JP2005314724A - Chromium nitrate aqueous solution and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide chromium nitrate aqueous solution with a very small content of residual organic carbon, and a method for manufacturing the same. <P>SOLUTION: In the chromium nitrate aqueous solution, the content of oxalic acid is ≤0.5 wt.%. In addition, the content of the total organic carbon is preferably ≤2.5 wt.% to chromium. Free nitric ions not bonded to Cr are preferably not substantially contained. The chromium nitrate aqueous solution is preferably manufactured by separately and simultaneously adding nitric acid and organic reducing agent to chromic acid aqueous solution. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an aqueous chromium nitrate solution and a method for producing the same.

Conventionally, chromium nitrate is produced by adding sulfuric acid to a sodium dichromate solution obtained by alkaline oxidation roasting of ore, reducing it with an organic substance to obtain a chromium sulfate solution, and adding caustic soda or soda ash to water A method is known in which a precipitate of chromium oxide or chromium carbonate is formed, filtered, washed with water, and then dissolved by adding nitric acid. Alternatively, high-carbon ferrochrome obtained by reducing chromium ore with an electric furnace using a carbon reducing agent is extracted with sulfuric acid, and this solution is electrolyzed to form metallic chromium, and nitric acid is added to metallic chromium to form chromium nitrate. Manufacturing methods are also known.

  In addition, a step of preparing a chromium trioxide-nitric acid mixed solution by mixing nitric acid with chromium trioxide at a production equivalent of chromium nitrate or more, a chromium trioxide-nitric acid mixed solution obtained by the above step, a monosaccharide, a disaccharide or Also known is a method in which an organic reducing agent composed of an alcohol, aldehyde, carboxylic acid or a mixture thereof derived from a carbohydrate selected from starches is excessively mixed to reduce chromium trioxide to form chromium nitrate. (For example, refer to Patent Document 1).

JP 2002-339082 A

  Among the conventional methods, the method of dissolving chromium hydroxide with nitric acid is very difficult to wash the chromium hydroxide precipitate obtained by adding caustic soda or soda ash to chromium sulfate, and impurities such as sodium or sulfate in chromium hydroxide Have problems that cannot be removed.

  In the method described in Patent Document 1, for example, mixing nitric acid with chromium trioxide to a chromium nitrate equivalent or more is effective in reducing hexavalent chromium. However, depending on conditions, the added reducing agent may react with nitric acid instead of chromium trioxide, and as a result, NOx may be generated. For this reason, not only a deNOx device is required, but a sudden reaction may result in a dangerous state.

  Accordingly, an object of the present invention is to provide chromium nitrate and a method for producing the same, which can eliminate the various disadvantages of the prior art described above.

  This invention achieves the said objective by providing the chromium nitrate aqueous solution characterized by content of oxalic acid being 0.5 weight% or less.

  Moreover, this invention provides the manufacturing method of the chromium nitrate aqueous solution characterized by adding nitric acid and an organic reducing agent separately and simultaneously to chromic acid aqueous solution.

  Furthermore, the present invention provides a chromium nitrate crystal characterized in that the oxalic acid content is 2% by weight or less based on chromium.

  Furthermore, the present invention adds a nitric acid and an organic reducing agent separately and simultaneously to a chromic acid aqueous solution to form a chromic nitrate aqueous solution, then heat-concentrates the chromic nitrate aqueous solution, and further cools the chromic nitrate crystals. The present invention provides a method for producing a chromium nitrate crystal characterized by being precipitated.

  The chromium nitrate aqueous solution and the chromium nitrate crystal of the present invention have a small amount of oxalic acid, and when this is used to perform metal surface treatment, an excellent gloss product can be obtained. Further, according to the production method of the present invention, chromium nitrate having a very low content of oxalic acid can be produced industrially advantageously.

  Hereinafter, the present invention will be described based on preferred embodiments thereof. The aqueous chromium nitrate solution of the present invention is characterized by a low level of oxalic acid, which is a kind of organic matter. Specifically, the chromium nitrate aqueous solution of the present invention has a low level in which the content of oxalic acid is 0.5% by weight or less, preferably substantially free. The content of oxalic acid is preferably at a low level, preferably 6% by weight or less, more preferably 2% by weight or less, and still more preferably substantially free from chromium. As a result of the study by the inventors, the chromium nitrate aqueous solution of the present invention having a low oxalic acid content can be used to obtain an excellent gloss product when the aqueous solution is used for metal surface treatment. found. In the technique described in Patent Document 1 described above, it exists in an aqueous solution due to the use of an organic reducing agent having a large number of carbon atoms such as starch and glucose in order to reduce hexavalent chromium. The amount of oxalic acid is higher than that of the present invention. Although there is no restriction | limiting in particular in the lower limit of the content of oxalic acid in the chromium nitrate aqueous solution of this invention, If the manufacturing method mentioned later is used, it can be made into the very low level which does not contain content of oxalic acid substantially.

