JP2007224340A - Production method of high purity silver ingot - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a production method for producing a high purity silver ingot by heating and melting silver powder including at least one kind of an impurity element selected from among lead, tellurium or selenium, the method in which each content of lead, tellurium and selenium in the silver ingot is stably reduced to ≤10 ppm, desirably to ≤1 ppm by single melting treatment. <P>SOLUTION: The silver powder containing at least one kind of the impurity element selected from among lead, tellurium and selenium is heated and melted in an oxidizing atmosphere by the procedure of (i) or (ii) in the coexistence of calcium phosphate to thereby remove the impurity element(s), and thereafter, the obtained molten silver is cast into a die: (i) adding calcium phosphate to the silver powder, then heating and melting the silver powder in a crucible; and (ii) heating and melting the silver powder in the crucible, thereafter adding calcium phosphate to the silver powder, and further heating and melting the silver powder. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for producing a high purity silver ingot (ingot), and more specifically, when producing a silver ingot by heating and melting silver powder containing at least one impurity element selected from lead, tellurium or selenium. In a single melting treatment of silver powder, the contents of lead, tellurium and selenium in the silver ingot are each stably reduced to 10 ppm or less, preferably 1 ppm or less. And a method for producing a high-purity silver ingot having excellent processability.

  Conventionally, high purity silver ingots having a purity of 99.999% or more (impurity element content <10 ppm) have been manufactured. In the casting process of the silver ingot, for example, the raw material silver powder is put in a graphite crucible and heated and melted, and the molten surface is cleaned by burner heating, and then charcoal is added to remove dissolved oxygen in the molten silver. Thereafter, a method of casting the obtained molten silver into a mold is performed except for charcoal.

  However, depending on the raw material, the impurity element content in the silver ingot sometimes exceeds 10 ppm. At this time, the content of lead, tellurium and the like has been increased. Conventionally, as a method of removing lead and tellurium remaining in a silver ingot and purifying the silver ingot, a method of remelting the silver ingot has been performed. However, in the method of remelting the silver ingot, not only the exact mechanism is unknown, but there is a problem that it takes time and labor and increases the cost.

Further, as a highly refined method, a method is proposed in which a silver raw material containing a trace amount of lead, sulfur, copper and the like is melted and vacuum impurity is removed under high vacuum to remove impurity elements (for example, see Patent Document 1). But,
There was a problem that required expensive capital investment and cost.

  From the above situation, the content of lead and tellurium is stably reduced during the melting and melting of silver powder and casting into a mold, that is, during one melting process without remelting the silver ingot. There is a need for a method of producing a pure silver ingot.

JP-A-9-256083 (first page, second page)

  In view of the above-mentioned problems of the prior art, the object of the present invention is to produce a silver ingot once by heating and melting silver powder containing at least one impurity element selected from lead, tellurium or selenium. It is an object of the present invention to provide a method for producing a high-purity silver ingot in which the content of lead, tellurium and selenium in a silver ingot is stably reduced to 10 ppm or less, desirably 1 ppm or less. Moreover, it is providing the method of manufacturing the high purity silver ingot which was further excellent in the surface state and workability simultaneously as needed.

  In order to achieve the above object, the present inventors have conducted extensive research on a method for producing a high-purity silver ingot using a silver powder containing an impurity element such as lead or tellurium. When the resulting molten silver is cast in a mold, a high-purity silver ingot from which lead, tellurium and selenium have been removed is obtained. Prior to casting, the molten silver is deoxygenated. As a result, it was found that a high-purity silver ingot excellent in surface condition and workability was obtained, and the present invention was completed.

That is, according to the first invention of the present invention, silver phosphate containing at least one impurity element selected from lead, tellurium or selenium is calcium phosphate in an oxidizing atmosphere in the following manner (a) or (b): A high-purity silver ingot manufacturing method is provided, in which the molten silver thus obtained is cast in a mold after being melted by heating and coexisting in the presence of
(A) Calcium phosphate is added to the silver powder and then heated and melted in a crucible.
(B) After the silver powder is heated and melted in a crucible, calcium phosphate is added and further heated and melted.

