Selenium extraction process

Various types of anode sludge, sulfuric acid sludge, and pulp washing sludge all contain selenium to varying degrees

Anode slime is vaporized; furthermore, crude selenium is usually refined through redox methods

Selenium is a semiconductor element that is primarily recovered through comprehensive recycling processes such as those involving anode sludge from copper-based smelters. Selenium dioxide is one of the most important selenium compounds, and it is widely used in many fields such as pharmaceuticals, chemicals, and metallurgy. One of its main uses is as an additive in the electrolytic production of manganese metal. Although the mechanism of action remains debated, it is generally agreed that selenium compounds increase the hydrogen overpotential, thereby raising the current efficiency of the electrolysis process from very low levels to over 80%~90%. They also have a positive effect on the crystal structure and deposition morphology of metallic manganese, and reduce the requirements for purifying the electrolyte. With the rapid development of the national economy, demand for steel is increasing steadily, particularly the demand for low-alloy steels (such as manganese steel). This has driven the growth of the metal manganese industry, resulting in a sharp increase in demand for selenium dioxide and a consequent significant rise in its price. Theoretically, more than 1.4 tons of selenium dioxide can be produced from 1 ton of selenium, and the price of selenium dioxide is also higher than that of pure selenium. Therefore, it is of great economic significance to use crude selenium to produce selenium dioxide or further refine it into pure selenium. This post was last edited by yutr on 2009-3-24 13:16]

Selenium is distilled from anode sludge, and selenium oxide is obtained through roasting acidic sludge – but how is pulp-sludge processed?

What is the selenium content in pulp sludge? Does anyone know?

I am humbly seeking advice from all of you; those who know the answer please do share it with me

Ordinary crude copper (non-alloyed copper) electrolytic anode sludge contains relatively high levels of selenium, and the process of roasting and removing selenium mainly involves the use of a sulfation rotary kiln (or resistance furnace). Selenium is volatilized as selenium dioxide with the flue gas from the rotary kiln, and selenium is obtained through water absorption. This is crude selenium; to obtain refined selenium, further purification is required. That’s more or less it.

The crude selenium obtained from the sulfation roasting and selenium distillation of copper anode slime generally contains 96%–98% selenium, along with impurities such as copper, lead, iron, and arsenic. The purification methods include distillation and chemical methods. Distillation is a method that utilizes the difference in volatility between selenium and impurities to control the distillation temperature and separate selenium from the impurities. The chemical methods are further divided into nitric acid oxidation and oxygen oxidation methods. Both methods can produce pure selenium with a purity of 99.99%. For detailed processes, please refer to relevant materials.

I want to use the full wet method to design the process; I’m not sure if that’s possible

The roasting method requires high standards for equipment; the all-wet method might be better. Has anyone tried it?

Due to the difference in volatility between the oxides of selenium and those of impurities present in crude selenium, selenium tends to oxidize into volatile selenium dioxide in the presence of oxygen and under heating conditions. Most of the impurities in crude selenium remain as non-volatile oxides such as copper oxide, iron oxide, and tellurium oxide, remaining in the residue. This process involves melting the crude selenium and then oxidizing it in an oxidation furnace; selenium appears as selenium dioxide and volatilizes, which is subsequently collected after cooling and settling in a sedimentation tank. Other gases such as nitrogen, carbon dioxide, and a small amount of sulfur dioxide enter the exhaust gas absorption tanks and purification bottles through the collection box, where they are further purified to recover the small amount of selenium dioxide contained within them. The main reactions involved are as follows: Se + O2 = SeO2↑, 2Cu2S + 3O2 = 2Cu2O + 2SO2↑, C + O2 = CO2↑. The vast majority of impurities remain in the oxidation slag after oxidation; only a small portion of these impurities become less dense due to oxidation, resulting in smaller particles and lower specific gravity. Coupled with intense oxidation inside the furnace and occasional large fluctuations in negative pressure, these impurities are carried by air currents to the selenium dioxide collection box or exhaust gas absorption tank. Selenium dioxide is then extracted from this collection box, and after cooling, weighing, and packaging, it becomes the final product – selenium dioxide. Selenium is generally extracted industrially from the anode sludge resulting from copper electrolytic refining. The main forms of selenium in anode slime are Cu2Se, Ag2Se, etc., with a content of 3–14%. The sulfation roasting method is currently widely used, and its main advantage is a high selenium recovery rate (>93%) ； Suitable for processing a variety of raw materials. 　　The production process for recovering selenium using the sulfation roasting method involves first dehydrating the anode slime at 140°C, then mixing it with concentrated sulfuric acid and feeding it into a rotary kiln for sulfation roasting. At 250°C, the following reaction takes place: Cu2Se + 6H2SO4 → 2CuSO4 + SeO2 + 4SO2 + 6H2O. When the temperature is raised to 700–750°C, selenium dioxide (SeO2) volatilizes (it sublimes at 315°C), while tellurium dioxide, due to its lower volatility, remains in the roasting residue along with the sulfates. The flue gas containing selenium dioxide that comes out of the roasting furnace enters the absorption tower, where SeO2 is absorbed by water to form hyposelenic acid (H2SeO3). This acid is then reduced to elemental selenium by sulfur dioxide (SO2) present in the flue gas: H2SeO3 + 2SO2 + H2O → Se + 2H2SO4. The precipitate is filtered, washed, and dried to yield crude selenium ash with a purity of 98–99%. The absorption solution still contains selenium in an amount of 3–10% of the selenium content in the raw material; Cu2Se precipitate can be obtained from it using the copper displacement method, and then it can be sent back to the sulfation roasting process for further treatment, or crude selenium can be directly precipitated from it using the SO2 reduction method. 　　In addition, there is the soda roasting method for recovering selenium. Production of high-purity selenium: The methods for purifying selenium include distillation and oxidation-reduction precipitation, with the latter being widely used to produce selenium with a purity of over 99.992%. The method involves first oxidizing the molten crude selenium with oxygen, causing selenium to turn into SeO2 and volatilize into an absorption tank, where it reacts with the ion-exchanged water there to form a selenous acid solution; subsequently, SO2 is introduced, resulting in the precipitation of pure selenium from the solution. 　　To produce high-purity selenium with a purity of over 99.999%, methods such as vacuum distillation, ion exchange, thermal decomposition of sulfides, and gas-phase ammonia reduction of selenium dioxide can be employed. This post was last edited by zhaowendao on 2009-4-28 18:40]