The production of ultra clean coal by sequential leaching with HF followed by HNO3

A UK bituminous coal with particle size <62 μm and containing 5.0 wt% ash and 2.4 wt% S was treated with a two-stage leaching sequence of aqueous HF followed by aqueous HNO₃. The ash and S contents reduced to 0.2 wt% and 1.3 wt%, respectively. In addition, the calorific value (CV) dropped from 31.5 to 29.5 MJ/kg, and the N content increased from 2.0 to 2.8 wt%, due to attack on the carbonaceous matrix during the HNO₃ leach. Interestingly, this attack only occurs when HNO₃ reacts with and dissolves pyrite, suggesting that it is the products of reaction between HNO₃ and pyrite which react with the coal. It is proposed here that localised concentrations of sulphuric and nitric acids at the pyrite site may react to form the powerful nitrating agent NO₂⁺ which then reacts with the carbonaceous coal matrix. The fluoride content of the coal increases from 80 to 3240 ppm after the HF leach, yet drops to 130 ppm following the HNO₃ leach. The mineral matter remaining in the coal after the leaching sequence consists largely of Fe, which is most likely present as finely disseminated unreacted pyrite.     

Production of glass from coal fly ash

The coal fly ash from a Chinese thermal power plant was vitrified after the addition of ∼10 wt% Na₂O. The glass products have suitable viscosity at 1200 °C and displayed a good chemical durability. The heavy metals of Pb, Zn, Cr and Mn were successfully immobilized into the glass as determined by the toxicity characteristic leaching procedure method. Results indicate an interesting potential for the coal fly ash recycling to produce useful materials.     

Ultra clean coal production by microwave irradiation pretreatment and sequential leaching with HF followed by HNO3

The goal of this project was to develop and test an innovative coal cleaning process to reduce the ash content of coal and produce ultra clean coal (UCC). Coal samples that were prepared from concentrates of Iran's Zirab and Tabas coal preparation plants were found to have initial ash contents of 8.31 and 10.36%, respectively. These coals were demineralized with the combination of microwave irradiation pretreatment and dual acid leaching processes with HF followed by HNO₃ in a batch reactor. For samples that were microwaved and leached with HF, the reduction in ash ranged from 22 to 76% and from 22 to 82% for Zirab and Tabas coal samples, respectively, depending on leaching conditions. Microwave irradiation pretreatment had a positive effect on demineralization with HF, especially for the coarse size fractions. For the fine size fractions, reduced leaching times were found to enhance the effect of microwave irradiation. The microwave-irradiated, HF-leached product was leached further using a 1.4 M HNO₃ solution at a leaching temperature of 65 °C to remove fluoride sediments and pyritic sulfur. The ash content of the Zirab coal was reduced from 2.57 to 0.69% by leaching with the HNO₃ solution for 1 h. The ash content of the Tabas coal was reduced from 2.44 to 0.39% by leaching with the HNO₃ solution for 3 h. The results show that microwave irradiation can be considered as a pretreatment process in the chemical leaching of coal to produce UCC.     

Generation of ultra-clean coal from Victorian brown coal — Sequential and single leaching at room temperature to elucidate the elution of individual inorganic elements

This paper addressed the probability of the generation of ultra-clean coal from chemical leaching of low-rank Victorian brown coal. Sequential leaching was employed to determine the modes of occurrence of the major elements in the two coals studied, including Na, K, Mg, Ca, Fe, Al, Ti, and Si. The results indicate that, the modes of occurrence of individual metals vary greatly with brown coal sample and elemental type. For one brown coal tested, it is dominated by water-soluble and ammonium acetate-soluble ion-exchangeable cations. Therefore, a single washing through the use of woody biomass-derived pyroligneous acid or citric acid easily reduced the concentrations of its overall ash and even sulphur and chlorine to meet the requirements for gas turbine fuel. The leaching of the organically bound cations in this coal was also very rapid and completed in 5 min. In contrast, another brown coal tested is mainly composed of quartz and/or clay compounds which remained intact even after being leached with 5 M nitric acid. These mineral grains possess two peak size ranges in the coal, 1.0-2.2 μm and 4.6-10 μm. The former size bin was embedded deeply in coal matrix, and hence, its leaching upon acids was very slow when compared with coarse particles which are mostly discrete grains residing separately from coal matrix. The Na-EDTA was found to be able to mobilise the small grains substantially through its Na ion to penetrate coal matrix to react with Al, forming acid-soluble Na aluminates. The ammonium acetate-insoluble Ti and Fe polyhedra were also mobilised by the EDTA. Accordingly, the overall ash content in coal residue accounts for ~ 1.5 wt.%, relative to 2.6 wt.% in the corresponding raw coal and 2.0 wt.% in the ammonium acetate-insoluble residue.     