  The amount of oxalic acid in the aqueous chromium nitrate solution of the present invention can be measured, for example, by ion chromatography.

  The aqueous chromium nitrate solution of the present invention is also characterized by a low level of total organic carbon (hereinafter also referred to as TOC). As a result of the study by the present inventors, in addition to the low level of oxalic acid content, when the TOC is low level, the chromium nitrate aqueous solution of the present invention is more excellent when used for metal surface treatment. It was found that a glossy product was obtained. Specifically, the chromium nitrate aqueous solution of the present invention has a low level of TOC of preferably 2.5% by weight or less, more preferably less than 0.3% by weight, based on chromium.

  TOC is the total amount of C remaining in the solution as an organic substance. In Patent Document 1 described above, it is described that 0.3% by weight or more of TOC is required in an aqueous chromium nitrate solution in order to reliably reduce hexavalent chromium to trivalent chromium. However, as a result of detailed studies on the TOC by the present inventors, it has been found that the smaller the TOC, the more excellent the gloss when an aqueous chromium nitrate solution is used as the metal surface treatment agent. And according to the manufacturing method mentioned later, even if there is little TOC, hexavalent chromium can be extinguished reliably. Although there is no restriction | limiting in particular in the lower limit of TOC, if it uses the manufacturing method mentioned later, it can be made into the very low level of 0.01 weight%.

  The TOC in the chromium nitrate aqueous solution of the present invention can be measured by, for example, a TOC500 type total organic carbon meter manufactured by Shimadzu Corporation.

  The aqueous chromium nitrate solution of the present invention has a low oxalic acid content and preferably is substantially free of hexavalent chromium in the aqueous solution despite the low level of TOC. Therefore, the chromium nitrate aqueous solution of the present invention has an advantage that the environmental load is small. Such an aqueous solution is suitably produced by a production method described later.

The chromium nitrate aqueous solution of the present invention contains a compound of the composition formula Cr (OH) x (NO ₃ ) y (wherein 0 ≦ x ≦ 2, 1 ≦ y ≦ 3, x + y = 3). In many cases, the aqueous chromium nitrate solution of the present invention is an aqueous solution of 25% by weight or more, preferably 35% by weight or more in terms of Cr (NO ₃ ) ₃ . If it exceeds 41% by weight, crystals will precipitate depending on the conditions. In addition to chromium nitrate represented by Cr (NO ₃ ) ₃ , the compound represented by the above composition formula includes basic chromium nitrate which is a compound obtained by substituting a part of this nitrate group with a hydroxyl group, such as Cr (OH ) _0.5 (NO ₃ ) _2.5 , Cr (OH) (NO ₃ ) ₂ , Cr (OH) ₂ (NO ₃ ) and the like.

  The compound represented by the composition formula may be present alone in the chromium nitrate aqueous solution of the present invention, or may be present in any combination of two or more. By combining two or more kinds, a solution suitable for a specific application can be prepared.

Since hexavalent chromium compounds have erosion and oxidation properties, impurity metal ions, especially Na and Fe, are inevitably mixed in a large amount in the chromium nitrate aqueous solution obtained from the hexavalent chromium compound. In contrast, the aqueous chromium nitrate solution of the present invention is also characterized by the extremely low content of these ions. Specifically, the impurity metal ions in the aqueous chromium nitrate solution are at a low level of preferably not more than 30 ppm, more preferably not more than 20 ppm per 40% by weight as Cr (NO ₃ ) ₃ . Regarding Fe, it is preferably 20 ppm or less, more preferably 10 ppm or less. Such a high-purity chromium nitrate aqueous solution is preferable because it has an advantageous effect that high-purity chromium hydroxide is obtained, particularly when it is used for producing chromium hydroxide used as a raw material for a chromium catalyst. . For example, ICP-AES is used for measuring the concentration of impurity metal ions.

