Oxidation induration process and kinetics of Hongge vanadium titanium-bearing magnetite pellets

The induration process and oxidation kinetics of Hongge vanadium titanium-bearing magnetite (HVTM) pellets have been investigated by employing X-ray diffraction, scanning electron microscope, energy-dispersive spectroscopy, thermogravimetric and differential thermal analysis and thermogravimetric and differential scanning calorimetry. The results indicated that HVTM was a high-chromium vanadium-bearing titanomagnetite containing 1.48?wt-% Cr₂O₃, and the crystal stock strength was 625?N. The compressive strength of HVTM pellets could be improved by increasing the roasting temperature and roasting time. Under the optimum conditions of oxidation roasting at 1200°C for 15?min, the compressive strength was found to be 2893?N. The phase transformations of the valuable elements could be described as follows: Fe₃O₄→Fe₂O₃; Fe₂VO₄→(Cr_0.15V_0.85)₂O₃; Fe_2.75Ti_0.25O₄→FeTiO₃→Fe₉TiO₁₅; FeCr₂O₄→(Fe_0.6Cr_0.4)₂O₄, Fe_0.7Cr_1.3O₃, (Cr_0.15V_0.85)₂O₃. Three stages were identified during the induration process: initial oxidation, later oxidation, and haematite re-crystallisation, poly-crystallisation and induration. The development of strength mainly occurred in the last stage. Kinetic parameters of the oxidation process were determined from heating experiments. The results showed that the average value of activation energy was calculated to be 69.33?kJ?mol^?1 by the Flynn–Wall–Ozawa methods. This study aims to provide theoretical and technical bases for the effective utilisation of HVTM ore for use in either blast furnaces or shaft furnaces.     

Oxidative roasting of covellite with minimal retardation from the CuO⋅CuSO4 film

Copper (II) sulfide can be efficiently converted to the oxide at lower temperatures than normally required in aerobic roasting by a new method involving programed environment roasting (PER). When heating was conducted in absence of oxygen up to about 650°C, and then nitrogen was replaced by air, the sulfide was easily converted to oxide without need for further increase in temperature. Traditional oxidative roasting of chalcocite required a temperature range of 800° to 850°C for conversion to tenorite. Unlike the situation with conventional roasting, CuSO₄ was not detected in the X-ray diffractogram of the product obtained with the PER method above 625°C. Also, the amount of CuO ⋅ CuSO₄ significantly decreased as the halt temperature in the PER process increased from 600° to 700°C. Apparently the shell of copper oxysulfate is impervious to oxygen and/or sulfur dioxide and delays the formation of tenorite until the sulfate and oxysulfate are decomposed. If the oxysulfate stage were bypassed with an inert atmosphere, then, even if small amounts of this salt were formed upon introducing the oxidant, it would decompose at an appreciable rate and the impedance of its thin film to gaseous transport would be considerably diminished. By contrast, the accelerating effect of externally added iron on the oxidative roasting of covellite was confined to the low temperature reactions and hence iron promoted the extent of sulfate formation. Iron did not, however, lower the thermal requirement for complete oxidation of CuS or Cu₂S because it had virtually no effect on the thermal decomposition of CuO ⋅CuSO₄.     

Oxidation Resistance of B,W2B5, and WC Powders

The oxidation of B, W₂B₅, and WC powders in air has been examined under isothermal conditions in the temperature range 800-1300 K (measurement times at each temperature 30, 60, 120, and 180 min) as well as under nonisothermal conditions on programmed heating with a Q-1500 derivative recorder with the simultaneous conduct of differential thermal analysis. All these specimens give parabolic curves in the interval 800-1300 K. The rectilinear character of the curves is due to the chemical interaction of the compound with atmospheric oxygen, while the parabolic form is due to diffusion of oxygen through the resulting layer of reaction products.     

