JPH10244242A - Process of removing hexavalent chromium from heat-resistant substance containing hexavalent chromium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a process for removing hexavalent chromium from a heat-resistant substance containing hexavalent chromium harmful for human bodies and reuse the heat-resistant substance from which hexavalent chromium is removed and made harmless or disposing the substance as an industrial waste. SOLUTION: In a process of heating a heat-resistant substance containing hexavalent chromium is heated up to the temperature of 1100 deg.C or higher and curing the same, the heat-resistant substance is formed into the reduced state at the time of curing, and cooled down to ordinary temperature, and the hexavalent chromium is removed from the heat-resistant substance containing the hexavalent chromium. The treatment is carried out by adjusting atmosphere, and a process of carrying out in atmosphere down to the given temperature and cooling at a reduced atmosphere or a process of heating and cooling all at a reduced atmosphere is applied.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to eliminating hexavalent chromium compounds harmful to the human body by reducing hexavalent chromium generated in industry using chromium oxide to trivalent chromium. is there.

[0002]

2. Description of the Related Art For example, chromium-containing refractories are currently used as indispensable materials in the field where high corrosion resistance is required. These chromium-containing refractories are widely used in iron and steel or non-ferrous processes, as well as in rotary kilns for cement firing, but have the disadvantage that they react with alkali salts to generate hexavalent chromium harmful to the human body.

Since hexavalent chromium is highly toxic and carcinogenic, legislation regarding the disposal of hazardous waste has already been strengthened in the United States. 6 for waste after use
It obliges detoxification treatment such as the chemical reduction method of valent chromium. Since May 1994, chromium-based refractories have to meet the standard of 5.0 mg / l in total chromium. In Japan, strict regulations for hexavalent chromium are set at 1.5 mg / l or less (Prime Ministerial Ordinance) as industrial waste.

[0004]

Chromium-containing refractories are:
Since it exhibits high corrosion resistance and spalling resistance, it is indispensable in the refractory related industry.
In particular, in the firing of cement, during use, a reaction represented by the following formula proceeds under the influence of the alkali atmosphere in the furnace and oxygen in the air, and hexavalent chromium harmful to the human body is generated. ^{_{2Cr 3+ 2 O 3 + 3O 2}} + 4K 2 O → 4K 2 Cr 6+ O 4

Heretofore, a method has been adopted in which used refractory bricks are solidified by mixing with a curing agent for the purpose of solidifying and recycling the used refractory bricks.
Sufficient results could not be expected, leaving problems such as the elution amount of chromium (1700 mg / l).

Therefore, research and development of alternative materials not containing chromium have been conducted, but they are still inferior in terms of high corrosion resistance and have not yet been replaced.

[0007]

Means for Solving the Problems The present inventors have focused on the environmental problem of chromium and, as a result, have found a method of high-temperature sintering by adjusting the atmosphere in which hexavalent chromium soluble in water is almost eliminated. The invention has been completed. That is, in the process of heating and cooling a heat-resistant substance containing hexavalent chromium to a temperature of 1100 ° C. or higher, the present invention provides a hexavalent chromium characterized by cooling the heat-resistant substance to a reduced state at the time of cooling to room temperature. This is a method for removing hexavalent chromium from contained heat-resistant substances.

The present invention enables the use of chromium-containing refractories without any limitation by removing hexavalent chromium from the used brick. Here, the method of removing hexavalent chromium is described mainly for refractories, but it can be similarly used in other industries using chromium oxide, for example, stainless steel industry.

Using high temperature X-ray diffraction, CaCO ₃ and Cr ₂ O
As a result of measuring the reaction with ₃ and the oxidation behavior, it becomes CaCrO ₄ which is a hexavalent chromium compound at 750 ° C. in the course of heating, and then changes to CaCr ₂ O ₄ which is a trivalent compound at 1150 ° C. There is no change at higher temperatures. When the temperature is lowered, CaCrO ₄ becomes hexavalent chromium again at around 1000 ° C., and thereafter becomes hexavalent chromium until room temperature. Therefore, it was found that when lowering the temperature by removing oxygen in a vacuum and nitrogen atmosphere when lowering the temperature, CaCr ₂ O ₄ was cooled to room temperature as CaCr ₂ O ₄ , which is a trivalent compound, without changing to CaCrO ₄ . In the present invention, hexavalent chromium has been successfully changed to trivalent chromium by firing in a reducing atmosphere utilizing this oxidation-reduction behavior.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The heat-resistant material in the present invention is a refractory material such as an oxide, a carbide, a nitride or a composite thereof, a ceramic or a metal, and is used in the form of a powder or a compact.

