JPH0353241B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to the production of high-purity selenium, and particularly to an improvement in a method for purifying crude selenium by vacuum distillation. Electrophotographic photoreceptors, which have seen increased demand in recent years,
There is a demand for highly purified selenium for use in the formation of optical semiconductor devices such as image pickup tubes and other uses. Selenium is contained in copper electrolytic anode slime, and the slime is usually sintered to form selenium oxide, then converted to an aqueous selenite solution, and then reduced with sulfur dioxide gas to obtain crude selenium. The crude selenium obtained at this stage has a purity of about 99 to 99.9% and cannot be put to practical use unless it is further purified. Since the crude selenium obtained in this way contains a large amount of water, it is dried by a method such as vacuum drying or hot air drying, and then purified by vacuum distillation or atmospheric pressure distillation to achieve high purity. Crude selenium is in the form of coke or powder, and a large amount of water is adsorbed, so it takes a long time to dry, and complete drying is difficult. Therefore, in the purification distillation process, stable operation is hindered because it takes a long time to reach a high vacuum. In order to solve the problems caused by water content, it is possible to finely grind crude selenium and dry it, but this does not shorten the drying time, and there is a risk of contamination with impurities during the grinding process, resulting in unavoidable losses. As an improvement of the crude selenium purification method, a method of continuously charging and dissolving crude selenium and subjecting it to vacuum distillation is disclosed, for example, in JP-A-56-129604 or JP-A-56.
- Proposed as No. 160307. However, according to these methods, the internal pressures of the melting furnace, distiller, and storage tank must be set in a well-balanced manner. In addition, since the internal pressure of the distiller must be kept at a relatively low degree of vacuum, about 1 torr or less, impurities can be removed at a level of 10 ^-2 to 10 ^-6 , similar to that obtained with batch distillation.
It is difficult to remove impurities as in the case of a vacuum as high as Torr. Moreover, these methods do not take into consideration the problem of adsorbed water on crude selenium. The present invention improves the shortcomings of the prior art and provides a selenium purification method that enables stable distillation of crude selenium under high vacuum in a short period of time and efficiently, and that can obtain high purity selenium of 99.999% or more. The purpose is to The present inventor focused on the fact that a large amount of water adsorbed and contained in crude selenium is the main cause that affects the rate of increase in the degree of vacuum in the still during vacuum distillation, and therefore the extension of the distillation time. Various pretreatments for the melt were investigated. As a result, it has been found that by selecting the residence time and heating temperature of crude selenium in the melting tank, the above-mentioned problems can not only be solved, but also the purity of the produced selenium can be unexpectedly improved. The present invention heat-treats the crude selenium melt in the crude selenium melting tank at a temperature range of 330°C to 500°C for at least 30 minutes, and then brings the temperature of the melt to a uniform temperature in the range of 250°C to 320°C. After cooling, distillation is performed using a vacuum distillation apparatus preheated to 250°C to 300°C. As mentioned above, the crude selenium raw material is in powder form and contains about 5% of adsorbed water as well as trace amounts of impurities such as sulfur, sodium, aluminum, and iron. It is believed that the heat treatment of the crude selenium melt in the melting tank according to the present invention evaporates and removes the volatile components of these impurities, or converts them into less volatile materials through thermal decomposition. Therefore, it is necessary that this heat treatment lasts for a sufficient period of time to allow the above-mentioned volatile matter removal or thermal decomposition reaction to take place sufficiently. The heat treatment temperature is approximately 330℃ considering the minimum required temperature during vacuum distillation.
The above is necessary. Appropriately, the upper limit temperature is about 500°C; temperatures higher than 500°C are not preferred because the vapor pressure of selenium itself becomes high, making operation difficult.
At a heat treatment temperature within the above range, sufficient desired effects can be obtained in at least 30 minutes, generally 2 to 3 hours, depending on the quality of the crude selenium raw material. Since the heat-treated crude selenium melt is at a sufficiently high temperature, it must be cooled before charging into the distiller to prevent bumping. Vacuum distiller operation is normally preferably carried out at a high vacuum of 10 ^-2 to 10 ^-6 Torr, with a temperature range of 330 to 450°C, so the temperature of the crude selenium melt should be below 320°C. be.
However, if the temperature is lowered too much, it will take time to raise the temperature, and considering that the freezing point is around 220°C, it is necessary to maintain it at 250°C or higher.