Effects of ternary additions on the thermoelastic martensitic transformation of NiAl

Nickel-rich β-NiAl alloys, which are potential materials for high-temperature shape-memory alloys, show a thermoelastic martensitic transformation, which produces their shape memory effect. However, the transformation to Ni₅Al₃ phase during heating of NiAl martensite can interrupt the reversible martensitic transformation; consequently, the shape memory effect in NiAl martensite might not appear after heating. The phase transformation process in binary Ni-(34 to 37)Al martensite was investigated by differential thermal analysis (DTA) method, and we found that the condition of reversible martensitic transformation was not the β → Ni₅Al₃ transformation, but rather the M → Ni₅Al₃ transformation occurring at 250 °C to 300 °C. Therefore, the transformation temperature of M → Ni₅Al₃ determined the highest operating temperature for the shape memory effect. For verifying the critical temperature, the phase transformation process was investigated for eight ternary Ni-33Al-X alloys (X=Cu, Co, Fe, Mn, Cr, Ti, Si, and Nb). Only Ti, Si, and Nb additions were found to be effective in dropping the M _s temperature, and they facilitated the shape memory effect in Ni-33Al-X alloys. In particular, the addition of Si and Nb raised the transformation temperature of M → Ni₅Al₃, a potentially beneficial effect for shape memory at higher temperatures. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.     

Observations of aging effects in a Cu-Sn shape memory alloy

A Cu-15.0 at. pct Sn alloy has been chosen as a model alloy for the study of aging effects in copper-based shape memory alloys. Different thermal aging treatments were carried out to determine the effects of both parent phase and martensite aging on the amount of shape recovery and the characteristic transformation temperaturesM _s ,A _s , andA _f . Aging of the martensite reduces both the amount of shape recovery and the extent of the reverse martensite → parent transformation. High martensite heating rates promote complete shape recovery and reverse transformation while the aging occurring during slow heating can inhibit or prohibit both. But irrespective of the martensite heating rate the transformation temperature hysteresis as given by (M _s -A _s ) is large for the Cu-15 pct Sn alloy compared to other shape memory alloys exhibiting thermoelastic behavior. On the other hand, some beneficial effects were noted when the Cu-15 pct Sn alloy was aged in the parent phase condition prior to subsequent transformation to martensite. TheM _s ,A _s , andA _f were lowered following prior parent phase aging, possibly because of a change in long range order, but prior parent phase aging was found to diminish the deleterious effect of martensite aging. Both shape recovery and the extent of the reverse martensite → parent transformation are enhanced by prior parent phase aging. The enhancement is greater the higher the aging temperature or the longer the aging time at a given temperature. J. D. STICE, formerly Research Assistant at the University of Illinois     

Deformation Induced Martensite Formation and its Effect on Transformation Induced Plasticity (TRIP)

The knowledge of the stress‐ and deformation‐induced martensite formation in metastable austenitic steels including the formation temperatures and amounts formed is of considerable importance for the understanding of the transformation induced plasticity. For this purpose a stress‐temperature‐transformation (STT) and a deformation‐temperature‐transformation (DTT) diagram have been developed for the steel X5CrNi 18 10 (1.4301, AISI 304). It is shown that the M_d‐temperature for γ→?, ?→α', γ→?→α’ and γ→α’ martensite formation is defined by two stress‐temperature curves which show a different temperature dependence. They specify the beginning and the end of the deformation‐induced martensite formation in the range of uniform elongation. The intersection point defines the corresponding M_d‐temperature. The stress difference which results from the stresses for the end and the beginning of the martensite formation shows positive values below the M_d‐temperature. It defines the amount of martensite being formed. When the M_d^γ→? temperature is reached and the formation of the first deformation‐induced amount of ?‐martensite appears, an anomalous temperature dependence of the maximum uniform elongation starts. The highest values of the maximum uniform elongation are registered for the tested steel in the immediate vicinity of the M_d^γ→α' or the M_d^γ→?→α' temperature ‐ similar as in other metastable austenitic CrNi steels. At this temperature the highest amount of deformation‐induced ?‐phase exists. The transformation plasticity in the test steel is considerably caused by the deformation‐induced ? and α’ martensite formation. Using the new evaluation method, the increase of plasticity ΔA (TRIP‐effect) and strength ΔR can be quantified.     

