Arsenic trioxide, also known as white arsenic, is a highly toxic substance with a chemical formula of As2O3 and a molecular weight of 197.8 g/mol. It exists in three forms: two crystalline and one amorphous.
- Octahedral (or cubic) crystalline form (arsenolita): This form is stable at room temperature and has a density of 3.86 g/cm3. It is formed by the condensation of As4O6 vapor.
- Monoclinic crystalline form (claudetita): This form is formed by the transformation of arsenolita at temperatures above 221 °C or by the condensation of As4O6 vapor at temperatures above 250 °C. It has a density of 4.15 g/cm3.
- Amorphous, glassy phase: This form is formed by the condensation of As4O6 vapor at temperatures above 250 °C. It has a density of 3.70 g/cm3.
Table of Contents
Arsenic trioxide sublimes, transitioning from solid to vapor without melting, at temperatures above 135 °C. The monoclinic form of arsenic trioxide melts at 312.3 °C under its own vapor pressure.
Arsenic trioxide is moderately soluble in water, with the amorphous, glassy form dissolving more readily than the crystalline form. The solubility of arsenic trioxide in water increases with temperature.
Ingestion of as little as 0.1 gram of arsenic trioxide can be fatal if it enters the stomach.
1. Production of Arsenic Trioxide
Arsenic trioxide production has a long history, dating back 500 years in China. The production process was simple, involving retorts with condensation chambers and starting with an ore containing 15% As.
In the early 18th century, a smelting facility for native arsenic was established in Germany. However, the demand for As2O3 remained low until the 19th century, when Great Britain became the second country to produce it. Great Britain was the leading producer of As2O3 from the mid-19th century to 1901.
Concerns about the release of As2O3-containing fumes led to legislation regulating production, which in turn led to a significant increase in worldwide As2O3 production.
The demand for arsenic surged in the early 20th century due to the boll weevil infestation in Mexico and the United States. Calcium arsenate was used to combat the boll weevil, driving up the demand for arsenic. New production plants emerged, particularly in the United States and Mexico.
One of the largest arsenic production plants operated from 1932 to 1962 in Sweden. However, the declining use of arsenic trioxide as a wood and plant preservative led to changes in the global structure of arsenic trioxide production. Major producers in Sweden and the United States ceased production, primarily due to environmental concerns.
China is now the world’s largest producer of both arsenic trioxide and arsenic metal.
1.1. Ore Dressing
Arsenic mining and processing is not a major industry, as arsenic is typically obtained as a byproduct of mining other metals. Arsenic-rich raw materials can vary widely in composition, but they are typically sulfides, often with pyrite as the main component.
During ore dressing, mixed arsenide-sulfide concentrates with high metal content are floated, while concentrating pure arsenic minerals is considered secondary and avoided.
Non-ferrous metal concentrates used for arsenic recovery have arsenic contents ranging from less than 1% to rarely 10%. The distribution of arsenic in these concentrates and waste products is based on the initial arsenic concentration in the ore.
In concentrating complex ores, a significant portion of the arsenic remains in the tailings. For example, in tin ore dressing, only 7.8% of the arsenic ends up in the tin concentrate.
In copper and copper-zinc ores, arsenic is concentrated in the copper concentrate, accounting for around 30% of the originally present arsenic in the ore. Consequently, copper concentrates typically contain 0.5% to 1% arsenic, while concentrates from complex ores can contain up to 5% to 8% arsenic.
One example of copper ore in which a substantial portion of the copper is chemically bound to arsenic is found in the Lepanto ore from the Philippines. The flotation product of this ore contains approximately 28% Cu, 32% S, and 9% As.
However, in most ores, only a small fraction of the copper is chemically bound to arsenic, resulting in copper concentrates with lower arsenic content. Enargite-containing copper concentrates are produced in locations like Cerro de Pasco, Peru, and Butte, Montana, USA.