In this case, the cooling temperature must be adjusted gradually in order to obtain a uniform distribution of melt temperature. For short periods of time, a uniform temperature of the melt cannot be achieved due to partial cooling. Generally, this purpose is achieved by maintaining the temperature at a constant temperature for 10 minutes or more, preferably 30 minutes. The crude selenium melt that has reached an appropriate temperature is supplied to a vacuum distillation apparatus, but in this case, the vacuum distillation apparatus must be preheated in advance. This is a necessary condition in order to perform the distillation operation efficiently in a short time and thereby obtain high purity selenium. The preheating temperature is 250-300°C for the same reason as the temperature of the charged selenium melt. Although the heat treatment of crude selenium in the melting tank is possible in the air, there is a risk of contamination with oxygen and other impurities, so it is preferable to perform the heat treatment in an inert gas atmosphere such as nitrogen gas. In addition, if the raw material crude selenium contains many impurities, sludge or floating substances may form at the bottom of the melting furnace. Preferably, scraping means are provided. Substantially the same effect can be obtained by performing preliminary melting heat treatment in a vacuum distiller without providing a separate melting tank, and then performing a vacuum distillation operation after cooling. has poor thermal efficiency, and operational difficulties such as the treatment of sludge affect the purity of purified selenium, so it is more convenient to separate the heating and cooling treatments of the melt. Implementation of the invention provides the following advantages over the prior art. (1) Low-boiling point impurities such as moisture and sulfur are removed by heat-treating the crude selenium melt in a melting tank in advance, so that low boiling point impurities such ^as water and sulfur are removed.
A high vacuum of 10 ^-6 Torr can be achieved in a short time, allowing stable high vacuum distillation operations. (2) High-vacuum distillation at 10 ^-2 to 10 ^-6 Torr improves efficiency in removing impurities and significantly improves purification accuracy. (3) Even crude selenium as a raw material with a somewhat high moisture content can be heat-treated, so there is no need for long preliminary drying, and at the same time there is no risk of contamination with impurities during the grinding process. The present invention will be further explained below using Examples (including Comparative Examples). Example The present invention is carried out using crude selenium recovered by a conventional method from the anode slime of copper electrolytic refining as a raw material. Crude selenium is in the form of wet (approximately 5% moisture) particles with a particle size of 5 to 20 m/m, and its impurity concentration is as shown in the table below on a dry basis. 8 kg of crude selenium as a raw material was placed in a quartz container and melted in a melting tank equipped with a nichrome wire heating device while flowing nitrogen gas at a rate of 500 ml/min. After holding the melting tank temperature at 400℃ for 2 hours, the temperature was lowered to 300℃,
This temperature was maintained for an additional 30 minutes. Next, the crude selenium melt was taken out from the melting tank along with the quartz container, and approximately
The mixture was placed in a vacuum distiller preheated to 300°C and subjected to vacuum distillation. The heating temperature of the distiller was 340°C, and the degree of vacuum was 0.05 Torr immediately after the vacuum pump was activated, and did not change during the distillation. Analytical samples were collected from the melted product after heat treatment in the melting tank and purified selenium from the distillation receiver, and compared with the impurity concentration of the crude selenium raw material. In addition, the degree of shortening of the operating time was compared with the vacuum distillation time (before the start of distillation and the distillation time) obtained in the following comparative example. The conditions for implementing the comparative example are as follows. 8 kg of crude selenium, the same raw material used in the example
After drying with hot air (30 m ³ /min) at 30 to 40°C for 10 hours, the mixture was placed in a distillation tank of an unpreheated vacuum distiller and distilled at 340°C. The degree of vacuum in the distiller was approximately 0.1 to 0.2 Torr for two hours after the vacuum pump was activated, and then rose to a maximum of 0.08 Torr. The results are summarized in the table below.

[Table] From the results in the above table, it can be seen that most of the impurities have already been removed by the heat treatment (400° C., 2 hours) of the crude selenium raw material according to the present invention. In addition, we were able to reduce the impurities of purified selenium purified by distillation to a total of about 1 ppm. This is less than half of the purified selenium obtained in the comparative example of the prior art, and shows that it is superior to the analytical value of redistilled selenium. Furthermore, it has been demonstrated that the time required for distillation is significantly improved due to the high vacuum degree and shortening of the arrival time obtained by the heat treatment of the melt according to the invention.

Claims (1)

[Claims] 1 Heat the crude selenium melt in the crude selenium melting tank to 330°C.
to 500°C for at least 30 minutes, and then increase the temperature of the melt to 250°C to 320°C.
After cooling to a uniform temperature in the range of 250℃ to 300℃
A method for purifying selenium, characterized by distilling it using a vacuum distillation apparatus preheated to ℃.