Silicate Formation Research Articles

Study on the synthesis of belite-ye’elimite-ternesite clinker

Compared with Portland cement (PC) clinker, belite-ye’elimite-ternesite (BYT) clinker has the advantages of low energy consumption and low carbon dioxide emissions. The phase composition and microstructure of BYT clinker were investigated in this paper. The results show that the synthesis parameters of BYT clinker are closely related to the designed phase composition. The synthesis temperature mainly depends on the designed content of ternesite (5CaO.2SiO2.SO3, C5S2$). Prolonging the heating time promotes the formation of dicalcium silicate (2CaO.SiO2, C2S), but has no effect on ye’elimite (4CaO.3Al2O3.SO3, C4A3$) and ternesite. The optimal heating protocol is at around 1210 °C for 1–2 h if the designed contents of C2S, ye’elimite and ternesite are in the range of 20–60 wt%, 20–60 wt% and 5–40 wt% , respectively. The formation of C2S was carried out by the diffusion of CaO with SiO2 as a core.

Utilization of Iron Ore Slag in The Manufacture of Calcium Silicate Boards

This study aims to determine the iron ore slag effect as an additive in particleboard based on the SNI 7705:2011 standard. Iron ore slag comes from the waste processing of iron ore into sponge iron. The iron ore slag is reduced to a size of 200 mesh. Particleboard made with the composition of slag and silica is 0:40, 8:32, 16:24, 20:20, 24:16, 32:8, and 40:0 wt%. Meanwhile, other materials were made permanent, namely PCC cement and lime 16 wt%, coconut fiber 3wt%, and water 3 wt%. They are pressed with 3 tons of pressure for 1 hour using a hydraulic press. Drying at room temperature for one day, under the hot sun for two days, then in an oven at 110 oC for 8 hrs. Analysis of the chemical composition of X-ray fluorescence and X-ray diffraction crystalline phase, SEM-EDS micro-photographs, physical tests including density and porosity, and mechanical compressive strength tests. The dominant composition of SiO2 and CaO affects the formation of silicon dioxide (SiO2), calcium silicate (CaSiO3), and dicalcium silicate (Ca2SiO4) phases. Silica has a positive effect on the compressive strength of particleboard but is different from Ca, which has an impact on reducing the compressive strength. The sem morphology shows that coconut fiber cannot withstand heating at 190 oC and results in agglomeration. The addition of 20% ore slag and silica has met the calcium silicate board SNI 7705-2011. These results can be used to develop slag waste from iron ore processing into much more useful objects.

Recycling of Ti-extraction blast furnace slag: Preparation of calcium silicate board with high slag content by steam pressure curing

Ti-extraction blast furnace slag (TEBFS) is an industrial solid waste, which is harmful to the environment and sustainable development of China's titanium industry. The efficient utilization of TEBFS is therefore urgently needed. In this study, calcium silicate boards were successfully prepared as partition materials. The effects of TEBFS content, cement content, calcium silicon mass (Ca/Si) ratio, and the Cl content of TEBFS on the material properties, pore structure, and phase composition of calcium silicate boards were explored. The results revealed that with an increase in TEBFS content, the average pore diameter of prepared samples gradually decreased. Furthermore, an excessive amount of cement and an excessively high Ca/Si ratio were not conducive to improving material properties. Optimum samples were prepared with 70 wt% TEBFS, 15 wt% Portland cement, and a Ca/Si ratio of 0.9. Under these conditions, the bulk density of the material was 1.64 g/cm3, the water absorption was 21.0%, and the flexural strength was 11.0 MPa. An identically prepared calcium silicate board using dechlorinated TEBFS had a flexural strength of 9.8 MPa, demonstrating the importance of chlorine in the calcium silicate formation process. This study provides an important reference for solving the waste management and recycling problems of TEBFS.

Preparation of triclinic alite with short soakings in a small experimental high-temperature furnace

The article deals with a laboratory preparation of triclinic modification of clinker mineral tricalcium silicate. A substantial part of the article is devoted to the technology and technique of firing a sample of tricalcium silicate, which would in the future allow the study of the development of the crystal lattice structure of this clinker mineral at short isothermal durations, in the order of minutes. As part of the research, a small high-temperature experimental furnace was designed and constructed. Based on the results, we can express the suitability and applicability of this furnace for the study of the formation of triclinic tricalcium silicate at short soakings.