Effect of TiO2‐content on Reduction of Iron Ore Agglomerates

The effect of titanium oxide on iron ore agglomerates is studied by the use of test sinter, test pellets and synthetic briquettes under laboratory conditions. Titanium favours secondary hematite rather than magnetite, which is the main phase in the sinter of Rautaruukki's Raahe plant. Additionally, the effects of sinter RDI and pellet LTD on the blast furnace process are evaluated using the test results of basket trials in LKAB's Experimental Blast Furnace. The effect of titanium in synthetic hematite is studied as hematite is reduced to magnetite in the RDI test. This occurrence causes deterioration in burden permeability. Synthetic titanium‐bearing iron oxides under controlled conditions are investigated at the University of Oulu. The effect of TiO₂, in solid solution in magnetite, on the magnetite to hematite oxidation is studied separately in order to simulate the final stage of the sintering process. In other experiments, hematite samples doped with various contents of TiO₂ are studied using thermogravimetry under a controlled gas atmosphere (CO/CO₂/H₂/N₂). The TiO₂ content of hematite has a clear effect on reduction degradation. Also increasing content of TiO₂ in solid solution in magnetite radically accelerates the oxidation rate. In the pilot tests, TiO₂ content has a similar negative effect on the reduction strength of both sinter and pellets     

Oxidation and Induration Characteristics of Pellets Made from Western Australian Ultrafine Magnetite Concentrates and Its Utilization Strategy

Western Australian magnetite concentrates normally have ultrafine granularity and much higher specific surface areas than Chinese magnetite concentrates owing to the significant pre-grinding and beneficiation for saleable iron grade.Such characteristics will inevitably affect the subsequent pelletization process.However,very few investigations have been done before.Thus,the oxidation and induration characteristics of pellet made from a Western Australian ultrafine magnetite concentrate were revealed by conducting routine preheating-roasting tests in an electric tube furnace and investigating the microstructure of fired pellets under an optical microscope in comparison with that of pellets made from typical Chinese magnetite concentrate.The liquidus regions of CaO-SiO_2-Fe_2O_3 and CaOSiO_2-Al_2O_3 ternary systems in air at various temperatures were calculated by FactSage software to explain the importance of liquid phase in the consolidation of fired pellets.The results show that pellet made from ultrafine magnetite concentrate possesses better oxidability and preheating performance than that made from Chinese magnetite concentrate.However,it has inferior roasting performance,usually requiring conditions of roasting at 1 280℃ for at least 30 min to acquire sufficiently high compressive strength,which are attributed to higher temperature sensitivity caused by its smaller particle size and less formation of liquid phase because of low impurities like CaO and Al_2O_3 in raw materials.Correspondingly,its roasting performance can be significantly improved by blending with Chinese magnetite concentrates or increasing the pellet basicity(wCaO/wSiO_2).By comprehensive evaluation,blending with Chinese iron ore concentrates is an appropriate way to utilize Western Australia ultrafine magnetite concentrates.     

Behavior of phase transformation of Baotou mixed rare earth concentrate during oxidation roasting

Based on the new process named “Combination Method” for metallurgy and separation of Baotou mixed rare earth concentrate (BMREC), the aim of this paper is to clearly elucidate the phase change behavior of BMREC without additives during oxidative roasting at 450–800 °C. The results indicate that the bastnaesite in BMREC is decomposed at 450–550 °C, the weight loss is about 10.3 wt%, and the activation energy (E) is 144 kJ/mol. The bastnaesite in BMREC is decomposed into rare earth fluoride, rare earth oxides (La₂O₃, Ce₇O₁₂, Pr₆O₁₁ and Nd₂O₃), and CO₂, particularly, with the increase of roasting temperature, bastnaesite in BMREC is more completely decomposed into LaF₃, which causes a decrease in leaching rate of La during the HCl leaching process. Additionally, the maximum cerium oxidation efficiency reaches about 60 wt% when the roasting temperature is equal to or above 500 °C, and the oxidation reaction rate of cerium increases with the increasing roasting temperature.     

Effects of Heating and Cooling Rates on Phase Transformations in 10 Wt Pct Ni Steel and Their Application to Gas Tungsten Arc Welding

10 wt pct Ni steel is a high-strength steel that possesses good ballistic resistance from the deformation induced transformation of austenite to martensite, known as the transformation-induced-plasticity effect. The effects of rapid heating and cooling rates associated with welding thermal cycles on the phase transformations and microstructures, specifically in the heat-affected zone, were determined using dilatometry, microhardness, and microstructural characterization. Heating rate experiments demonstrate that the Ac₃ temperature is dependent on heating rate, varying from 1094 K (821 °C) at a heating rate of 1 °C/s to 1324 K (1051 °C) at a heating rate of 1830 °C/s. A continuous cooling transformation diagram produced for 10 wt pct Ni steel reveals that martensite will form over a wide range of cooling rates, which reflects a very high hardenability of this alloy. These results were applied to a single pass, autogenous, gas tungsten arc weld. The diffusion of nickel from regions of austenite to martensite during the welding thermal cycle manifests itself in a muddled, rod-like lath martensitic microstructure. The results of these studies show that the nickel enrichment of the austenite in 10 wt pct Ni steel plays a critical role in phase transformations during welding.     