Sulphur reduction evaluation of selected high-sulphur Chinese coals

Under a collaborative project between China and UK partners, investigation was carried out on high-sulphur coals from Beisu Coal Mine in Shandong Province and Dizong Coal Mine in Guizhou Province to evaluate the sulphur reduction potential by coal preparation. Extensive evaluation of raw coal samples was carried out for design of optimum sulphur reduction processes. This paper discusses the background to the project and describes the evaluation process. This involved selection and sampling of the coals, determination of their washability characteristics and application of process prediction models to the washability data to assess their sulphur reduction potential. This paper demonstrates the potential of model predictions for the development of optimum desulphurisation processes using latest separation processes and plant design. The study has shown that over 50 wt% of sulphur can be removed from Beisu coal (ash 17.9 wt%, sulphur 4.8 wt%) and over 60 wt% of sulphur from Dizong coal (ash 33.5 wt%, sulphur 5.1 wt%) using latest coal cleaning processes.     

Exploring the possibilities of using Brazilian subbituminous coals for blast furnace pulverized fuel injection

The current study investigates the combustion and blast furnace injection performance of three Brazilian subbituminous coals (Mina do Recreio) and their beneficiation products using laboratory scale combustion tests. The coals have relative high ash yields (up to 40 wt%) that were reduced stepwise to levels as low as 12 wt%, dry basis. The reduction of ash yields is paralleled by a significant decrease in sulphur and inertinite contents.The combustion tests were performed in a drop tube reactor operating at 1300 °C using two different atmospheres (2.5 and 5% O₂). The chars exhibited preferentially rounded shapes with thick walls and abundant secondary porosity for the 2.5% O₂ chars, whereas the 5% O₂ chars showed very thin walls as a consequence of extensive burnout. The intrinsic reactivities of both set of chars were similar. The differences in conversion between the two working atmospheres were 24-37% and roughly tend to increase with increasing mineral matter content. Conversions as high as 76-81% were reached operating under 5% O₂ indicating that the coals are easy to burn. The small differences in burnout among the coals and their beneficiation products cannot be clearly attributed neither to mineral matter or inertinite content. A rough inverse relationship was found between the intrinsic reactivity of the chars and the inertinite content of the parent coal indicating that the char material derived from inertinite was intrinsically less reactive than that derived from vitrinite. These differences were no longer relevant at high temperature.Blast furnace injection performance was studied through thermobalance experiments using CO₂ atmosphere and 1050 °C temperature. It is apparent that the beneficiation process has no effect on the reactivity of the coals from Recreio Mine. The only exception is the low ash coal-2-LabB (11.5 wt%), for which a higher reactivity is indicated. The reactivity tests show also that the coals have adequate properties to be used together with imported coal blends in pulverized coal injection in the blast furnace (PCI).     

Characterization and evaluation of fly-ash from co-combustion of lignite and wood pellets for use as cement admixture

Conventional coal fly-ash (CFA) and two coal-biomass fly-ashes (CBFAs) were obtained at a thermoelectric power station (Atikokan, Ontario) from combustion of undiluted lignite coal and co-combustion of lignite coal with up to 66% wood pellets (on a thermal basis). Fly-ashes were characterized and analyzed for use as cement admixtures. Co-combustion did not markedly change the fly-ash composition, owing to an extremely low ash content of wood pellets compared to lignite coal; toxic metals and minor elements were within ranges reported for other coal fly-ashes. All fly-ashes had losses on ignition (LOI) <1 wt% and therefore complied with ASTM LOI regulations for use in concrete. All fly-ashes contained major amorphous phases, along with quartz and periclase. Partial substitution of cement with fly-ash (up to 40 wt%) had a moderate effect on the entrained air content of mortars (up to 2.5%), but this difference vanished upon addition of air entraining agent (0.6 mL/kg of cementitious material). Substituted mortars exceeded 75% of the strength of ash-free mortar after 28 days of curing (therefore meeting ASTM requirements for strength development), and by 90 days, met or surpassed 100% of the strength of ash-free mortar. Amending mortar with 20 wt% CFA or CBFA had no effect on its durability following repeated freeze-thaw cycles when air content was kept constant. Also, no micromineralogical differences were observed between hydrated CFA- and CBFA-amended mortars, with fly-ash particles reacting with Ca ions originating from dissolution of cement clinker or calcium hydroxide.     