Effects of Silicic Acid on Leaching Behavior of Arsenic from Spent Calcium-Based Adsorbents with Arsenite

The spent adsorbents that remain after being used to purify As-contaminated water constitute waste containing a large amount of As. These spent adsorbents, after being disposed, are likely to come into contact with silicic acid leached from the soil or cementitious solidification materials. Thus, it is crucial the evaluate the effects of silicic acid on spent adsorbents. In this study, the effects of silicic acid on spent Ca-based (CaO and Ca(OH)2) adsorbents with arsenite were investigated. The As leaching ratio for the spent adsorbents decreased with an increase in the initial concentration of silicic acid in the liquid. Under the tested conditions, the As leaching ratio decreased from 8–9% to less than 0.7% in the presence of silicic acid at an initial Si-normalized concentration of 100 mg/L. The primary mechanism behind the inhibition of As leaching by silicic acid was determined to be re-immobilization via the incorporation of arsenite during the formation of calcium silicates. In the presence of silicic acid, spent Ca-based adsorbents with arsenite had a lower As leaching ratio than those with arsenate. Therefore, spent Ca-based adsorbents with arsenite were found to have a higher environmental stability than those with arsenate.

Chemical study of bottom ash sintering in combustion of pelletized residual agricultural biomass

Although residual agricultural biomass is a high-potential energy source, its use is curtailed by the characteristics of its ash (quantity and composition). This ash can cause problems during combustion, including bottom ash sintering, which is the main focus of this study. Results of bottom ash proportion and sintering degree yielded by combustion tests carried out with four different agropellets (one was exclusively woody, and the others were blended with various herbaceous components –mixed agropellets) under different operating conditions in a laboratory fixed-grate reactor were presented in a previous paper. The inlet air temperature and the excess air were the two controlled operating parameters in the tests. To bring the analysis further, samples of bottom ash collected during these tests have been characterized using SEM-EDS and P-XRD techniques. The results confirmed that, given its high content of alkali metals and its high ratio (K+Na)/(Si+P), the differences in sintering behavior presented by the four agropellets are related to the formation of alkali metal silicates and phosphates; Si+P is the limiting and determining content that condition the sintering severity. Regarding the influence of operating conditions, only small variations in the composition of both the sintered and not sintered fractions of the bottom ash have been detected when varying inlet air temperature and excess air in the analyzed range. Nevertheless, these SEM-EDS results have led to the detection of two opposing phenomena related to increased excess air: higher ash entrainment and lower vaporization, which are associated with the increase of air velocity and the reduction of combustion temperature, respectively. Whereas in the mixed agropellets and in the range of excess air tested these two effects offset each other, woody agropellet is significantly affected by entrainment due to its lack of sintered matter. These results provide useful information towards a solution of the problem posed by sintering in the combustion of residual agricultural biomass.

Photoactive bismuth silicate catalysts: Role of preparation method

The published papers highlight the photocatalytic properties of bismuth silicates, especially Bi2SiO5. Meanwhile, there is limited information on the formation of bismuth silicates that hinders the controlled synthesis of pure phases of bismuth silicates or composites on the basis thereof. The study is focused on the influence of preparation conditions on the interaction of initial reagents at each stage of synthesis taking into account the roles of solvent (water or ethylene glycol (EG)), precipitation agent, Si-containing component as well as the calcination procedure. The key role in these transformations is played by the stable Bi(III)–EG complex that participates in the Bi3+ reduction to metallic state under solvothermal conditions. The phase formation process is accomplished during the thermal treatment of the samples synthesized from EG solution. A four-phased composite material demonstrates the highest photocatalytic performance in photodegradation of Rhodamine B and phenol solutions via selective deethylation and mineralization processes, respectively.