Pseudoelasticity and the strain-memory effect in Cu-Zn-Sn alloys

Pseudoelasticity and the strain-memory effect have been studied in alloys with compositions in the range Cu-33 to 35 wt pct Zn-3 to 3.5 wt pct Sn, having a retainedβ structure and a martensitic transformation below room temperature. The alloys show maximum pseudoelasticities of 8.5 pct for single crystals and 4.5 pct for polycrystals at temperatures close toA _f . In single crystals high elasticity is retained to at least 100°C aboveA _f but in polycrystals it decreases rapidly aboveA _f . The strain-memory effect occurs on deformation belowM _s with subsequent heating betweenA _s andA _f . The two effects are complementary, such that when one is large the other is small and vice versa. The total pseudoelastic and strain-memory recoveries are normally close to 100 pct. Both effects can be explained on the basis of the formation of a particular variant of the martensite giving significant elongations to the specimens. For pseudoelasticity, the initial structure is theβ phase and the oriented martensite reverts to theβ phase on removal of the stress. In the strain-memory effect the initial structure is oriented thermal martensite and the oriented martensite disappears only on heating to betweenA _s andA _f so that the martensite reverts to theβ matrix. L. C. BROWN, currently on leave from the Department of Metallurgy, University of Melbourne, Victoria, Australia     

Oxidation of titanium-vanadium slags with the participation of Na2O and its effect on the behavior of vanadium

The oxidation of titanium-vanadium slags with the participation of Na₂O is studied by X-ray diffraction. The slags have the following phase compositions: anosovite, anosovite-spinellide, and spinellide. An alkaline component in the form of Na₂CO₃ is introduced into a slag either before oxidizing roasting or during slag production from a concentrate (alkaline slag forms). The general laws of the processes of slag oxidation in the temperature range 600–1100°C and the effect of these processes on the behavior of vanadium are found. The degree of formation of watersoluble vanadium compounds during roasting is shown to depend substantially on the SiO₂ content in the slag. When the slags are roasted, SiO₂ intensely combines with Na₂O to form sodium silicates or aluminosilicates depending on the slag composition, which restricts the formation of sodium vanadates. In anosovite-spinellide slags (with a high Al₂O₃ content), the maximum degree of vanadium oxidation with the formation of water-soluble sodium vanadates is achieved at a temperature of about 1000°C. In anosovite slags (with a low Al₂O₃ content), the maximum degree of formation of water-soluble vanadium compounds is achieved at a temperature below 900°C. At higher temperatures, the major portion of vanadium transforms into an acid-soluble form.     

Selective separation of iron and scandium from Bayer Sc-bearing red mud

In this study, we investigated the separation of iron and scandium from Sc-bearing red mud. The red mud object of our study contained 31.11 wt% total iron (TFe), 0.0045 wt% Sc, hematite (Fe₂O₃) and ferrosilite (FeO·SiO₂) as the main Fe-bearing minerals. The Sc-bearing red mud was treated by a novel deep reduction roasting and magnetic separation process that includes the addition of coke and CaO to extract Fe and enriching Sc from the Sc-bearing red mud. The addition of coke and CaO enhances the transformation of hematite (Fe₂O₃) to metallic iron (Fe⁰) and magnetite (Fe₃O₄) as well as the transformation of ferrosilite into metallic iron (Fe⁰). The test results show that utilizing the new process a Fe concentrate with a TFe content of 81.22 wt% and Fe recovery of 92.96% was obtained. Furthermore, magnetic separation tailings with Sc content of 0.0062 wt% and Sc recovery of 98.65% were also obtained. The test results were achieved under the following process conditions: roasting temperature of 1373 K, roasting time of 45 min, calcium oxide dosage of 20 wt%, coke dosage of 25 wt%, grinding fineness of 90% < 0.04 mm, and magnetic field intensity of 0.24 T. The major minerals in the Fe concentrate are metallic iron (Fe⁰) and magnetite (Fe₃O₄). The main minerals in the magnetic separation tailings with a low TFe content of 2.62% are CaO·SiO₂, Na₂O·SiO₂, FeO·SiO₂, Ca₃Fe₂Si₃O₁₂, CaAl₂SiO₆ and CaFe(SiO₃)₂.     