  According to a second invention of the present invention, in the first invention, the calcium phosphate is added in an amount of 0.5 to 15% by weight based on the silver powder. A method is provided.

According to a third aspect of the present invention, in the first aspect, the oxidizing atmosphere is formed while controlling the oxygen partial pressure of the molten silver within a range represented by the following formula. A method for producing a pure silver ingot is provided.
−0.7> log Po ₂ > −7
(However, in the formula, Po ₂ represents the oxygen partial pressure in the molten silver in atm units, and is converted to a temperature reference of 1200 ° C.)

  According to a fourth aspect of the present invention, there is provided the method for producing a high purity silver ingot according to the first aspect, wherein the crucible is made of a non-reducing material.

  According to a fifth aspect of the present invention, there is provided the method for producing a high purity silver ingot according to the first aspect, wherein the temperature for the heat melting is 970 to 1300 ° C.

  According to a sixth aspect of the present invention, there is provided the method for producing a high purity silver ingot according to the first aspect, further comprising performing deoxidation treatment of the molten silver prior to casting.

When producing a silver ingot by heating and melting silver powder containing at least one impurity element selected from lead, tellurium or selenium in the first to fifth inventions, the method for producing a high purity silver ingot of the present invention comprises: A high-purity silver ingot can be produced in which the content of the impurity elements contained in the silver ingot is stably reduced to 10 ppm or less, desirably 1 ppm or less, by a single melting treatment of silver powder. Industrial value is extremely high.
In the sixth invention, lead and tellurium are removed by performing deoxygenation of the molten silver prior to casting, and a high-purity silver ingot having an excellent surface condition and workability is produced. So it is more advantageous.

Hereinafter, the manufacturing method of the high purity silver ingot of this invention is demonstrated in detail.
The method for producing a high-purity silver ingot according to the present invention comprises the steps of (a) or (b) below, in a oxidizing powder containing silver powder containing at least one impurity element selected from lead, tellurium or selenium. It is characterized in that it is melted by heating in the presence of coexistence to remove the impurity elements, and then the obtained molten silver is cast into a mold.
(B) Calcium phosphate is added to the silver powder and melted by heating in a crucible.
(B) After the silver powder is heated and melted in a crucible, calcium phosphate is added and further heated and melted.

In the present invention, it has an important significance in casting after being melted by heating in the coexistence of calcium phosphate (Ca ₃ (PO ₄ ) ₂ ) in an oxidizing atmosphere in the manner of (b) or (b). As a result, it is possible to produce a high-purity silver ingot in which the content of the impurity elements in the silver ingot is stably reduced to 10 ppm or less, desirably 1 ppm or less, by a single melting treatment of silver powder. That is, by melting in an oxidizing atmosphere, lead, tellurium and selenium in the molten silver are oxidized, and by absorbing these oxides in calcium phosphate, these impurity elements are reduced to a target content or less. It seems that the reduction is realized.

More specifically, since lead, tellurium, and selenium in silver powder exist in a metallic state, they exist in a molten state in silver while remaining in a metallic state in a reducing atmosphere. On the other hand, in an oxidizing atmosphere, lead, tellurium and selenium have a property of having a stronger affinity for oxygen than silver, but they have an equilibrium constant at a melting temperature of 1200 ° C. (5.6 × 10 ⁵ , 2. (Estimated as 9 × 10 ² )), it can be said that when the content of lead, tellurium and selenium contained in silver is a trace amount on the order of ppm, it is difficult to be oxidized.
By the way, there is a method of separating lead from silver, generally called the ash-blowing method, but this method penetrates only calcium oxide into calcium phosphate by utilizing the fact that the surface tension of lead oxide produced by oxidation is lower than that of silver. This is a physical separation method in which separation is performed. Therefore, as described above, when the lead content is low, it is difficult to oxidize itself, so that it is considered difficult to separate by the ash blowing method.