Changes to the organic functional groups of an inertinite rich medium rank bituminous coal during acid treatment processes

A South African inertinite rich medium rank bituminous coal was subjected to a series of acid treatment steps using concentrations of HF and HNO₃ ranging between 1.5 and 4 M, temperatures between 65 and 80 °C and reaction time between 1.5 and 3 h for each acid. A HF and HCl procedure was used to reduce mineral matter to less than 2.5%. Analyses indicate that oxygen and nitrogen containing species are incorporated in the coal structure during the HF/HNO₃ leaching processes, but not during the HCl/HF/HCl procedure. The HF/HNO₃ methods lead to increased amounts of = N-OH groups formed on the remaining coal structure, whereas the HCl/HF/HCl procedure did not. The -COOH content upon HCl/HF/HCl treatment increased slightly possibly due to protonation of COO⁻ groups by HCl. The results show that the CO₂ reactivity doubled after the HF/HNO₃ procedures and increased by a factor of 2.5 for the HCl/HF/HCl treatment. Surface areas of the treated coal samples increased with the highest increase for the HCl/HF/HCl method. The increased surface areas may cause the increased CO₂ reactivity. The results indicate that the described acid treatment procedures with mixtures containing nitric acid have slight, but measurable, effects on the coal structure.     

Effect of catalyst addition on gasification reactivity of HyperCoal and coal with steam at 775-700 °C

HyperCoal is a clean coal with ash content <0.05 wt%. HyperCoal was prepared from a bituminous coal by solvent extraction method with an extraction yield of 66.7%. Effect of K₂CO₃ loading and temperature on steam gasification rate of HyperCoal and parent raw coal was investigated. Experiments were carried out at 775 and 700 °C for both HyperCoal and coal with 0-25% catalyst loading. Gasification rates increased with increased catalyst loading and reached a value above which rates did not change with catalyst loading. The catalyst loadings were 6% for HyperCoal and 20% for coal. The difference is most likely due to the difference in ash content of HyperCoal and coal and not due to the effect of H₂ inhibition on the rate. In the presence of catalyst effect of temperature was only on gasification rate and total gas yield and gas composition were unaffected.     

Split and collectorless flotation to medium coking coal fines for multi-product zero waste concept

The medium coking coal fines of − 0.5 mm from Jharia coal field were taken for this investigation. The release analysis of the composite coal reveals that yield is very low at 10.0% ash, about 25% at 14% ash and 50% at 17% ash level. The low yield is caused by the presence of high ash finer fraction. The size-wise ash analysis of − 0.5 mm coal indicated that − 0.5 + 0.15 mm fraction contains less ash than − 0.15 mm fraction. Thus, the composite feed was split into − 0.5 + 0.15 mm and − 0.15 mm fractions and subjected to flotation separately. The low ash bearing fraction (− 0.5 + 0.15 mm) was subjected to two stages collectorless flotation to achieve the concentrate with 10% ash. The cleaner concentrate (18.9%) with 10% ash was recovered which has an application in metallurgical industries. The concentrate of 30.2% yield with 12.5% ash could be achieved in one stage collectorless flotation which is suitable for use in coke making as sweetener. As the − 0.15 mm fraction contains relatively high ash, collector aided flotation using sodium silicate was performed to get a concentrate of 23.6% yield with about 17% ash. The blending of this product with cleaner tail obtained from − 0.5 + 0.15 mm produces about 35.0% yield with 17% ash and that can be utilized for coke making. The reject from the two fractions can be used for conventional thermal power plant or cement industries using a 23.5% ash after one stage collector aided flotation and the final tailings produced content ash of 61.6% can be used for fluidization combustion bed (FBC). This eventually leads to complete utilization of coal.     

