Quality Improvement of Low-Grade Calcium Carbonate Using Induced Roll Magnetic Separator

Calcium carbonate (CaCO3) is an essential raw material in the manufacture of goods and industrial products like cement, rubber, paper, paints, food, and medicines. For this compound to be economically valuable, however, its quality needs to meet the standard market requirements. Among the various impurities found in natural CaCO3-bearing ores, iron (Fe) is one of the most problematic. In this study, the upgrading of low-grade CaCO3 from a processing plant in Thailand by magnetic separation was investigated. Detailed characterization of the low-grade material was also carried out to identify the solid-phase partitioning of Fe. The results showed that Fe was mainly associated with magnetite and pyrrhotite in the ore, and during processing, additional Fe was introduced from the ball milling process. To improve the quality of this low-grade CaCO3, the effects of magnetic field intensity, feed rate, and repetition on the induce roll magnetic separation were investigated. Based on the results, higher magnetic field intensity, lower feed rate, and more repetition are required for the upgrading of low-grade CaCO3

Batch Studies On Arsenic Adsorption Onto Lignite, Bentonite, Shale And Iron Sand: Effects Of Ph, Time, Particle Size And Sulfate Concentration

Arsenic (As) is a toxic element found in both natural and anthropogenic sources. High concentration of this element was recently uncovered in the groundwater of Sumbawa Island, Indonesia. To mitigate this problem, As adsorption potential of natural geological materials like lignite, bentonite, shale, and iron sand obtained in Indonesia were evaluated by batch experiments. Arsenic adsorption onto these materials was investigated as a function of solution pH, particle sizes of adsorbents and coexisting sulfate concentration. In addition, batch leaching experiments were performed to elucidate the stability of geogenic As present in all adsorbents at different pHs. The results showed that among these natural materials tested, lignite was the most effective adsorbent of As(V) followed by bentonite, shale and then iron sand, and that the amounts of As(III) adsorbed onto all adsorbents were lower than those of As(V).This indicates that As(III) is more mobile in comparison to As(V). The adsorption isotherms of As(III) and As(V) conformed to nonlinear types, either Langmuir or Freundlich. It was found that adsorption of As onto these natural adsorbents was pH-dependent. This could be attributed to the changes in the surface charges of the adsorbents with pH. With respect to the adsorbent particle size, the amount adsorbed somewhat increased with decreasing particle size, which could be explained by the larger surface area of the smaller particles. Acidic (pH 10) conditions destabilized the geogenic As content of the adsorbents, indicating that the effectiveness of these natural materials as adsorbents is greatly limited by the pH of the contaminated system. Keywords: Adsorption, arsenic, natural geological materials, particle size, p

Editorial for Special Issue “Novel and Emerging Strategies for Sustainable Mine Tailings and Acid Mine Drainage Management”

Climate change is one of the most pressing problems facing humanity this century [...

Mobilization and speciation of arsenic from hydrothermally altered rock containing calcite and pyrite under anoxic conditions

The effects of water residence time and anoxic conditions on the mobilization and speciation of arsenic (As) in a calcite- and pyrite-bearing altered rock excavated during a road-tunnel project has been evaluated using batch and column laboratory experiments. Higher infiltration rates (i.e., shorter water residence times) enhanced the leaching of As due to the higher pH values of the effluents and more rapid transport of dissolved As through the columns. The concentration of As in the effluent also increased under anoxic conditions regardless of the water residence time. This enhanced leaching of As under anoxic conditions could be attributed to a significant pH increase and decreased Fe oxyhydroxides/oxides precipitation compared to similar experiments done under ambient conditions. Processes that controlled the evolution of pH and the temporal release mechanisms of As under anoxic conditions were identical to those previously observed under ambient conditions: the dissolution of soluble phases, pyrite oxidation, co-precipitation and/or adsorption/desorption reactions. Speciation of As in the column experiments could partly be attributed to the pH-dependent adsorption of As species onto Fe oxyhydroxides/oxides precipitates. Moreover, apparent equilibriums of the total As and arsenite (As[III]) concentrations were delayed under anoxic conditions in both batch and column experiments