The aqueous chromium nitrate solution of the present invention is also characterized by being substantially free of free nitrate ions not bound to Cr. The fact that the oxalic acid content is not more than the above-mentioned value and does not substantially contain free nitrate ions means that the chromium nitrate aqueous solution of the present invention is stored for a long time at a high concentration of, for example, 40% Cr (NO ₃ ) _3. This is preferable because the advantageous effect that the precipitation of crystals can be suppressed is exhibited.

  The aqueous chromium nitrate solution of the present invention can be preferably used for, for example, metal surface treatment, chromate, and catalyst. In particular, when used for metal surface treatment, there is an advantage that a product with excellent gloss can be obtained.

  Next, the suitable manufacturing method of the chromium nitrate aqueous solution of this invention is demonstrated. The production method is characterized in that nitric acid and an organic reducing agent are separately and simultaneously added to the chromic acid aqueous solution.

  First, the chromic acid aqueous solution, which is a raw material, is obtained by dissolving chromic trioxide obtained by performing various purification treatments using, for example, sodium chromate obtained by alkaline oxidation roasting of chrome ore in water. It is done. The chromic acid aqueous solution thus obtained is a chromic acid aqueous solution prepared by adding chromium hydroxide or chromium carbonate obtained by adding caustic soda or soda ash to chromium sulfate, or high carbon ferrochromium with sulfuric acid or hydrochloric acid. Impurities such as Fe, Na, Mg, Al, Ca, Ni, Mo, and W are extremely small compared to the chromic acid aqueous solution obtained by dissolution.

  The aqueous chromic acid solution may be a solution in the reaction system, and chromium trioxide can be used in the initial reaction. However, in many cases, an aqueous solution prepared by adding and dissolving water is used. Although there is no restriction | limiting in particular in the density | concentration of chromic acid aqueous solution, It is preferable that it is 20 to 60 weight% as a general range.

  The organic reducing agent added to the chromic acid aqueous solution is not particularly limited as long as it is substantially decomposed into carbon dioxide gas and water in the reduction reaction described later and substantially no organic decomposition product remains. For example, monohydric alcohols such as methyl alcohol and propyl alcohol, and dihydric alcohols such as ethylene glycol and propylene glycol are preferably used. As other organic reducing agents, monosaccharides such as glucose, disaccharides such as maltose, and polysaccharides such as starch can be used. When a saccharide having a large number of carbon atoms is used, an organic decomposition product tends to remain, and it is not easy to reduce the content of oxalic acid. Moreover, it is not easy to make TOC containing oxalic acid low. Therefore, in this production method, it is preferable to use a monohydric or dihydric alcohol that is a reducing agent that is difficult to produce oxalic acid and that can easily reduce the TOC. Moreover, when monohydric or dihydric alcohol is used, there also exists an advantage that the reductive reaction near a stoichiometric amount is easy to be obtained. From these viewpoints, it is particularly preferable to use methyl alcohol or ethylene glycol, and it is particularly preferable to use methyl alcohol.

  The organic reducing agent may be added to the chromic acid aqueous solution without being diluted as it is, or may be added in a state diluted with water. When diluting in water, the concentration of the organic reducing agent is preferably about 10 to 30% by weight from the viewpoint of operability and reaction management.

  As nitric acid added to the chromic acid aqueous solution together with the organic reducing agent, industrial ones can be used, and either synthetic nitric acid or by-product nitric acid may be used. Usually, those having a concentration of 67.5% by weight and a specific gravity of 1.4 are used. However, it is not limited to this. These raw materials are desirably used at as high a concentration as possible for the purposes of the present invention.

  Nitric acid and organic reducing agent are added simultaneously and separately to the aqueous chromic acid solution. There is no restriction | limiting in particular in the addition rate of nitric acid and an organic reducing agent. “Separately” means that nitric acid and an organic reducing agent are not added in a mixed state. Mixing an organic reducing agent and nitric acid is dangerous because they react to generate NOx. Further, when a method of adding a reducing agent to a mixed solution of nitric acid and chromic acid as in the method described in Patent Document 1, depending on conditions, the added reducing agent reacts with nitric acid instead of chromic acid, and NOx. May occur. Therefore, not only a device for deNOxing is necessary, but a sudden reaction may cause a dangerous state.

