Chemical Phase Analysis of Arsenic Ores—A Study on Selective Separation Conditions of Orpiment and Realgar

DONG Yani; XIONG Ying; PEI Ruohui; ZHANG Xiao; XIE Guangjin; CUI Changzheng

doi:10.15898/j.ykcs.202208050145

Rock and Mineral Analysis > 2024 > 43(2): 270-280. > DOI: 10.15898/j.ykcs.202208050145

DONG Yani，XIONG Ying，PEI Ruohui，et al. Chemical Phase Analysis of Arsenic Ores—A Study on Selective Separation Conditions of Orpiment and Realgar[J]. Rock and Mineral Analysis，2024，43（2）：270−280. DOI: 10.15898/j.ykcs.202208050145

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Chemical Phase Analysis of Arsenic Ores—A Study on Selective Separation Conditions of Orpiment and Realgar

Shaanxi Experimental Research Institute of Geology and Mineral Resources Co., Ltd., Xi’an 710054, China

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Received Date: August 04, 2022
Revised Date: March 06, 2024
Accepted Date: March 15, 2024
Available Online: April 28, 2024

Abstract

HIGHLIGHTS
(1) The separation conditions of orpiment and realgar samples were determined by single mineral selective separation technique.

(2) Ammonia was a good leaching agent for the selective separation of orpiment and realgar, and the leaching rates were 92.2% and 2.4%, respectively.

(3) Iodine can oxidize realgar into arsenic oxide soluble in alkali, and the leaching rate of realgar was increased from 75.8% to 92.2%.

ABSTRACT

As a rich symbiotic mineral resource, orpiment (As₂S₃) and realgar (AsS/As₂S₂/As₄S₄) are common sulfide minerals of arsenic. However, due to their similar chemical properties, the chemical phase analysis method of arsenic has always taken these two minerals as the same phase analysis and determination, and cannot obtain their respective contents. In the research, the orpiment and realgar arsenic ores of Shimen in Hunan Province were taken as the object. The separation conditions of orpiment and realgar samples were studied by single-mineral selective separation technology. The single mineral of orpiment and realgar was leached with 4mol/L ammonia solution and a 35℃ water bath for 6h. The extraction rate of orpiment and realgar was 92.2% and 2.4% respectively. Realgar was leached with sodium hydroxide solution, the extraction of realgar was 75.8%. Due to the spatial structure of realgar being oxidized, the addition of iodine can oxidize it into arsenic oxide that is easily soluble in alkali, increasing the extraction rate to 92.2%. The building of the selective separation conditions lays the foundation for the development of chemical phase separation methods for arsenic ores mainly composed of orpiment and realgar. The BRIEF REPORT is available for this paper at http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202208050145.
- orpiment,
- realgar,
- chemical phase analysis,
- ammonia,
- sodium hydroxide,
- selective separation conditions
BRIEF REPORT
Significance: Orpiment and realgar are common arsenic sulfide minerals^[2] and are produced together in low-temperature hydrothermal deposits and sulfur volcanic fumaroles, so orpiment is a symbiotic mineral of realgar. It is difficult to achieve the separation of orpiment and realgar due to its symbiosis and similar physicochemical properties. At present, there are several methods for arsenic removal in industry, including roasting^[4], hot pressing, and bacterial oxidation. The roasting method causes significant pollution; The hot pressing method requires high technical requirements, which need a corrosion-resistant equipment and has the risk of high pressure and high oxygen; The bacterial oxidation method has the advantages of low investment, simple process, and environmental friendliness. Liu Rongli et al.^[5] conducted an experiment on the treatment of Pingding gold mine in Gansu Province, which contains both orpiment and realgar; Liu Shugen et al.^[6] proposed the application of bacterial oxidation arsenic removal technology in the treatment process of arsenic containing solid waste residue; Yuan Qiuhong et al.^[7] studied the biological leaching mechanism of different bacterial strains on orpiment. The results of such studies indicate that bacterial oxidation methods can not effectively separate orpiment and realgar from minerals or solid waste residue, and their respective leaching rates cannot be obtained. Meanwhile, these exist a disadvantage of long leaching cycles. Thus, the establishment of an efficient and reasonable chemical phase separation and analysis method for the symbiotic ore of orpiment and realgar has guiding significance for the exploration, evaluation, beneficiation process research and industrial application of arsenic ore.