Experimental and theoretical investigation of the formation of two-dimensional Fe silicate on Pd(111)

A single layer of Fe silicate was grown on Pd(111) and analyzed experimentally and theoretically. Following sequential deposition of SiO and Fe and annealing above 900 K in O2, an incommensurate but well-ordered, low-defect density layer was observed with low-energy electron diffraction and scanning tunneling microscopy (STM). The STM images revealed a moiré pattern due to the lattice mismatch between the relaxed oxide layer and the substrate, while high-resolution images showed a honeycomb structure consistent with a silicate layer with six-membered rings of corner-sharing SiO4 tetrahedra at its surface. Reflection-absorption infrared spectroscopy revealed a single peak at 1050 cm−1 due to Si–O–Fe linkages, while x-ray photoelectron spectroscopy data indicated a Si/Fe ratio of one, that the Fe were all 3+, and that the Si atoms were closest to the surface. Consistent with these experimental observations, first principles theory identified a layer with an overall stoichiometry of Fe2Si2O9 with the six-membered rings of SiO4 tetrahedra at the surface. One of the oxygen atoms appears as a chemisorbed atom on the Pd surface, and, thus, the layer is better described as Fe2Si2O8 atop an oxygen-covered Pd surface. The Fe2Si2O8 is chemically bound to the Pd surface through its oxygen atoms; and the passivation of these bonds by hydrogen was investigated theoretically. Upon hydrogenation, the adsorbed O atom joins the Fe silicate layer and thermodynamic analysis indicates that, at room temperature and H2 pressures below 1 atm, Fe2Si2O9H4 becomes favored. The hydrogenation is accompanied by a substantial increase in the equilibrium distance between the oxide layer and the Pd surface and a drop in the adhesion energy to the surface. Together the results indicate that a highly ordered 2D Fe silicate can be grown on Pd(111) and that subsequent hydrogenation of this layer offers potential to release the 2D material from the growth substrate.

Recycling hazardous steel slag after thermal treatment to produce a binder for cemented paste backfill

Steel slag is a solid waste during steel production, which poses substantial environmental impacts. The present work explored the feasibility of preparing an eco-friendly binder for cemented paste backfill (CPB) using steel slag after being thermal treated. Steel slag was calcined at 600, 800 and 1000 °C. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) were performed to study the structure modification on steel slag after thermal treatment. Steel slag calcined at 1000 °C (SS100) showed the highest pozzolanic reactivity due to the formation of dicalcium silicate (C2S). SS1000 was then used to prepare binders for CPB supplemented by metakaolin (MK) and silica fume (SF). The results showed that samples with 15 wt% MK and 5 wt% SF addition (P3) demonstrated the optimal performances. After curing for 28 days, the CPB samples (CPB3) using P3 as a binder showed the highest compressive strength (2.88 MPa) at a binder/tailings ratio of 1/4, reaching around 88.7% of that of the CPB samples made using ordinary Portland cement (OPC). Leaching tests showed that all the toxic metals were stabilized in CPB3, which posed no environmental risk.

Potassium Ions Regulated the Disproportionation of Silicon Monoxide Boosting Its Performance for Lithium-Ion Battery Anodes

Silicon monoxide (SiO) has captured great attention as one of the most promising anode materials due to its great cycling stability as well as high theoretical capacity. However, hindered by its low initial Coulombic efficiency, the possible large-scale application remains an urgent issue. Herein, through a facile method, potassium ions are introduced into the disproportionation process of SiO to promote the transition from amorphous SiOx into the cristobalite phase with higher crystallinity. The cristobalite phase exhibits lower reaction activity with lithium ions, which reduces the formation of lithium silicate. As a consequence, the cycling stability and the initial Coulombic efficiency (ICE) are improved. In addition, it is proved in our study that increasing the heating temperature is helpful to improve the ICE and cycling stability of SiO. This work provides a facile disproportionation method for the microstructure regulation of SiO, offering a possible avenue for the development of large-scale applications of the SiO anode.

Porosity of Calcium Silicate Hydrates Synthesized from Natural Rocks.

In this work, the suitability of natural raw materials with various modifications of SiO2—granite sawing waste (quartz) and opoka (a mixture of cristobalite, tridymite, quartz, and an amorphous part)—for the 1.13 nm tobermorite and xonotlite synthesis is examined, and their specific surface area, pore diameter and volume, and the predominant pores are determined. Hydrothermal syntheses were carried out at 200 °C for 12 and 72 h from mixtures with a molar ratio of CaO/SiO2 = 1.0. X-ray diffraction analysis, simultaneous thermal analysis, and scanning electronic microscopy were used, which showed that in the lime–calcined opoka mixture the formation of crystalline calcium silicate hydrates takes place much faster than in the lime–granite sawing waste mixture. The high reactivity of amorphous SiO2 results in the rapid formation of 1.13 nm tobermorite and xonotlite (12 h). According to Brunauer, Emmet and Taller (BET) analysis data, this product features a specific surface area of ~68 m2/g, a total pore volume of 245 × 10−3 cm3/g, and has dominating 1–2.5 nm and 5–20 nm diameter pores. This porosity of the material should provide good thermal insulation properties of the products made from it as no air convection occurs in the fine pores.

