Antimony Trioxide: Production, Uses and Toxicology

1.1. Roasting of Antimony Sulfide

Antimony trioxide can be produced from sulfide compounds by two methods: volatilizing roasting and nonvolatilizing roasting. Volatilizing roasting produces volatile antimony trioxide, which can be separated from the residue. Nonvolatilizing roasting produces nonvolatile Sb₂O₄, which remains mixed with the residue.

Volatilizing roasting is the preferred method because it yields a purer product. However, it is more challenging to control the temperature and draft in the furnace during this process.

The fundamental reaction for volatilizing roasting is:

2 Sb₂S₃ + 9 O₂ → 2 Sb₂O₃ + 6 SO₂

Excess oxygen can lead to the formation of Sb₂O₄:

Sb₂S₃ + 5 O₂ → Sb₂O₄ + 3 SO₂

At temperatures exceeding 560 °C, the reaction rate notably diminishes. During roasting, Sb₂O₄ can also react with Sb₂S₃ to produce antimony trioxide:

Sb₂S₃ + 9 Sb₂O₄ → 10 Sb₂O₃ + 3 SO₂

Therefore, careful engineering of the process is necessary to ensure rapid and preferential formation of Sb₂O₃. The temperature must be adequately high to promote volatilization, while the oxygen supply must be limited to prevent the formation of Sb₂O₄.

However, excessively high temperatures can lead to the melting of some of the charge, enveloping the sulfide grains and impeding their oxidation. During the condensation of Sb₂O₃, it is essential to maintain a low oxygen content in the gas phase to prevent the oxidation of Sb₂O₄.

The temperature employed is dependent on the sulfide content of the ore. Low-grade ores can be roasted between 850 and 1000 °C. For rich ores, the upper temperature limit is the melting point of the sulfide (546 °C).

In practice, temperatures should not exceed 400 °C. Control of the oxygen supply is achieved by incorporating wood charcoal or coke breeze into the charge and admitting only the necessary air to form carbon monoxide and antimony trioxide.

Carbon monoxide serves to suppress the formation of Sb₂O₄. Nevertheless, the complete prevention of Sb₂O₄ formation is challenging.

The yield of the process varies depending on the specific method employed and the sulfide content of the ore, ranging from 60% to 90% or more.

Commercial Sb₂O₃ should contain a minimum of 99.5% antimony trioxide. Quality is also assessed based on its crystalline fineness, adhesion to surfaces, and the absence of caking. A reddish tinge in the oxide suggests the presence of Sb₂S₃, while a yellowish shade is undesirable and indicative of selenium and PbO contamination.

The acceptable arsenic content should be around 0.1%.

Refined commercial antimony trioxide can be produced through a secondary roasting process, where crude oxide undergoes controlled roasting conditions. This separation process is especially critical for obtaining high-quality Sb₂O₃, with arsenic oxide having little value.

Mercury-containing Sb ores typically require roasting at temperatures below 400 °C to volatilize the mercury, with precise temperature control necessary to avoid overheating and the volatilization of Sb compounds.

The roasted ore, still containing Sb₂S₃, is then slowly cooled under oxygen exclusion before undergoing reduction or iron precipitation in open-hearth furnaces.

Modern roasting furnaces commonly employ rotary kilns. These kilns can achieve antimony trioxide yields of 95 – 98% with high throughput, independent of the ore’s nature. They are suitable for oxide-sulfide ores, reducing the Sb to the metal initially and subsequently oxidizing it to Sb₂O₃, free of nonvolatile impurities and off-gases produced during reduction.

Some rotary kilns use hot air for heating, which reduces gas flow rates and minimizes gangue entrainment but may still lead to temperature fluctuations resulting in charge melting.

Crude Sb₂O₃, produced at approximately 81% Sb, 0.3% As, and 0.15% Ni, can be obtained through a volatilizing roast of sulfide concentrate in a rotary kiln. Temperatures during this process range from 1100 – 1200 °C.

The oxidation of Sb₂S₃ to Sb₂O₃ partly occurs in the gas phase. Notably, this conversion process utilizes the Chemetron Process, which incorporates unique features in oxidation and cooling control.

While Sb₂O₃ is continuously removed from the kiln, nonvolatile elements remain within the kiln and are removed periodically during deslagging campaigns. Critical process control parameters include kiln-feed rates, burner positioning, and cooling rates, which influence product

1.2. Recovery of Antimony Oxide

Antimony trioxide, also known as antimony(III) oxide, is a widely used flame retardant. Its demand has increased in recent years, and it is primarily produced by roasting antimony sulfide ores or other antimony-containing raw materials.

To obtain high-purity antimony trioxide, especially to separate arsenic from antimony trioxide, a selective volatilization process is used. This involves preferentially vaporizing the more volatile arsenic trioxide and then selectively condensing the less volatile antimony tetroxide at high temperatures. This process results in the condensation of arsenic trioxide at lower temperatures.

Another method for recovering antimony trioxide from complex antimony sulfide ores is to react them with calcium chloride. This reaction takes place in an oxidizing atmosphere at around 500 °C, as shown in the equation below:

Sb₂S₃ + 3 CaCl₂ + 6 O₂ → 2 SbCl₃ + 3 CaSO₄

During this process, over 90% of the antimony is volatilized as SbCl₃, which can then be purified by distillation and hydrolysis to produce antimony trioxide.

2. Uses of Antimony Oxide