Comparison of calculated mercury emissions from three Alberta power plants over a 33 week period - Influence of geological environment

Feed coals and fly ashes from two coal-fired power plants burning Alberta subbituminous coal were analyzed for C, Cl, Hg, and S and calorific values (for feed coal only), every week for a period of 33 weeks. The feed coals used in this study were deposited in brackish water and are compared to the coals deposited in a freshwater environment. The Hg and char (unburnt carbon) content of the fly ash was monitored to determine the variation of Hg and its possible relationship to the char types in the fly ash. The feed coals have Hg content of 0.026-0.089 mg/kg and their fly ash contains 0.02-0.243 mg/kg of Hg. The C content of the fly ashes ranges from 0.15% to 0.51%. The carbon/char was separated from the fly ash using HF and HCl. Reactive vitrinitic (formed from woody part of plants) and less reactive inertinitic (natural char) chars were separated by density separations of various specific gravities using ZnBr₂.The char is mostly reactive vitrinitic (67-80 vol.%). Both stations have similar range of C content for their respective fly ashes. However, station 2 shows a much wider range of Hg in fly ash compared to station 1. In general, the fly ash from coal deposited under brackish water environment (stations 1 and 2) appears to have same or higher Hg content for lower C content compared to the fly ash from coal deposited under fresh water environment.The calculated emitted Hg for the period of 33 weeks for station 1 is estimated to be 64-90% of the total input of Hg with an average of 74%. The calculated emitted Hg shows a more complex pattern for station 2 and falls into two groups; with group (a) showing higher enrichment index for both Hg and S. The calculated emitted Hg for this group is 43-74% with an average of 57%, indicative of more Hg being captured by fly ash, possibly due to interaction between Hg and S. In the second group (b) the emitted Hg is calculated to be 74-95% with an average of 85%. The relative enrichment of both Hg and S in group (b) is low compared to group (a), indicative of possible slight paleo-weathering of the feed coal.The present study indicates that geological parameters such as paleo weathering and also depositional environment of the feed coal may influence the Hg content of fly ash.     

Study of the characteristics of the ashing process of straw/coal combustion

An experimental study was performed to examine the ashing process during straw/coal co-combustion to determine the effect of the blending ratio on ash products. A total of eleven blending samples with coal contents varying from 5 wt.% to 90 wt.% along with pure wheat straw and pure coal samples were tested to determine the ash fusion points, oxide contents and mineral contents. Blends with coal contents between 5 wt.% and 15 wt.% were able to inhibit ash and reduced ash quantity. Blends with coal contents greater than 20 wt.% promoted ash and increased ash quantity. Thermal decomposition processes during the combustion and ash pyrolysis of the blends with 10 wt.% and 40 wt.% and the samples of pure coal and pure wheat straw were simulated using a Thermal Gravity Analyser. The results indicated that the ashing processes of the blends were influenced by the coupling reactions of the minerals in the straw and coal. When using a blend of 10 wt.% coal, more potassium (K) was accelerated into gaseous products during the volatile releasing and firing stage, which caused an ash quantity reduction effect. K₂O content was lowest in this sample, and a minimum amount of K compounds was detected. With a blend of 40 wt.% coal, because the coupling reactions of Ca and Al produced stable minerals of CaAl₈Fe₄O₁₉ and KSi₃AlO₈, less CaCO₃ and CaSO₄ were produced. Thermal decomposition at the ash pyrolysis stage was very weak and resulted in much less gaseous products than what would be expected at high temperatures; therefore, more ash residues remained.     

Study on the washability of the Kaitai coal, Guizhou Province China

Tonnage of coal samples were collected from Kaitai Coal field, Puan County, Guizhou Province and sieved into different particle size catalogs. The analysis of the overall coal suggested that the coal has low ash but high sulfur content of 3.17 wt.% with medium to high volatile content. The heating value of the coal is 31.668 mJ/kg.The coal sample with different particle size ranges were tested for float-sink using gravity separation method, in which ZnCl₂ solutions with different density are used. It is showed that decreasing sulfur to 2.12 wt.% can give coal yield of 94.55%, suggesting that the coal's floatability is good with sulfur. The coal yield is only 85.4% when reducing sulfur to 1.5 wt.%, and 76.7% when sulfur is decreased to 1.2% through the ZnCl₂ float-sink process. The δ ± 0.1 is 20.55, which is in the 20.1-30 range, suggest that Kaitai coal is difficult to float-sink for depyritisation.Characterization of the floated coal at different sizes showed that the organic sulfur may mainly be present in the small size, the pyrite sulfur is mainly present in the coal with bigger particle size, which can be easily removed through float-sink process. The ash in the small particle sized coal is mainly from kaolinite and quartz, while the pyrite is the main ash contributor to the coal with big size.     