Arsenopyrite is often found in pyritic copper ores in regions such as the Iberian Peninsula, the Balkans, Sweden, and New Brunswick, Canada. Arsenopyrite can be recovered through selective flotation.
Copper pyrites are separated by flotation using higher xanthates, while other sulfide minerals are retained with the aid of lime. After acidification and heating of the pulp, the pyrites are floated, and subsequently, after further acidification and activation with copper sulfate, the arsenopyrite is floated.
Alternatively, arsenopyrite may be selectively floated from the pyrites pulp after heating and activation with copper sulfate.
In the case of many precious metal ores, gold and silver often coexist with arsenopyrite (FeAsS), löllingite (FeAs2), and pyrites or other sulfides. While these metals are not chemically bound to arsenic but occur as native gold and in non-arsenical mineral forms, selective flotation for separation is often unsatisfactory due to extensive intergrowth of the minerals.
In such cases, a combined concentrate of sulfides and arsenides with an arsenic content ranging from a few percent to over 30% is floated.
Research efforts continue to improve the separation of refractory gold and arsenical minerals by ore dressing techniques, although there are limitations, especially with refractory gold ores.
An example of the dressing of a combined precious metal and arsenic ore is the ore from Salsigne, France. This ore, which contains arsenopyrite and pyrites, along with precious metals, copper, and a small amount of bismuth, yields a metal concentrate with approximately 23% As, 27% S, 34% Fe, 0.7% Cu, 0.6% Bi, 55 ppm Au, and 115 ppm Ag.
Lead and copper concentrates rich in arsenic are produced in various regions, including Mexico, South America, and South West Africa, while cobalt concentrates are found in North Africa, and precious metal concentrates are produced in Canada, South America, and other locations.
A notable trend in mining and minerals processing is the production of “dirtier” concentrates from complex ores, resulting in higher arsenic content in copper and zinc concentrates.
This shift is driven by new environmental regulations, which require increased efforts and costs to manage and stabilize arsenic in byproducts for safe disposal. For instance, the tolerable arsenic content in zinc concentrates for hydrometallurgical zinc recovery is limited to about 0.8%.
In the case of nickel concentrate at the Luikonlahti Concentrator of Partek in Finland, arsenic contents can vary up to 5%.
To transform high-arsenic nickel concentrates into marketable products, research has focused on separating pentlandite from arsenic-containing niccolite and gerdorffite using advanced flotation techniques involving grinding, aeration, and NaCN depression.
1.2. Pretreatment of Arsenical Materials
1. Arsenic removal from nonferrous metal concentrates
Arsenic is a major concern in nonferrous metal smelting due to its adverse effects on production. It elevates costs, disrupts metal extraction, reduces product quality, poses environmental risks, and creates disposal challenges. Therefore, it is important to remove arsenic early in the smelting process.
Roasting is a commonly used method for dearsenifying nonferrous metal concentrates. The roasting process converts arsenic into volatile compounds, which can then be removed from the concentrate. Roasting can be conducted in a variety of furnaces, including multiple-hearth furnaces and fluidized-bed reactors.
3. Hydrometallurgical alternatives
In addition to roasting, there are a number of hydrometallurgical alternatives for pretreating arsenical materials. Hydrometallurgical methods offer advantages such as selectivity and the generation of stable arsenic compounds suitable for disposal.
Two hydrometallurgical alternatives are oxidative pressure leaching and biochemical pretreatment. Oxidative pressure leaching is used to treat refractory gold ores. This process involves oxidizing sulfides like pyrites and arsenopyrites to sulfates, making gold accessible for subsequent leaching.
The process operates at elevated temperature and pressure, with oxygen consumption and acid production. Iron and arsenic are precipitated as iron arsenates.
Biochemical pretreatment uses bacteria like Thiobacillus ferrooxidans to selectively oxidize gold ore. These bacteria thrive in acidic conditions and break down sulfide minerals, releasing metals into solution. For refractory ores, this oxidation liberates gold from the mineral matrix, making it amenable to recovery.