However, in practice, when the amount of calcium phosphate added is sufficiently large relative to lead, tellurium, etc. in the molten silver, these oxides (lead oxide, tellurium oxide, etc.) are part of the calcium oxide (CaO). And CaO—PbO, CaO—TeO _{2 and} other melts with a melting point of 1000 ° C. or lower, and their activities are significantly reduced, so that Po ₂ is oxidized at an oxygen concentration of 10 ⁻⁷ or more. Was recognized. Here, since the CaO—PbO-based and CaO—TeO ₂ -based melts have a sufficiently large amount of calcium phosphate, they are absorbed therein and easily separated.

  In the present invention, the heat melting is performed by adding calcium phosphate to silver powder and then heat-melting in a crucible, or by adding silver phosphate after heat-melting silver powder in a crucible and further heat-melting. In either way, a similar effect is obtained for the removal of lead, tellurium and selenium in silver. Here, the crucible is installed in a heating furnace such as a kerosene or gas combustion heating furnace or an electric furnace, and the inside of the furnace is made an oxidizing atmosphere.

  Silver powder used in the method of the present invention is not particularly limited, and silver powder containing at least one impurity element selected from lead, tellurium or selenium obtained from a silver refining process is used. In particular, electrodeposited silver or reduced silver, which is a raw material for high-purity silver ingots, is used, and in particular, silver powder containing tens and several ppm of lead and tellurium, respectively, is preferably used. Here, the electrodeposited silver can be obtained, for example, by electrolytic purification in a nitric acid bath using, as an anode, a crude silver ingot having a silver quality of 80 to 90% by weight. Reduced silver is obtained, for example, by wet-purifying silver chloride to remove impurity elements, and then reducing the silver chloride by adding a reducing agent in an aqueous alkaline solution.

  It does not specifically limit as calcium phosphate used for the said method, Commercially available powder is used. The addition amount is not particularly limited, and is preferably 0.5 to 15% by weight, more preferably 0.5 to 2% by weight with respect to the silver powder. That is, when the addition amount of calcium phosphate is less than 0.5% by weight, the addition amount is small as described above, leading to insufficient oxidation of lead, tellurium and selenium and insufficient absorption. On the other hand, if the amount of calcium phosphate added exceeds 15% by weight, the drug cost increases, and the amount of separated calcium phosphate waste increases.

The oxidizing atmosphere used in the above method is not particularly limited, but in order to oxidize lead, tellurium and selenium contained in trace amounts in the molten silver, the molten silver in the crucible should be an oxidizing atmosphere. Is essential. As the atmosphere in the heating furnace, since silver does not oxidize in the air at normal pressure, it can be performed in an air atmosphere that does not require special adjustment. However, in order to ensure an oxidizing atmosphere of the molten silver in the crucible, and to strictly control and confirm the end point, the oxygen partial pressure of the molten silver is appropriately measured and controlled within the range shown in the following formula. It is desirable.
−0.7> log Po ₂ > −7
(However, in the formula, Po ₂ represents the oxygen partial pressure in the molten silver in units of atm and is converted to a temperature reference of 1200 ° C.)

  The crucible used in the above method is not particularly limited, but those using non-reducing materials such as ceramics such as alumina and magnesia are preferable. Usually, a graphite crucible is often used for melting silver, but a crucible in which an oxidizing atmosphere can be easily obtained is preferable.

  The temperature of the heat melting used in the above method is not particularly limited, and is a temperature at which lead and tellurium can be sufficiently removed and a silver ingot can be cast, and in particular, 970 to 1300 ° C. is preferred.

  In the method of the present invention, if necessary, the deoxidation treatment of the molten silver can be performed prior to casting. As a result, lead and tellurium are removed, and a high-purity silver ingot excellent in surface condition and workability is produced.