The reaction formula of this production method when, for example, methyl alcohol is used as the organic reducing agent is as follows (wherein x represents a number of 0 or more and 3 or less).
2H ₂ CrO ₄ + (6-2x) HNO ₃ + CH ₃ OH → 2Cr (OH) x (NO ₃ ) 3-x + CO ₂ + 7H ₂ O

  As shown in the above reaction formula, the theoretical amount (stoichiometric amount) of nitric acid required to convert chromic acid to chromium nitrate is a, and the theoretical amount (stoichiometric amount) of the organic reducing agent required to reduce chromic acid. When the theoretical amount) is b, it is preferable to add both so that the relationship of a <b is always satisfied while nitric acid and the organic reducing agent are added simultaneously and separately. This produces an advantageous effect of suppressing generation of NOx by reacting nitric acid that is not bonded to trivalent chromium and the reducing agent. The relationship between a and b is more preferable when a / b is less than 1, particularly 0.9 or less.

  Addition of nitric acid and an organic reducing agent to the chromic acid aqueous solution starts a redox reaction. The reaction proceeds rapidly with a considerable exotherm. The reaction temperature is usually 90 to 110 ° C. The generated water vapor is cooled by a condenser and refluxed into the reaction system.

  In this production method, it is preferable to add only the organic reducing agent to the chromic acid aqueous solution prior to adding nitric acid and the organic reducing agent simultaneously and separately. This is because the a / b in the reaction system can always be less than or equal to the above value by preceding the organic reducing agent and finishing adding nitric acid after the addition of the organic reducing agent.

  Nitric acid is added together with the organic reducing agent being added to the chromic acid aqueous solution. Thereby, both are added simultaneously and separately.

After completion of the reaction, it can be aged for a while and used as it is. The aging is preferably performed at 90 to 110 ° C. for 30 minutes or more. Such ripening can reduce the Cr ⁶⁺ present in the solution to substantially zero and the oxalic acid content to 0.5% or less (less than 6% by weight with respect to chromium). The main purpose. If necessary, an organic reducing agent is further added to completely reduce the remaining Cr ⁶⁺ . Further, if necessary, nitric acid may be added to finely adjust the molar ratio of chromium ions to nitrate ions.

  The aqueous chromium nitrate solution obtained by this production method has a low level of oxalic acid and is substantially free of hexavalent chromium. The obtained chromium nitrate aqueous solution can be heated and concentrated if necessary, and cooled to obtain crystals of chromium nitrate. The resulting chromium nitrate crystals have a low level of oxalic acid content of 2% by weight or less, preferably 0.5% by weight or less, and more preferably substantially free of chromium. Moreover, hexavalent chromium is not substantially contained.

  In the heating concentration, moisture in the chromium nitrate aqueous solution may be removed. The heat concentration may be performed during the reaction even after the reaction is completed. In the case of concentrating by heating during the reaction, it is efficient and industrially advantageous to concentrate by condensing the generated water vapor with a condenser and extracting the water out of the reaction system.

  The present invention will be specifically described below with reference to examples. Unless otherwise specified, “%” means “% by weight”.

[Example 1]
251.6 g of water was added to a glass reaction vessel with a condenser, and further 168.6 g of chromic trioxide was added and dissolved with sufficient stirring to obtain a 40% chromic acid aqueous solution. As the reducing agent, an 18% aqueous solution of methyl alcohol obtained by adding 119.2 g of water to 27.0 g of 99.5% methyl alcohol was used. This aqueous methyl alcohol solution was added to the aqueous chromic acid solution at a rate of 1.22 g / min using a metering pump. This addition rate is a rate at which 146.2 g of 18% methyl alcohol aqueous solution is added in 2 hours.

12 minutes after the start of the addition of the aqueous methyl alcohol solution, 470.6 g of a 67.5% nitric acid aqueous solution was added separately from the aqueous methyl alcohol solution at a rate of 3.92 g / min. This addition rate is a rate at which the aqueous nitric acid solution is added in 2 hours, similar to the addition time of the aqueous methyl alcohol solution. The ratio a / b of the theoretical amount a of nitric acid and the theoretical amount b of methyl alcohol at the end of the addition of the aqueous methyl alcohol solution was 0.9. The addition of the aqueous nitric acid solution was completed 12 minutes after the completion of the addition of the aqueous methyl alcohol solution. Thereafter, aging was continued for 30 minutes. The temperature at this time was 105 ° C. After aging, the remaining Cr ⁶⁺ was checked, and an aqueous methyl alcohol solution was added to continue aging. The reaction was terminated after confirming that the color development of Cr ⁶⁺ was lost by the diphenylcarbazite method. Nitrous acid gas was not generated during the reaction. The composition of the obtained chromium nitrate aqueous solution was as follows.