A computational study on the mixing and reduction of slags from ferrochrome and stainless steel production

Large amounts of slags are formed in the production of ferrochrome and stainless steels. Although there are many suitable applications for ferrochrome slag products, some chromium is lost with the slag. On the other hand, utilisation of some stainless steel slags are hindered by the volumetric changes of the dicalcium silicate, which is formed during the solidification and cooling of slags. Furthermore, there is some chromium also in stainless steel slags and its recovery would significantly improve the material efficiency and decrease the environmental impact of the stainless steel production. The purpose of this study was to estimate the possibilities to overcome these hindrances by mixing different slags with different ratios thus creating slags in which no dicalcium silicate is formed during solidification and from which chromium could be reduced in one process step before solidification of the slag. Carbon, methane and ferrosilicion were considered as possible reductants in this study. The study was conducted in three stages in which thermochemical simulations were used to estimate the solidus and liquidus temperatures of different slag mixtures, amounts of reductants required to reduce chromium from the slag mixtures and finally the needs for additional energy in the reduction process. The results of thermodynamic simulations were used to define suitable slag mixing ratios for chromium recovery taking avoidance of dicalcium silicate formation, distribution of chromium between different phases as well as the amounts of different possible reactants required for chromium recovery into account. According to the results, at least 30% of ferrochrome production slags are required in the mixtures in order to avoid the formation of dicalcium silicate, whereas the excessive use of ferrochrome slags increases the amount of solid spinel phase in the system thus making the chromium recovery more challenging.

Geopolymer concrete with treated recycled aggregates: Macro and microstructural behavior

Geopolymer concrete is a more environmentally friendly alternative than conventional concrete because its production does not need cement. Instead, waste materials such as fly ash, rice husk ash, slag, etc. are used. This research studies the effect of the activator concentration (6–10 M), curing temperature (40–80 °C) and incorporation ratio of thermally and mechanically treated recycled aggregate (0–100%) on changes in the macrostructural properties of geopolymer concrete. All raw materials were previously characterized by thermogravimetric and differential thermal analysis, X-ray fluorescence and X-ray diffraction. Furthermore, a microstructural characterization was carried out on two geopolymer concrete mixes used as reference: one made with a natural aggregate and the other with a treated recycled aggregate. The interfacial transition zone between the treated recycled aggregate/natural aggregate and geopolymer paste was studied using SEM-EDS. A beneficial effect of the treated recycled aggregates on the compressive strength of geopolymer concrete was observed due to formation of silicate hydrated gel (CSH) and calcium aluminate silicate hydrated gel (CASH). However, a slight increase in the water absorption of geopolymer concrete and the mass of peels after 28 freezing and thawing cycles occurred because of the presence of residual porous cement mortar on the treated recycled aggregate surface.

Spectral and mineralogical alteration process of naturally-heated CM and CY chondrites

Spectral and mineralogical analyses were performed using nine naturally hydrated and dehydrated carbonaceous chondrite samples which were classified into heating stages (HS) from I to IV based on previous X-ray diffraction results. In-situ heating of samples at 120–400 °C was performed during spectral measurements and successfully removed absorbed water and part of rehydrated water from chondrite samples. Reflectance spectra of HS-I samples show the positive slope in visible (Vis)-infrared (IR) range and the significant 0.7- and 3-μm absorption bands. The 0.7-μm band appears in only HS-I sample spectra. With increasing temperature of heating, (1) Vis-IR slope decreases, (2) the 3-μm band becomes shallower, and (3) Christiansen feature (CF) and Reststrahlen bands (RB) shift toward longer wavelength. TEM-EDX analyses showed that the matrix of strongly-heated chondrites consists of tiny olivine, low-Ca pyroxene, and FeNi metallic particles mostly smaller than 100 nm in diameter, instead of Fe-rich serpentines and tochilinite observed in the HS-I chondrite. Therefore, in proportion to the heating degree, amorphization and dehydration of serpentine and tochilinite from HS-I to HS-II may cause the 0.7- and 3-μm band weakening, spectral bluing and darkening of chondrite spectra. In addition, formation of secondary anhydrous silicates and FeNi-rich metal grains at HS-IV would be responsible for the 3-μm band depth decrease, spectral reddening and brightening, CF peak shift, and RB changes of chondrite spectra. Those spectral changes in response to mineralogical alteration processes will be useful to interpret planetary surface composition by remote-sensing observations using ground-based or airborne/space telescopes or spacecraft missions.