4. Choice of pretreatment method
The choice of pretreatment method depends on the specific characteristics of the raw materials and the desired product quality. Factors such as mineralogy, arsenic concentration, and the presence of other valuable metals influence the selection of the most suitable pretreatment process.
1.3. Production of Refined As2O3
1.3.1. Raw materials
Refined arsenic trioxide (As2O3) is produced from arsenic-rich dust or slurry, which is typically a byproduct of roasting and smelting arsenic-containing ores and concentrates. These raw materials come primarily from copper smelters, but also from lead, cobalt, and other smelting processes. In some cases, arsenic-rich ore is roasted specifically to recover arsenic.
1.3.2. Preliminary concentration
If the starting material has a low arsenic content, it is first concentrated using a multistage process. In the first stage, the material is roasted to produce As2O3 content ranging from 5% to 80%. This As2O3 is sublimed and separated from the roasting gases, leaving behind impurities in the roasted material.
Reducing agents like sulfides or charcoal are added to the furnace charge to create a reducing atmosphere that breaks down metal arsenates and prevents the formation of additional arsenates.
High-purity crude oxide can be refined using either dry or wet methods.
The dry refinement process involves heating crude arsenic in a reverberatory furnace. The resulting gases pass through a dust-settling chamber to a series of arsenic separation chambers known as “kitchens,” followed by a bag filter.
Maintaining temperatures around 295°C in the dust-settling chamber ensures efficient sublimation. Arsenic trioxide accumulates in various forms in the kitchens, with the crystalline form predominating.
The wet refinement process exploits the solubility of As2O3 in water at different temperatures, as well as the low solubility of impurities. Crude oxide with an As2O3 content ranging from 80% to 90% is pressure leached in steam-heated autoclaves with water or a circulating solution.
As2O3 dissolves, while impurities form a slightly soluble sludge, which is separated from the leaching solution. The solution is vacuum-cooled, and crystallization is controlled to produce a relatively coarse product, which is separated, washed, dried, and packaged.
The mother liquor is recycled. This process yields wet-refined arsenic containing over 99% As2O3.
Treatment of arsenic-rich sludge
Various techniques have been developed for the treatment of arsenic-rich sludge from off-gas cleaning, particularly in mining operations. These methods aim to extract arsenic efficiently and reduce environmental impact.
2. Uses of Arsenic Trioxide
Arsenic is mainly used in the form of compounds, with arsenic trioxide being the most important starting material. Arsenic compounds are used in a variety of industries, including:
- Forestry: Arsenic trioxide is the primary raw material for the production of wood preservatives, such as chromated copper arsenate (CCA). CCA is an effective wood preservative, but it has been banned in many countries due to environmental concerns. The United States and Malaysia are the largest consumers of CCA.
- Agriculture: Arsenic compounds are used as herbicides and insecticides for cotton, coffee, and rice. However, the use of arsenic in agriculture is declining due to concerns about its toxicity.
- Industrial chemicals: Arsenic compounds are used in the purification of electrolytes in the electrolytic recovery of zinc and in metal pickling baths containing phosphoric acid.
- Glass industry: Arsenic compounds are used as fining agents and decolorizers in glass production.
- Medicine: Arsenic compounds were once used to treat a variety of medical conditions, such as syphilis, malaria, and leukemia. However, their use in medicine has declined due to the development of safer and more effective treatments.
- Electronics: Arsenic compounds are used in the production of semiconductors, such as gallium arsenide, which is used in lasers, LEDs, and solar cells.
- Pyrotechnics: Arsenic compounds are used in fireworks to create green and white sparks.
- Alloying: Arsenic is added to copper and lead alloys to improve their machinability and strength.
- Animal feed: Arsenic compounds were once added to animal feed as a growth promoter and to prevent disease. However, this practice is now banned in most countries.
- Arsenic and Arsenic Compounds; Ullmann’s Encyclopedia of Industrial Chemistry. – https://onlinelibrary.wiley.com/doi/10.1002/14356007.a03_113.pub2