[Example 2]
In a glass reaction vessel with a condenser, 280.2 g of a 60% chromic acid aqueous solution and 140.0 g of water were placed and stirred sufficiently to obtain a 40% chromic acid aqueous solution. As a reducing agent, a 20% ethylene glycol aqueous solution in which 121.9 g of water was added to 91.7% ethylene glycol 31.7 g was used. This ethylene glycol aqueous solution was added to the chromic acid aqueous solution at a rate of 1.28 g / min using a metering pump. This addition rate is a rate at which 153.6 g of 20% ethylene glycol aqueous solution is added in 2 hours.

Twelve minutes after the start of the addition of the ethylene glycol aqueous solution, 470.6 g of a 67.5% nitric acid aqueous solution was added separately from the ethylene glycol aqueous solution at a rate of 3.92 g / min. This addition rate is a rate at which the nitric acid aqueous solution is added in 2 hours, similar to the addition time of the ethylene glycol aqueous solution. At the end of the addition of the ethylene glycol aqueous solution, the ratio of the theoretical amount a of nitric acid to the theoretical amount b of ethylene glycol, a / b, was 0.9. The addition of the nitric acid aqueous solution was completed 12 minutes after the end of the addition of the ethylene glycol aqueous solution. Thereafter, aging was continued for 30 minutes. The temperature at this time was 105 ° C. After aging, the remaining Cr ⁶⁺ was checked, and further aging was continued by adding an ethylene glycol aqueous solution. The reaction was terminated after confirming that the color development of Cr ⁶⁺ was lost by the diphenylcarbazite method. Nitrous acid gas was not generated during the reaction. The composition of the obtained chromium nitrate aqueous solution was as follows.

Example 3
A 60% chromic acid aqueous solution (70.0 kg) and water (35.0 kg) were placed in a glass reactor with a condenser, and the mixture was sufficiently stirred to obtain a 40% chromic acid aqueous solution. As a reducing agent, an 18% aqueous solution of methyl alcohol obtained by adding 29.9 kg of water to 6.8 kg of 99.5% methyl alcohol was used. This aqueous methyl alcohol solution was added to the aqueous chromic acid solution at a rate of 306 g / min using a metering pump. This addition rate is a rate at which 36.7 kg of 18% aqueous methyl alcohol solution is added in 2 hours.

12 minutes after the start of the addition of the aqueous methyl alcohol solution, 117.6 kg of a 67.5% aqueous nitric acid solution was added separately from the aqueous methyl alcohol solution at a rate of 980 g / min. This addition rate is a rate at which the aqueous nitric acid solution is added in 2 hours, similar to the addition time of the aqueous methyl alcohol solution. The ratio a / b of the theoretical amount a of nitric acid and the theoretical amount b of methyl alcohol at the end of the addition of the aqueous methyl alcohol solution was 0.9. The addition of the aqueous nitric acid solution was completed 12 minutes after the completion of the addition of the aqueous methyl alcohol solution. Thereafter, aging was continued for 30 minutes. The temperature at this time was 105 ° C. After aging, the remaining Cr ⁶⁺ was checked, and an aqueous methyl alcohol solution was added to continue aging. The reaction was terminated after confirming that the color development of Cr ⁶⁺ disappeared by the diphenylcarbazite method. During the reaction, 50 kg of condensed water was extracted from the bottom of the condenser and concentrated. After completion of the reaction, the reaction mixture was cooled to room temperature, seed crystals were added, and stirring was continued for a whole day and night. The precipitated crystals were separated by a centrifugal separator, and 25 kg of chromium nitrate crystals were recovered. The obtained crystal was confirmed to be Cr (NO ₃ ) ₃ .9H ₂ O by X-ray diffraction. The composition of the obtained chromium nitrate crystals was as follows.

Example 4
A glass reaction vessel with a condenser was charged with 280.2 g of 60% liquid chromic acid and 280.2 g of water, and sufficiently stirred to obtain a 30% chromic acid aqueous solution. As a reducing agent, a 34% ethylene glycol aqueous solution in which 60.2 g of water was added to 91.7% ethylene glycol 31.7 g was used. This ethylene glycol was added to the chromic acid aqueous solution using a metering pump at a rate of 0.77 g / min. This addition rate is a rate at which 91.9 g of 34% ethylene glycol is added in 2 hours.