Thermal decomposition and oxidation of bastnaesite concentrate in inert and oxidative atmosphere

To clearly elucidate the oxidative roasting behaviors of the bastnaesite, the thermal decomposition and oxidation of the bastnaesite concentrate in inert and oxidative atmosphere have been investigated in detail. Experimental data indicated that the initial decomposition temperature of the concentrate under N_2 atmosphere is 150 ℃ higher than that under O_2 atmosphere,most likely because the oxidation of the cerium induces the decomposition of the concentrate. For the roasted samples under N_2 atmosphere at500 ℃ and above,the oxidation efficiency of the cerium is 19.8%-26.8% because of the fact that rareearth fluorocarbonate is first decomposed to form rare-earth oxyfluoride and CO_2, and the cerium oxyfluoride is then partially oxidized by the CO_2 gas. The rest cerium in these samples can be further oxidized in air at room temperature, with the oxidation efficiency of the cerium gradually increasing to above 80% in 7 d. This can be attributed to the obvious changes in the inner morphology of the roasted samples under N_2 atmosphere at high temperatures, which largely induce the diffusion of the air and improves the oxidation activity of CeOF, and further induces the oxidation of CeOF by the air. XRD and XPS techniques were used to further verify the significant differences in the thermal decomposition behaviors of the bastnaesite concentrate under N_2 and O_2 atmosphere. Moreover, no oxidation of Pr~(3+) to Pr~(4+) in the roasted samples under both N_2 and O_2 atmosphere is observed. This gives an overall understanding of the oxidative roasting of the bastnaesite concentrate without additives.     

Oxidation of iron ore at moderate and high temperatures

The oxidation of magnetite and titanomagnetites in iron-ore sinter at moderate (400–1000°C) and high (1000–1350°C) temperatures is subjected to physicochemical analysis. The oxidation kinetics is studied on briquets of Olkhovsk magnetite concentrate and Kachkanar titanomagnetite concentrate, as well as samples of unfluxed Kachkanar pellets and pellets fluxed to a basicity of 1.3. At moderate temperatures, the limiting stage in oxidation is the diffusion of the reagent to sections of the surface smaller than the total spherical surface. At high temperatures, in both isothermal and nonisothermal conditions, the limiting stage in oxidation is the diffusion of oxygen in pellet pores. From the kinetic equations for isothermal oxidation, the apparent activation energy with the specified degree of conversion is calculated; its variation is associated with change in the type of reagent diffusion through the layer of reduction products. The apparent diffusion coefficients of oxygen in Kachkanar pellets are determined at 500–1000°C. A method has been developed for determining the degree of pellet oxidation as a function of the time and the temperature in nonisothermal conditions. This method may be used to calculate the oxidation of the pellets in roasting on conveyer machines. The results may be used to determine the degree of oxidation in the roasted pellet bed and to optimize the heat-treatment parameters in roasting systems.     

Phase Composition of Oxide Scales during Reheating in Hot Rolling of Low Carbon Steel

Low carbon steel was oxidized over the temperature range 1000‐1250°C in O₂‐CO₂‐H₂O‐N₂, O₂‐H₂O‐N₂, and O₂‐CO₂‐N₂ gas mixtures. Oxidation times were 12‐120 min. and the scales were 50‐2000 μm thick. The variations of these parameters were chosen to elucidate the phase composition of oxide scales under conditions similar to those of reheating furnaces in hot strip mills, using either thin slab casting or conventional casting and rolling technology. Two types of scales have been observed which are influenced by the furnace atmosphere, oxidation time, and temperature. The first type is a crystalline scale with an irregular outer surface, composed mostly of wustite (FeO), and a negligible amount of magnetite (Fe₃O₄). The second type is the classical three‐layer scale, composed of wustite (FeO), magnetite (Fe₃O₄), and hematite (Fe₂O₃). In general, the experiments showed that an increase in oxidation time decreased the percentage of wustite while the percentages of magnetite and hematite increased. A rise in oxygen concentration in the gas mixture increased the percentages of magnetite and hematite, confirming earlier experimental findings. In water vapour‐free atmospheres O₂‐CO₂‐N₂, the oxide scales had a low percentage of wustite, and high percentage of magnetite and hematite. Carbon dioxide showed a small influence at 1100°C, and a negligible one at 1250°C.     