Influence of metallurgical slag additives on proppants synthesis processes based on drilling muds

The slags formed during the smelting of ferrous alloys consist of more than 75% oxides of calcium, iron and silicon that predetermines broad prospects for their use as an alternative to natural raw materials. Considering the significant amount of alkaline earth oxides (calcium and magnesium) in the metallurgical slag, it seems promising to use it as a fluxing additive in highalumina aluminosilicate materials, one of which is proppants – granular aluminosilicate materials designed to enhance oil production by hydraulic fracturing. The aim of this work is to study the effect of metallurgical slag additives on the processes of proppant synthesis based on drill slurry. The composition of the initial drill slurry from the Morozovskoye deposit and the fluxing additive of metallurgical slag of JCS “TagMet” was studied. Based on the composition of the metallurgical slag, an assumption was made about its fluxing activity in the synthesis of silicate materials, which is due to a large amount of alkaline earth metal oxides, a low intensity of the crystalline phase and the presence of preliminary heat treatment. Five compositions of raw mixtures for the production of proppants were developed with the addition of two types of fluxing additives: metallurgical slag and sodium hydroxide. Qualitative (texture and color) and quantitative (density and sintering coefficient) parameters of the degree of sintering and melting of mixtures were determined. It was shown that both additives have a fluxing effect on drill slurry. It was found that the introduction of metallurgical slag leads to a change in texture to a more sintered one, as well as an increase in density and sintering coefficient. The introduction of sodium hydroxide has a more active effect on drill slurry, which leads to glass formation of the surface and further melting of the material. Physicochemical processes under the fluxing effect of the selected additives were considered, which is explained by the formation of more low-melting silicates, which reduce the final melting temperature of the mixture. The work was performed at YuRGPU (NPI) with the financial support of the Russian Science Foundation under the Agreement No. 20-79-10142 “Development of an effective technology for synthesis of aluminosilicate proppants using waste of drilling oil and gas wells of the Southern Federal District” (head – A. A. Tretyak).

Investigation of control methods for agglomeration and slagging during combustion of olive cake in a bubbling fluidized bed combustor

Olive cake is an agricultural residue with a high calorific value and high availability. It should be considered in energy production for use in waste management and the waste-to-energy approach. However, there are some ash-related problems encountered during olive cake combustion with a silica bed due to its high alkali content. In this study, two control methods (silica bed with olive cake leaching, and limestone-bed material) were investigated to prevent ash-related problems. The combustion tests were conducted in a bubbling fluidized bed. Bottom ashes and slags were analyzed to determine elemental composition and phase composition. Agglomeration was detected at 850 °C for olive cake combustion by itself. Removal of the alkalis in the olive cake by leaching prevented silicate formation and produced a noticeable decrease in the rate of silica bed agglomeration and slag formation during combustion of the leached olive cake. Using limestone as the bed material instead of silica sand was even more effective than leaching of olive cake in terms of preventing slagging. The use of limestone as fluidizing bed material causes Ca and Si to form compounds with the high melting point. The positive effect of these control methods on agglomeration and slag formation should make it possible to use olive cake in energy production without any operational problems.