Twelve minutes after the start of the addition of the aqueous ethylene glycol solution, 392.2 g of 67.5% nitric acid was added separately from the aqueous ethylene glycol solution at a rate of 3.27 g / min. This addition rate is a rate at which nitric acid is added in 2 hours, similar to the addition time of the aqueous ethylene glycol solution. The ratio, a / b, of the theoretical amount a of nitric acid a and the theoretical amount b of ethylene glycol at the end of the addition of the ethylene glycol aqueous solution was 0.9. The addition of nitric acid was completed 12 minutes after the end of the addition of the ethylene glycol aqueous solution. Thereafter, aging was continued for 30 minutes. The temperature at this time was 105 ° C. After aging, the remaining Cr ⁶⁺ was checked, and further aging was continued by adding an ethylene glycol aqueous solution. The reaction was terminated after confirming that the color development of Cr ⁶⁺ was lost by the diphenylcarbazite method. Nitrous acid gas was not generated during the reaction. The composition of the obtained chromium nitrate aqueous solution was as follows.

[Comparative Example 1]
251.6 g of water was added to a glass reaction vessel with a condenser, and further 168.6 g of chromic trioxide was added and dissolved with sufficient stirring to obtain a 40% chromic acid aqueous solution. As the reducing agent, a 26% glucose aqueous solution in which 107.2 g of water was added to 39.0 g of 97% glucose was used. This aqueous glucose solution was added to the aqueous chromic acid solution using a metering pump at a rate of 1.22 g / min. This addition rate is a rate at which 146.2 g of a 26% glucose aqueous solution is added in 2 hours.

12 minutes after the start of the addition of the aqueous glucose solution, 470.6 g of a 67.5% aqueous nitric acid solution was added separately from the aqueous glucose solution at a rate of 3.92 g / min. This addition rate is the rate at which the aqueous nitric acid solution is added in 2 hours, similar to the addition time of the aqueous glucose solution. At the end of addition of the aqueous glucose solution, the ratio of the theoretical amount a of nitric acid to the theoretical amount b of glucose, a / b, was 0.9. The addition of the aqueous nitric acid solution was also completed 12 minutes after the completion of the aqueous glucose solution. Thereafter, aging was continued for 30 minutes. The temperature at this time was 105 ° C. After aging, the remaining Cr ⁶⁺ was checked, and an aqueous glucose solution was added to continue aging. The reaction was terminated after confirming that the color development of Cr ⁶⁺ disappeared by the diphenylcarbazite method. Nitrous acid gas was not generated during the reaction. The composition of the obtained chromium nitrate aqueous solution was as follows.

[Performance evaluation]
The chromate treatment solution was erected using the chromium nitrate aqueous solutions obtained in Examples 1, 2, and 4 and Comparative Example 1, and the chromate treatment was performed by immersing and drying test pieces of galvanized steel sheets. The degree of gloss after the treatment was evaluated. The results are shown below.

As is clear from the above results, it can be seen that when the chromium nitrate aqueous solution of the example (product of the present invention) is used, the gloss by the chromate treatment is excellent.

Claims (9)

  A chromium nitrate aqueous solution, wherein the content of oxalic acid is 0.5% by weight or less.   The aqueous chromium nitrate solution according to claim 1, wherein the total organic carbon is 2.5% by weight or less based on chromium. _{3. The} aqueous chromium nitrate solution according to claim 1, wherein impurity metal ions in the aqueous solution are Na ≦ 30 ppm and Fe ≦ 20 ppm per 40 wt% as Cr (NO ₃ ) ₃ .   The chromium nitrate aqueous solution according to any one of claims 1 to 3, which is substantially free of free nitrate ions not bonded to Cr.   A method for producing a chromium nitrate aqueous solution, wherein nitric acid and an organic reducing agent are separately and simultaneously added to the chromic acid aqueous solution.   When the theoretical amount of nitric acid required to convert chromic acid to chromium nitrate is a, and the theoretical amount of organic reducing agent required to reduce chromic acid is b, nitric acid and organic so that a <b is satisfied. The method for producing an aqueous chromium nitrate solution according to claim 5, which is added with a reducing agent.   The method for producing an aqueous chromium nitrate solution according to claim 5 or 6, wherein the organic reducing agent is a monohydric alcohol or a dihydric alcohol.   A chromium nitrate crystal having an oxalic acid content of 2% by weight or less based on chromium. A feature is that a nitric acid and an organic reducing agent are separately and simultaneously added to a chromic acid aqueous solution to form a chromic nitrate aqueous solution, and then the chromic nitrate aqueous solution is heated and concentrated, and further cooled to precipitate chromium nitrate crystals. A method for producing chromium nitrate crystals.