A high-temperature furnace is usually used to heat a heat-resistant substance containing hexavalent chromium to a temperature of 1100 ° C. or higher. High temperature furnaces can include electric furnaces, heavy oil furnaces, light oil furnaces, and propane furnaces.

In order to bring a heat-resistant substance into a reducing state, the atmosphere in the furnace is usually brought into a reducing state. There is a method to make the atmosphere all reducing. In the present invention, any method may be used. However, in the case of a heat-resistant substance which may be oxidized at a high temperature, it is preferable that the entire atmosphere from heating to cooling be reduced. As a method for reducing the atmosphere, a hydrogen gas or a carbon monoxide gas, a gas obtained by diluting the same with an inert gas such as a nitrogen gas or a helium gas, or a substance containing carbon is charged into a high-temperature furnace to perform a monoxide oxidation. There is a method of generating carbon gas. Carbon-containing materials include coal, coke, graphite, activated carbon, natural organic polymers and synthetic organic polymers.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below with reference to embodiments. Example 1 Tables 1 to 3 show the results of a hexavalent chromium removal test on brick waste from which hexavalent chromium was generated from bricks after use of a rotary kiln for cement firing. Table 1 shows the analysis results of Cr ⁶⁺ of the powder sample ground and processed to 180 μm or less. Tables 2 and 3 show the C of the sample whose hexavalent chromium was removed by adjusting the atmosphere using a cut sample of 11 to 13 mm square and 25 to 30 mm in length.
It is an analysis result of ^{r6 +} . An atmosphere adjusted only during cooling and an atmosphere adjusted from room temperature are shown. Ai in the table
"r" is a result of heating and cooling in an air atmosphere.
The heating temperature was 1300 ° C and the cooling time to room temperature was 3 hours and 5 hours.

[0014]

[Table 1]

[0015]

[Table 2]

[0016]

[Table 3]

Example 2 The size of the brick waste was about 110 × 150
Table 4 shows the results of analyzing Cr ⁶⁺ of the brick heated to 1300 ° C. in a CO atmosphere with a size of × 112 mm and cooled. The treated brick was cut into 27 pieces and analyzed. Here are some of them. 6 of untreated bricks used under the same conditions for reference
The amount of chromium is indicated.

[0018]

[Table 4]

Example 3 The sample of the brick subjected to the CO atmosphere adjustment treatment of Example 1 was analyzed for Cr ⁶⁺ based on the industrial waste analysis method. The result is 1.5m as determined by the Environment Agency.
Hexavalent chromium content of treated brick is 0.04mg / l against g / l regulations
And 0.09 mg / l.

[0020]

According to the present invention, the recycling of the inorganic oxide containing hexavalent chromium, which has not been reused, is promoted. The disposal of hazardous industrial waste, which was impossible due to the high concentration of hexavalent chromium, has become possible. Since the above effects can be expected, the use of chromium oxide-containing products which have been shunned in the past is promoted.

Continuing on the front page (72) Inventor Hideo Asakura 707 Ibe, Bizen-shi, Okayama Prefecture Inside Shinagawa White Brick Co., Ltd. ) Inventor, Mitsuaki Inoue 936, Ibe, Bizen-shi, Okayama, Japan In Ceramics Co., Ltd. (72) Inventor Akira Yamaguchi Okisho-cho, Showa-ku, Nagoya-shi

Claims (1)

[Claims] 1. A heat-resistant substance containing hexavalent chromium at 1100 ° C.
A method for removing hexavalent chromium from a heat-resistant material containing hexavalent chromium, wherein the heat-resistant material is reduced to a normal temperature during cooling in the process of heating to and cooling to the above temperature.