Methods: The arsenic ores of Shimen in Hunan Province were collected and analyzed. The single minerals of orpiment and realgar were selected from the 0.25mm test samples, and the single minerals were carefully ground to 0.100-0.074mm with agate mortar (the particle size was observed with a microscope while grinding). The single mineral selective separation technology was used to separate the single mineral samples of 0.100-0.074mm. Ammonia was used to extract orpiment. Sodium hydroxide solution was used to extract realgar, and oxidant iodine was added to oxidize realgar into arsenic oxide that was easily soluble in alkali, thus effectively improving the leaching rate of realgar.

Data and Results: Through the single mineral optimization experiment, the optimal leaching conditions of orpiment were as follows: water bath temperature 35℃, ammonia concentration 4mol/L, and leaching time 6h. Under these conditions, the leaching rates of orpiment, realgar and arsenopyrite were 92.2%, 2.4% and 3.2%, respectively. Four kinds of leaching agents, namely ammonia carbonate, sodium carbonate, sodium hydroxide (NaOH) and potassium hydroxide, were selected to carry out leaching tests on realgar arsenopyrite. During the leaching of the ammonia carbonate solution, the leaching rate of realgar and arsenopyrite was 1.5% and 1.2%, respectively; for the leaching agent of sodium carbonate, the leaching rate of realgar and arsenopyrite was 64% and 1.0%, respectively; in potassium hydroxide solution, black precipitate was formed and the leaching rate of arsenopyrite was 2.5%, indicating that potassium hydroxide could not effectively leach realgar; in NaOH solution, the leaching rate of realgar was 70.2% and that of arsenopyrite was 2.2%, so NaOH solution was selected as an extracting agent of realgar. Further condition optimization showed that when the concentration of NaOH solution was 2mol/L and boiling water bath for 2h, the highest leaching rate of realgar was 75.8%, and the leaching rate of arsenopyrite was 3.7%. Realgar is susceptible to oxidation due to its spatial structure, in which the sulfide of As(Ⅱ) is often oxidized to the oxide of As(Ⅲ, Ⅴ) and released in alkaline solution, which can improve the solubility of realgar. Therefore, the effects of different oxidants on the extraction rate of realgar were further explored. H₂O₂ was used to oxidize realgar, and the leaching rate of realgar was 94%, where the single mineral of arsenopyrite gradually changed from gray-black to yellow while leaching realgar, indicating that the surface of arsenopyrite was severely oxidized by H₂O₂. At this time, the leaching rate of arsenopyrite was as high as 50%, so the oxidant with weak oxidation should be selected for the test. I₂ or KMnO₄ were used to oxidize realgar. In the presence of oxidants I₂ and KMnO₄, the extraction rate of realgar was more than 90%, but during the oxidative leaching process, the solution with KMnO₄ appeared as a dark purple precipitate and had poor operability, so I₂ was selected as the oxidant in this experiment. The oxidation of I₂ increased the leaching rate of realgar to 92.2%, and 26.3% of the arsenopyrite was leached. This condition had the best effect on the leaching of realgar, but the experimental effect on arsenopyrite was not ideal. For the arsenic ore dominated by orpiment, the content of arsenopyrite was small or trace, and the leaching of arsenopyrite did not affect the selective separation results of realgar. Under the condition of selective separation, the relative deviation between the analytical values of orpiment and realgar samples and the measured values of X-ray diffractometer and automatic mineral analyzer was less than 14.0%.

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