Ash chemistry and mineralogy of an Indonesian coal during combustion: Part 1 Drop-tube observations

The paper reports a systematic and comprehensive laboratory investigation into the ash chemistry and mineralogical changes undergone by a low-rank Indonesian coal during combustion. Combustion experiments conducted in a drop-tube furnace included ash formation experiments (using cyclone and filter arrangement) under closely controlled conditions in the range of 1200–1400 °C and deposition experiments at a probe temperature of 750 °C. Tests conducted with raw coal, coal/additive mixtures and washed coal indicated significant changes in ash characteristics. Of the ash formation and deposit samples examined, the raw coal + bauxite showed the lowest glass content and high contents of corundum indicating low ash deposition propensities. When compared to the ash formation samples, the deposit samples showed even significantly lower glass contents and were enriched in quartz. With the exception of the raw coal + bauxite sample, all are characterized by high silica and iron and moderate aluminium contents. In contrast, the raw coal + bauxite sample have low silica and much higher alumina contents which is in agreement with XRD observations. QEMSCAN™ results showed that the ash particles are sparsely distributed suggesting lack of a deposit initiation layer. Experimental observations suggest that the use of raw coal with bauxite would appear to offer the best performance with respect to handling ash-related issues. Present findings are of practical significance to power utilities employing Indonesian coal as there is no comprehensive work reported in the literature on ash chemistry and mineralogy of such coals.     

Leaching behavior of heat-treated waste ash

The leaching behavior of heat-treated waste ash was studied to verify the possibility of the thermal treatment of waste incineration ash in existing incinerators and boiler combustion chambers. The influence of temperature, oxygen concentration and treatment time on the leaching behaviors of harmful heavy metals, especially lead (Pb) and chromium(VI) (Cr (VI)) were studied to clarify effective treatment conditions to suppress leaching. By examining the leaching behavior of Pb and Cr from ash heat-treated under various conditions, it was found that leaching can be suppressed by heat-treating the ash under conventional combustion conditions of around 900-1000 °C at 5-10% oxygen concentration. The leaching behaviors of Pb and Cr (VI) from real ash with different particle sizes and from model samples were also investigated in detail to find an effective method to suppress Pb leaching. It was found that the formation and growth of gehlenite (SiO₂·2CaO·Al₂O₃) in the ashes led to the decrease in the amount of Pb leaching. Therefore it was considered that the addition of an inorganic matrix with a high silica content that can promote gehlenite growth in the ash, for example, coal ash or waste glass is effective to suppress of Pb leaching.     

Studies of relationship between petrography and elemental analysis with grindability for Kentucky coals

The effects of macerals, ash, elemental analysis and moisture of wide range of Kentucky coal samples from calorific value of 23.65-34.68 MJ/kg (10,170-14,910 (BTU/lb)) on Hardgrove Grindability Index (HGI) have been investigated by multivariable regression method. Two sets of input: (a) macerals, ash and moisture (b) macerals, elemental analysis and moisture, were used for the estimation of HGI. The least square mathematical method shows that increase of the TiO₂ and Al₂O₃ contents in coal can decrease HGI. The higher Fe₂O₃ content in coal can result in higher HGI. With the increase of micrinite and exinite contents in coal, the HGI has been decreased and higher vitrinite content in coal results in higher HGI. The multivariable studies have shown that input set of macerals, elemental analysis and moisture in non-linear condition can be achieved an acceptable correlation, R = 90.38%, versus R = 87.34% for the input set of macerals, ash and moisture. It is predicted that elemental analysis of coal can be a better representative of mineral matters for the prediction of HGI than ash.     

Bio-oil derived from empty fruit bunches

The fast pyrolysis of washed and unwashed empty fruit bunches (EFB), a waste of the palm oil industry, is investigated in this study. Firstly, the composition and particle size distribution of the washed and unwashed feedstock were determined and the thermal degradation behaviour was analysed by TGA. Then a 150 g/h fluidised bed bench scale fast pyrolysis unit was used to study the impact of key variables: reactor temperature in the range 425-550 °C and feedstock ash content in the range 1.03-5.43 mf wt%. The properties of the liquid product were analysed and compared with wood derived bio-oil and petroleum fuels. It was found that the maximum ash content of washed feedstock that still yields homogenous liquids is less than about 3 mf wt%. The experiments also indicated that the fast pyrolysis of washed EFB with a low ash content gave similar yields as commonly obtained for wood.     

