysis of the influence of the former on the mucking velocity. The experimental set was composed by 11 blasts made in an open pit mine. Non-parametric, distribution-free statistics is applied. The mucking rates (in bank cubic meters per unit of bucket volume and per hour) appear to be clearly dependent on the explosive energy and powder factor, both magnitudes combined as energetic powder factor (energy per unit of rock volume). The mucking rate is a non-monotonic function of such factor in the range of the blasts analyzed. The determination of the function is optimum when the fraction of the explosive below grade is not considered and when the explosive energy is qualified as that delivered at a partial detonation regime. Different types of loaders seem appropriate for different energy levels. At low levels both front loaders and rope shovels obtain small productivities without a statistically significant difference in their performance. At high energy levels, front loaders qualify higher than rope shovels. Mucking velocities can be equally small at low energy powder factors and at high ones if rope shovels are involved.

Jan 1, 2008

By Jacob W. Young

A casual reading of the geological literature extant to-day would give one the impression that carbon is an element which by some chance or another always existed at or near the surface of the earth, and was never of a magmatic origin. The carbonaceous matter in shales is said to be of vegetal derivation; petroleum is stated to have been derived from vegetable matter or from marine animal bodies; if carbon or bituminous matter is found in igneous rocks or associated with metallic ores, the statement is made that deeply buried carbonaceous sedimentaries were undoubtedly the source from which such carbon or bituminous matter was derived. This, I do not consider a reasonable or common sense view of the matter. Carbon is one of the elements of which the earth is composed. In the table giving the periodicity of the elements it comes sixth in the order of increasing atomic weights. It is an element formed like all the rest were formed, and must be as common to the interior of the earth as the rest of the eighty-odd elements so far discovered. The spectroscopic analysis of light from the sun, and the stars, shows the presence of carbon in the sun and in the colder stars. Thus with carbon, an element, found throughout the universe as well as on the earth, it is entirely reasonable to consider it as present in the interior of the earth with the other elements. Such being the case, the magmatic origin for the various forms of carbon in rocks and ores is a rational and logical conclusion. If but a small fraction of thought and effort was made along these lines, which is made in fostering the idea of the organic origin of the carbon found in the earth&apos;s crust, untold advances in this phase of geological science would result.

Jan 1, 1925

By D. Kosior

Influence of frothers (a-terpineol and n-octanol) concentration on kinetics of a three phase contact (TPC) formation by a bubble colliding with a hydrophobic surface (Teflon) of different roughness was studied using a high-speed camera (1040 frames per second). It was found that the Teflon surface roughness and the frother concentration strongly affected time of the TPC formation (tTPC) and time of drainage (tD) of the liquid film separating the colliding bubble from the solid surfaces. With increasing Teflon surface roughness, from ca 1 to 80 µm, the time of the TPC formation was shortened by over the order of magnitude, from ca 100 to a few milliseconds only. At low a-terpineol and n-octanol concentrations, thetTPC values were shorter than in distilled water due to lowering the bubble velocity in the frother presence and diminished bouncing of the colliding bubble. However, at high frothers concentrations the tTPC values were prolonged, despite that the bubble velocity was similar. The prolongation of the tTPC at high frother concentrations indicates that air (micro- and/or nano-bubbles) was present at the Teflon surfaces of different roughness. The mechanism of the tTPCprolongation at high frother dosage is described.

Sep 1, 2012

By N. S. Dalal, X. Shi

"With a view of elucidating the role of glutathione (GSH) in the biochemical pathways of the chromate-exposure related carcinogenesis, we carried out electron spin resonance (ESR) spectroscopic investigations of the chromate-GSH redox reactions. The ESR measurements, employing spin-traps, provide evidence for the involvement of the glutathione (GS•) radical, as well as an isolable Cr(V)-glutathione intermediate. These results indicater a new mechanism for the reduction of chromate by GSH in cellular environment and help understand the (unexpected increase in Cr(VI)-induced DNA strand breaks at elevated GSH levels.We report here indirect electron spin resonance (EGR evidence for the involvement of the glutathionyl (GS) radical and direct evidence for a Cr(V) intermediate in the reactions of glutathione ¬(GSH) with chromate for the first time.. This work was undertaken to clarify the biochemical pathway of the reduction of chromate by GSH, since this reduction process is thought to be a critical step in the pathogenesis of the chromate-exposure related carcinogenesis. This follows from the literature reports which show that (a) Cr(VI)compounds have been proven to be carcinogens in laboratory animals as well as in human studies2-4, (b) Cr(VI) and not Cr(III) compounds are mutagenic 5-7, (c) Cr(VI) and not Cr(III) compounds can penetrate cell membranes 8.9 (d) the final state of Cr(VI) in cellular reductions is Cr(III)', (e) by itself chromate (or Cr(VI)) cannot be directly mutagenic since Cr(VI) is known not to interact with isolated DNA under physiological conditions 10, suggesting the significance of Cr(VI).reduction processes. One of the major reductants in cellular environments is thought to be the GSH, both outside and inside the ce11s 11-13. Other evidence for the role of GSH in the chromate toxicity is seen from recent studies showing that exposure of hamster cells to non-toxic levels of added selenite increases the levels' of GSH as well as the Cr(VI)-induced DNA strand beaks 14, and thatsuch DNA strand breaks in hepatocytes also change in direct proportion to their GSH content 15,16. These observations wereinterpreted as implying that the reduction of chromate by GSH, to some reactive intermediate is an important step in the chromatecarcinogenicity 15,16. The following reaction steps were,suggested 15, 16"

Jan 1, 1988

By A. López-Valdivieso, M. A. Corona-Arroyo, M. Llamas-Bueno

Magnetite (Fe3O4) ore is concentrated by low-magnetic-field drums to recover a magnetite concentrate that is low in silica (SiO2). The presence of SiO2 in the magnetite concentrate for steel production increases the steel processing costs, so a major challenge in magnetic concentration is to lower the SiO2 grade in the Fe concentrate. This work presents studies that were carried out on the removal of SiO2 from the magnetite concentration at the plant scale. Studies were performed with a three 36 × 96″ drum unit processing 46 ton/h of rougher magnetite concentrate. These studies showed that silica appears in the magnetite concentrate by three mechanisms, namely SiO2 entrainment in Fe3O4 chains, heterocoagulation between Fe3O4 and SiO2 particles, and mineral locking of SiO2 to Fe3O4. The SiO2 entrainment mechanism had the highest contribution (75%) to the SiO2 recovered in the Fe concentrate. Electrokinetic studies on Fe3O4 and SiO2 showed that heterocoagulation occurs because of the low negative zeta potential of the minerals at the pH of the plant slurry. This study also showed that a high percentage of ultrafine size SiO2 particles trapped in Fe3O4 agglomerates was not removed by the dilution water for the slurry fed to the drum. To lower the SiO2 recovery in the drum magnetic concentration process, work should be directed towards the removal of trapped SiO2 in Fe3O4 agglomerates.