JPS5931039B2 - Acid digestion treatment method using a tantalum acid digestion pot - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for acid digestion treatment of waste organic polymer materials using a tantalum acid digestion pot, and in particular, to prevent corrosion and hydrogen absorption embrittlement of the tantalum acid digestion pot that occurs during the acid digestion process. It is about the method.

The waste organic polymer substances to be subjected to acid digestion treatment in the present invention include various natural or synthetic rubber products, various natural or synthetic fiber products such as cotton, wool, nylon, and vinylon, and synthetic resins such as polyethylene and polypropylene. It includes all kinds of waste organic polymer materials (hereinafter simply referred to as organic materials) such as films, sheets, and various other thermoplastic or thermosetting resin molded products. The processing will be explained using an example. Facilities that handle nuclear fuel or other radioactive isotopes use various equipment and furnishings to protect the human body. Rubber products made by (co)polymerizing chlorine-containing vinyl monomers such as vinyl chloride, vinylidene chloride, and chloroprene are typical examples. Volume reduction and stabilization
There is a way to compact and store it. As a means to be adopted for volume reduction and stabilization, a method is being considered in which the above-mentioned organic matter is put into a high-temperature, high-concentration sulfuric acid bath and acid-digested.

This method consists of the following two steps, and the conditions are extremely harsh. (Decomposition process of organic matter using sulfuric acid (in the case of polyvinyl chloride) (C2H3Ct)n+ nH2SO4-→2nC+NS
O2 + 2nH20 + NHCt (2) Carbon oxidation step by adding sulfuric acid 3C + 2HN03 + 4N0 + 3C02 + 2H20 The first step in the above is usually carried out in a sulfuric acid bath at 200°C or higher for about 5 hours, followed by the addition of nitric acid, and then about 2 The second step is carried out over a period of time.

Generally, platinum, high-silicon cast iron, glass-lined steel, tantalum (including alloys), etc. are considered as materials that can withstand such severe conditions.

However, platinum is expensive, high-silicon cast iron is brittle, and glass-lined steel has the disadvantage that it is difficult to process parts with complex shapes and may crack when subjected to thermal history. . The remaining tantalum (including alloys; hereinafter simply referred to as Ta) is shown in a close-up. However, in the case of Ta, there is a problem of corrosion, albeit in a very small amount, and there is also a concern that it absorbs hydrogen generated as a result of the corrosion and becomes brittle.

Therefore, it is insufficient as a material for a limited acid digester unless these problems are solved. There has been very little previous research on corrosion prevention of Ta, and it is a virtually unexplored field, but one way to prevent hydrogen absorption embrittlement is to attach platinum foil to a part of the surface of Ta. Methods are known that provide a type of sacrificial cathode effect.

However, corrosion and erosion of platinum progresses extremely quickly, requiring repeated replacement, which is disadvantageous in terms of both workability and economy. The present invention has been made in view of these circumstances, and it is an object of the present invention to provide a method that can simultaneously and effectively prevent corrosion deterioration and hydrogen absorption embrittlement of Ta without any problems in terms of workability and economy. This is the purpose.

Therefore, the gist of the present invention is to perform acid digestion by adding nitric acid, nitrous acid, or salts thereof to a high-temperature, high-concentration sulfuric acid bath prior to the decomposition process of organic matter.

That is, the present invention is based on the knowledge that corrosion of Ta is suppressed and hydrogen absorption is prevented by adding nitric acid or nitrous acid in the form of free acid or salt to a high-temperature, high-concentration sulfuric acid bath. The structure and effects of the present invention will be clarified below based on experimental examples. FIG. 1 is an explanatory diagram showing the outline of a corrosion test apparatus. Several (three in the figure) mantle heaters 2 are arranged on a tray 1, each of which has a separable flask 3 equipped with a cooling condenser 5.
is enshrined here.

Each flask 3 contains a sulfuric acid solution 9 with a different concentration, and has a fixed size (2 mm x 20 mm x 2 in the experiment).
A Ta plate 10 (0 mm) was immersed. The cooling water is introduced along the arrow A, passes through the stop valve 8, and then flows through the arrows B, C,
The water passes through each condenser 5 along line D and is further discharged in the direction of arrow E. Furthermore, 4 is a temperature sensor, 6 is a water cutoff sensor, and 7 is a liquid leakage sensor, and each is placed in the control section surrounded by a chain line to ensure safe operation. Each flask 3 has 9
Addition of nitric acid, nitrous acid and their salts, containing 5% sulfuric acid;
A holding experiment was conducted for 500 hours while maintaining the sulfuric acid bath temperature at 250° C. by dividing the samples into groups with no additives and those with no additives.

After the experiment was completed, the test piece was taken out, dried, weighed, and the hydrogen concentration in Ta was analyzed. The average corrosion rate was determined from the weight change before and after the test and is shown in Table 1. In other words, the effect of addition varies depending on the type of salt, but in all cases it shows a remarkable reduction effect compared to the case without addition, and the effect has almost no dependence on the amount added. Ta.

Among various metal salts, alkali metal salts and ammonium salts, which are thought to have relatively good ionizability, have a more pronounced effect when using salts of elements belonging to the copper group, alkaline earth metals, iron group, etc. showed that. It has also been found that there is no substantial difference between nitrate and nitrite. The types of salts are not limited to those exemplified above, but include Mg, Ca, Cd
, Mn, At, In, etc., addition as a double salt or complex salt such as molybdenum nitrate or gadolinium nickel nitrate is not excluded. The nitric acid or nitrous acid or their salts added in the present invention have a broad meaning as described above, but the timing of adding them to the sulfuric acid bath is determined at the stage of preparing the sulfuric acid bath before the decomposition process of organic matter, It may be selected arbitrarily from the stage of adding organic substances, the stage of sulfuric acid bath temperature reaching a high temperature, etc., but it goes without saying that the earliest possible stage is desired.

It is also understood from the above description that the concentration is not limited. Since the present invention is constructed as described above, T.
Corrosion of a and hydrogen absorption can be simultaneously and reliably suppressed.

Therefore, volume reduction and stabilization by acid digestion of organic matter can now be carried out safely using a Ta acid digestion kettle, making it possible to greatly advance the treatment technology of organic matter contaminated with radioactive elements.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing an outline of the experimental apparatus. 9...Sulfuric acid solution, 10...Ta plate.

Claims (1)

[Claims] 1. A method for acid digestion treatment of an organic polymer substance, which includes the steps of decomposing the organic polymer substance contained in a tantalum acid digestion pot in a high-temperature, high-concentration sulfuric acid bath, and further oxidizing it by adding nitric acid. , Acid digestion treatment using a tantalum acid digestion pot, characterized in that, prior to the decomposition step, one or more selected from the group consisting of nitric acid, nitrous acid, nitrates, and nitrites is added to the sulfuric acid bath. Method.