Ash deposition behavior of upgraded brown coal in pulverized coal combustion boiler

Ash with a low melting point causes slagging and fouling problems in pulverized coal combustion boilers. Ash deposition on heat exchanger tubes reduces the overall heat transfer coefficient due to its low thermal conductivity. The purpose of this study is to evaluate the ash deposition for Upgraded Brown Coal (UBC) and bituminous coal in a 145 MW practical coal combustion boiler. The UBC stands for Upgraded Brown Coal. The melting temperature of UBC ash is relatively lower than that of bituminous coal ashes. Combustion tests were conducted on blended coal consisting 20 wt.% of UBC and 80 wt.% of bituminous coal. Before actual ash deposition tests, the molten slag fractions in those coal ashes were estimated by means of chemical equilibrium calculations. The calculation results showed the molten slag fraction for UBC ash reached approximately 90% at 1523 K. However, that for blended coal ash decreased to 50%. These calculation results mean that blending UBC with bituminous coal played a role in decreasing the molten slag fraction. This phenomenon occurred because the coal blending led to the formation of alumino-silicates compounds as a solid phase. Next, ash deposition tests were conducted using a practical pulverized coal combustion boiler. A water-cooled stainless-steel tube was inserted in locations at both 1523 K and 1273 K in the boiler to measure the amount of ash deposits. The results showed that the mass of ash deposition for blended coal did not greatly increase, compared with that for bituminous coal alone. Therefore, appropriately blending UBC with bituminous coal enabled the use of UBC without any ash deposition problems in practical boilers.     

Olive cake combustion in a circulating fluidized bed

In this study, a circulating fluidized bed of 125 mm diameter and 1800 mm height was used to find the combustion characteristics of olive cake (OC) produced in Turkey. A lignite coal that is most widely used in Turkey was also burned in the same combustor. The combustion experiments were carried out with various excess air ratios. The excess air ratio, λ, has been changed between 1.1 and 2.16. Temperature distribution along the bed was measured with thermocouples. On-line concentrations of O₂, SO₂, CO₂, CO, NO_x and total hydrocarbons were measured in the flue gas. Combustion efficiencies of OC and lignite coal are calculated, and the optimum conditions for operating parameters are discussed. The combustion efficiency of OC changes between 82.25 and 98.66% depending on the excess air ratio. There is a sharp decrease observed in the combustion losses due to hydrocarbons and CO as the excess air ratio increases. The minimum emissions are observed at λ=1.35. Combustion losses due to unburned carbon in the bed material do not exceed 1.4 wt% for OC and 1.85 wt% for coal. The combustion efficiency for coal changes between 82.25 and 98.66% for various excess air ratios used in the study. The ash analysis for OC is carried out to find the suitability of OC ash to be used as fertilizer. The ash does not contain any hazardous metal.     

Effect of zirconia content on the mechanosynthesis and structural features of fluorapatite-based composite nanopowders

The influence of zirconia content on the mechanosynthesis of fluorapatite–zirconia composite nanopowders was investigated. The structural features of the specimens with different amounts of monoclinic zirconia (0–20 wt%) were examined after 5 h of mechanical activation. Results indicated that the formation of fluorapatite–zirconia composite was strongly influenced by the zirconia content. In the presence of 5–10 wt% monoclinic zirconia, fluorapatite–zirconia composite nanopowders were produced after 5 h of milling. With increasing zirconia content to 20 wt%, there was no trace of fluorapatite–zirconia composite. In the absence of zirconia, the average crystallite size, lattice strain and the volume fraction of grain boundary of fluorapatite were about 34 nm, 0.469% and 8.38%, respectively. These values reached 24 nm, 0.754% and 11.71% with the addition of 10 wt% monoclinic zirconia. In the presence of 10 wt% monoclinic zirconia, the fraction of crystalline phase considerably decreased after 5 h of milling. Results revealed that the lattice parameter deviations were affected by the zirconia content. Based on SEM observations, no significant differences in the size distribution and morphology of the agglomerates were observed.