Mobilization and speciation of arsenic from hydrothermally altered rock in laboratory column experiments under ambient conditions

This paper describes the mobilization and speciation of arsenic (As) found in hydrothermally altered rock under oxic column conditions. The altered rock sample was obtained from a tunnel project located in the Nakakoshi area of Hokkaido, Japan, whose geology is represented by slate, shale and sandstone. This area underwent silicification, pyritization and argillic alteration resulting in As-enrichment of the rock. Results of the column experiments show that the infiltration rate, bulk density and rock bed thickness affected the duration of water residence, which in turn influenced the pH of the rock-water system. Coexisting ions most notably calcium (Ca2+) at amounts greater than ca. 50 mg/L retarded the mobilization of As. Mobilization of As from the rock with time occurred in two stages: stage 1 (weeks 1-20) with higher As leaching and stage 2 (weeks 20-76) characterized by nearly constant As release. In addition, pore water As concentrations revealed that the columns developed into two regions: the top half where most of the leaching occurred and the bottom part dominated by adsorption. Thus, the mechanisms controlling the mobilization of As from the rock is a combination of one or more of the following processes: dissolution of soluble As-bearing fractions, pyrite oxidation and adsorption reactions. Arsenite (As[III]) was the dominant species in the effluent at the start of the experiment in columns with shorter water residence time and lower pH conditions (<8). On the other hand, arsenate (As[V]) was the major inorganic species released from the rock at higher pH (8-9.5) and when the system was close to equilibrium. Speciation of As with depth also indicated that As[III] disappeared around the bottom half of the columns, probably as a result of adsorption and/or oxidation. Arsenic speciation is partially controlled by the pH dependent adsorption of As species. The important adsorbent phases in the rock included Fe-Al oxides/oxyhydroxides, clay minerals and organic matter, which permitted the columns to attenuate additional As loadings including As[III]. Implications of these results on the design of a novel disposal method for these altered rocks include the enhancement of As adsorption through the addition of natural or artificial adsorbents and the utilization of a covering soil with low permeability to minimize rainwater infiltration into the rock

Combined neutralization-adsorption system for the disposal of hydrothermally altered excavated rock producing acidic leachate with hazardous elements

Hydrothermally altered rock excavated in a tunnel project produces acidic leachate containing hazardous elements that include arsenic (As), lead (Pb), copper (Cu) and zinc (Zn). To mitigate this problem, this paper evaluated a combined neutralization-adsorption system that used readily available and cheap reagents like calcium carbonate (CaCO3) and partly-weathered volcanic ash. Batch neutralization experiments showed that CaCO3 was effective in raising the pH of the leachate around neutral while the batch adsorption experiments illustrated that the volcanic ash sample collected near the tunnel project area was highly capable of adsorbing arsenate (As[V]), Pb, Cu and Zn. Under column conditions, the amount of hazardous elements released from the rock increased by several folds and their breakthrough curves had flushing-out trends. The mechanisms of As and heavy metals release probably include the dissolution of soluble phases and pyrite oxidation. Addition of CaCO3 in the column experiments based on estimates from the batch results underestimated the amount of neutralizer needed to adjust the effluent pH to around 8, resulting only in slight increase of the pH. Nevertheless, the presence of CaCO3 drastically reduced the amount of hazardous elements released from the altered rock especially during the initial stages of the column experiments. Combining neutralization and adsorption effectively reduced the amount of As and heavy metals in the effluent throughout the duration of the column experiments, which is attributed to the slight neutralizing effect of volcanic ash that raised the pH around circumneutral as well as its rich Al and Fe oxyhydroxide/oxide contents. The combined system immobilized the hazardous elements through a combination of co-precipitation and adsorption reactions and showed potential as an alternative method for the disposal of altered rocks producing acidic leachate