A study of electrical resistivity,internal friction and shear modulus on an aged Ti49Ni51 alloy

The 400°C aged Ti₄₉Ni₅₁ alloy can exhibit the transformation sequence of B2 →r premartensite R-phase →r martensite. In the early aging stage, only the premartensitic transformation is observed due to the M_s point being deeply depressed by the coherent stress of Ti₁₁Ni₁₄ precipitates. In the later aging stage, internal friction peaks associated with premartensitic and martensitic transformations are all observed on both heating and cooling. The sharp peaks associated premartensitic transformation on heating is believed to be related to the “bias” effect of the coherent stress induced by the Ti₁₁Ni₁₄ precipitates. The serrations of internal friction appearing significantly in the temperature around −30 to −80°C are found to be caused by the stress induced accomodation of R-phase or martensite variants, and are not associated with the transformation. The Ti₁₁Ni₁₄ precipitates can enhance the amount of martensite formed by unit of temperature or time during the martensitic transformation.     

Solidification of the eutectic Ga–Sn alloy

Cyclic thermal analysis is used to study the effect of overheating of the eutectic Ga–8.5 mol % Sn melt on the presolidification supercooling. It is found that, when the liquid eutectic is overheated above the eutectic temperature (T_e = 293.5 K) and is subsequently cooled, the dependence of the presolidification supercooling on the overheating temperature exhibits monotonic ascending behavior. The maximum supercooling after heating of the melt to 339 K is 26 K. The kinetic and thermodynamic parameters of eutectic solidification are calculated using the thermal analysis curves measured during melting.     

Characterization of the Martensitic Transformation in NiPtAl Alloy Using Digital Holographic Imaging

Surface reliefs due to phase transformations in a 56.8Ni-5.6Pt-37.6Al at. pct alloy were characterized in situ using digital holographic imaging during thermal cycling from room temperature up to 405 K (132 °C). The 3D images of the surface revealed that the austenite plates formed during heating are exactly the same for each cycle, which is not the case for the martensite plates formed during cooling. The martensite start temperature was found to vary by up to ~ 20 K from one grain to another within the same specimen. The absence of Ni₃Al γ′ precipitates, due to the relatively high Al content, results in the propagation of the martensitic transformation over grains up to a millimeter in size. Bright-field optical imaging showed the formation of large martensite plates in some grains, with cracks perpendicular to these plates, upon cycling. Cracks were also observed at grain boundaries and could be related to the height variations across the grain boundaries.     

Sintering characteristics and kinetics of acidic haematite ore pellets with and without mill scale addition

Haematite ore pellets require very high induration temperature (>1573?K) while, magnetite ore pellets require much lower temperature due to the oxidation of magnetite during induration. Mixing of some magnetite in haematite ore can improve the sintering property of pellets during induration. Mill scale is a waste material of steel plant which contains mainly FeO and Fe₃O₄. It can also be blended in haematite ore pellet mix which can enhance diffusion bonding and recrystallisation bonding and facilitate sintering at the lower temperature like magnetite ore. The extent of improvement in sintering property, sintering mechanism and its kinetics in the presence of mill scale is very imperative to study. In current study, the sintering characteristics of acidic iron ore pellet with 15% mill scale and without mill scale has been studied separately through microstructure observation, apparent porosity measurement and volume change. The volume changes due to heating at varying temperature and time has been measured by mercury displacement method and the data has been exploited for sintering kinetics study, wherein, extent of sintering α has a power relation with time. Several kinetics parameters such as time exponent (n), rate constant (k) and activation energies have been estimated for above two pellets and compared. While acidic pellet without mill scale requires 385?k?cal?mol^?1, acidic pellet with 15% mill scale requires only 310?k?cal?mol^?1 activation energy.