Phase equilibrium occurring during low-carbon iron-based melt deoxidation with silicostrontium

At the moment, to improve quality of metal (especially low-alloyed), out-of-furnace steel processing technologies are used with complex alloys utilization, which include alkaline earth metals (ALM) in addition to silicon. Study of strontium additives effect on deoxidation and liquid steel modification processes is one of the promising areas of research in field of metallurgical technologies. Thermodynamic modeling of phase equilibria in Fe – Sr – Si –C– O system melt was carried out using method of constructing surface of components solubility in metal. Solubility surface determines stability limits of non-metallic phases formed during deoxidation, depending on composition of liquid metal of the studied system. The calculation was carried out using equilibrium constants of reactions occurring in the melt during deoxidation, as well as the first order interaction parameters (according to Wagner) of elements in liquid iron. Activity of the oxide melt components was determined using theory of subregular ionic solutions. Activity of the gas phase was calculated taking into account partial pressures. Simulations were performed for two temperatures (1550 and 1600 °C) for fixed carbon concentrations (0 (no carbon in liquid iron) and 0.1 % (low-carbon metal melt)). It has been shown that, in comparison with silicon, strontium is stronger deoxidizing agent in liquid metal. According to the simulation results, liquid oxide non-metallic inclusions of variable composition or strontium ortho- and metasilicates Sr2SiO4 and SrSiO3 (with an increase in strontium concentration) should be the main oxide phases in deoxidation products. Decrease in the temperature of liquid metal leads to changes in phase formation (formation of SrSiO3 silicate becomes possible).

The Composition, Structure, and Origin of Carbonate Concretions Sampled in the South Kambalnyi Central Thermal Field, Kamchatka

Carbonate concretions are formed at the base of a sequence of hydrothermal clay in the South Kambalnyi Central Thermal Field situated in the southern part of the Kambalnyi volcanic mountain range, Kamchatka. The concretions have complex chemical and mineral compositions: apart from aragonite which is the main component of each layer, the chemical compounds identified there include oxides of iron and silicon, sulfates of calcium and barium, sulfides of iron and other metals, carbonates of iron and manganese, siliceous ferromanganese formations, nitrogen compounds, and phosphates of calcium and rare metals. The concretions have diverse structures and textures that indicate a multiphase character of formation for these mineral aggregates. It is thought that their formation was due to the discharge of deep-seated alkaline metalliferous solutions in a zone of rock argillization of the South Kambalnyi Central Thermal Field.

Behavior of light elements in iron-silicate-water-sulfur system during early Earth\u2019s evolution

Hydrogen (H) is considered to be one of the candidates for light elements in the Earth’s core, but the amount and timing of delivery have been unknown. We investigated the effects of sulfur (S), another candidate element in the core, on deuteration of iron (Fe) in iron–silicate–water system up to 6–12 GPa, ~ 1200 K using in situ neutron diffraction measurements. The sample initially contained saturated water (D2O) as Mg(OD)2 in the ideal composition (Fe–MgSiO3–D2O) of the primitive Earth. In the existence of water and sulfur, phase transitions of Fe, dehydration of Mg(OD)2, and formation of iron sulfide (FeS) and silicates occurred with increasing temperature. The deuterium (D) solubility (x) in iron deuterides (FeDx) increased with temperature and pressure, resulting in a maximum of x = 0.33(4) for the hydrous sample without S at 11.2 GPa and 1067 K. FeS was hardly deuterated until Fe deuteration had completed. The lower D concentrations in the S-containing system do not exceed the miscibility gap (x < ~ 0.4). Both H and S can be incorporated into solid Fe and other light elements could have dissolved into molten iron hydride and/or FeS during the later process of Earth’s evolution.

COMBINED EFFECT OF PYROMETALLURGICAL AND HYDROMETALLURGICAL PROCESSES ON QUARTZ ORE PURIFICATION

Recent advances in high-tech applications have highlighted the growing demand on highly pure silicates like quartz. Therefore, purification of quartz ore was determined as the subject of this study performed by pyrometallurgical followed by hydrometallurgical processes. In this research, the effect of thermal treatment (TT) followed by oxalic acid (OA) bleaching of quartz was examined to have a better understanding on the relationship between Fe remaining in concentrate and colour response. The level of TT temperature was found to have a significant effect on the purification of quartz by OA. The maximum Fe rejection rate was observed to occur both for non-treated and TT quartz up to 250°C. TT between 400°C and 900°C showed poor purification performance: decreasing L* value, and increasing a* and b* values. It is important to note that further increase in TT temperature to 1100°C resulted in the poorest bleaching: Fe rejection rate decreased, but colour response improved providing the highest L* value and the lowest a* and b* values. This finding was explained by the formation of dissolution resistant iron silicates. Moreover, the rate of Fe removal from quartz ore and differences observed in its colour response by OA bleaching were explained by changes in crystalline structure and formation of microcracks.