The roles of pyrite and calcite in the mobilization of arsenic and lead from hydrothermally altered rocks excavated in Hokkaido, Japan

This paper describes the enrichment of hydrothermally altered volcanic and sedimentary rocks with arsenic (As) and lead (Pb), and the effects of pyrite and calcite on the mobilities and release mechanisms of these toxic elements under oxic and anoxic conditions. Enrichment of the altered rock with As and Pb predominantly occurred in precipitated pyrite grains and not on the alumino-silicate minerals making up the matrix of the rock. Arsenic was incorporated in pyrite grains formed during alteration in both volcanic and sedimentary rocks, but Pb was only found in the pyrite grains of the volcanic rock samples. When in contact with water, altered volcanic rocks had acidic pH while altered sedimentary rocks had alkaline pH. The mobilities of both As and Pb from the altered rocks were enhanced at acidic and alkaline pH and a minimum was observed in the circumneutral pH under both oxic and anoxic conditions. The absence of O2 retarded the oxidation of pyrite most notably in the alkaline region but not in the acidic and circumneutral pH. The absence of CO2 increased the pH of samples with significant calcite content but did not affect those containing substantial amounts of pyrite. Increasing the CO2 also had insignificant effect on the concentrations of As and Pb in the leachate. The mechanisms controlling the mobilization of As and Pb from these rocks like dissolution of soluble secondary minerals, pyrite oxidation and adsorption were all related to pyrite while the pH of the rock when in contact with water was controlled by pyrite and calcite. Thus, excavated waste rocks that have been altered can be grouped based on the relative abundance of pyrite and calcite and their pH when in contact with water

Acid mine drainage sources and hydrogeochemistry at the Yatani mine, Yamagata, Japan: A geochemical and isotopic study

This paper describes the geochemistry of groundwater and its flow system in the closed Yatani mine in southern Yamagata Prefecture, Japan. The mine is located in a sulfide deposit containing pyrite and has been generating acid mine drainage (AMD). The study was intended to elucidate the formation of AMD and its flow patterns using geological, hydrological, geochemical, and isotopic techniques. The results indicate that AMD is formed by the interaction of groundwater with sulfide minerals, sand slime, and tailings back-filled into excavated mine areas. Groundwater recharge areas were identified on the mountain slope at an elevation of similar to 900 m. The formation of AMD in the drifts and shaft was more extensive than that in the deeper drainage levels. Principal component analysis was applied to the hydrogeochemical data to identify the causes of AMD formation. The first, second, and third principal components reveal that the increased ion concentrations in mine drainage are a result of water mineral reactions in excavated mine areas, the contribution of groundwater in deep reductive environments, and isotopic fractionation during precipitation, respectively. A promising method of reducing AMD formation is to prevent contact between dissolved oxygen and sulfide minerals by increasing the drainage level or by filling the shallow underground excavated area with cementitious materials

Removal of Arsenic, Boron, and Selenium from Excavated Rocks by Consecutive Washing

This paper describes the leaching behavior and release mechanisms of arsenic (As), boron (B) and selenium (Se) from excavated rocks using sequential extraction for solid-phase fractionation, batch experiments with pH variation, and consecutive batch experiments with changes in the solid-liquid mixing ratios. Arsenic in the excavated rock was mostly found with the sulfides/organic matter fraction while majority of the leachable B and Se were associated with the exchangeable phases. The leaching of As was strongly pH dependent, Se was pH dependent only around the acidic region, and B was pH independent. Consecutive washing technique with deionized water effectively lowered the B and Se concentrations in the leachate below the drinking water standards of Japan, but was inefficient in the removal of As. Arsenic exhibited non-conservative leaching behavior and its movement was affected by processes like dissolution, precipitation and pyrite oxidation. In contrast, B and Se behaved more conservatively, resulting in their easy removal from the excavated rock by simple washing and dilution