  Conventionally, in a casting process for industrially producing a silver ingot, it has been essential to suppress oxygen dissolution in heat melting of a silver raw material. Insufficient suppression of oxygen dissolution causes blistering due to bubbles in the resulting silver ingot, resulting in deterioration of the surface condition, and there are bubbles remaining inside, resulting in poor workability. , Can not be commercialized. That is, it is known that silver has a property of absorbing a large amount of oxygen in a molten state and releasing the absorbed oxygen upon solidification.

  The deoxygenation treatment used in the above method is not particularly limited, and is performed by maintaining the inside of the molten silver in a reducing atmosphere. For example, simple methods such as charcoal, coke, activated carbon, etc. A method of adding a reducing agent such as carbon having good reactivity or a carbon-containing compound that generates carbon at the heating and melting temperature such as sugar and starch.

  The casting temperature is not particularly limited, and a temperature at which the casting operation of the silver ingot can be performed is selected. Among these, the temperature above the melting point (962 ° C.) of silver having a low oxygen content is particularly preferable. Near is preferred.

Hereinafter, the present invention will be described in more detail by way of examples and comparative examples of the present invention, but the present invention is not limited to these examples. In addition, the analysis method of the metal used by the Example and the comparative example was performed by the ICP emission analysis method.
Moreover, in an Example and a comparative example, the schematic structure of the crucible furnace used for melting of silver powder is shown in FIG. In FIG. 1, a crucible furnace 1 includes an electrically heated furnace body 2 and a semi-sealed lid 3, and a crucible 4 is installed inside.

Example 1
500 g of reduced silver powder containing 15 ppm of lead and 2 ppm of tellurium and 5 g of calcium phosphate were placed in an alumina crucible, and the crucible was placed in the crucible furnace. The inside of the crucible furnace was heated in an air atmosphere and held at 1200 ° C. for 2 hours. At this time, when the partial pressure of oxygen was measured using an expendable oxygen probe, the partial pressure of oxygen (logPo ₂ ) was −5. Immediately thereafter, the molten silver was cast into a heat-resistant cast iron mold. After casting, it was gradually cooled using a propane gas burner while pouring the molten metal surface. Thereafter, lead and tellurium of the obtained silver ingot were analyzed. As a result, the contents of lead and tellurium were 1 ppm and <1 ppm, respectively.

(Example 2)
Except for using reduced silver powder containing 19 ppm lead, 2 ppm tellurium and 9 ppm selenium, the same procedure as in Example 1 was performed, and the surface of the obtained silver ingot was observed and lead, tellurium and selenium were analyzed. As a result, the contents of lead, tellurium, and selenium were 2 ppm, <1 ppm, and <1 ppm, respectively.

(Example 3)
500 g of reduced silver powder containing 15 ppm of lead and 2 ppm of tellurium and 10 g of calcium phosphate were placed in an alumina crucible, and this crucible was placed in a crucible furnace. The inside of the crucible furnace was heated as an atmospheric atmosphere and held at 1100 ° C. for 1 hour. At this time, when the partial pressure of oxygen was measured using an expendable oxygen probe, the partial pressure of oxygen (logPo ₂ ) was −5. Next, charcoal powder was added in the crucible to such an extent that the surface of the molten silver was covered and held for 15 minutes. Then, after remaining charcoal powder was separated, molten silver was cast into a heat-resistant cast iron mold. After casting, it was gradually cooled using a propane gas burner while pouring the molten metal surface. It should be noted that the generation of bubbles due to oxygen release during casting of the molten silver was very small, and the surface of the ingot thus obtained was in a very good state with no blistering and hole opening due to bubbles. Thereafter, lead and tellurium of the obtained silver ingot were analyzed. As a result, the lead and tellurium contents were both 1 ppm or less.

Example 4
500 g of reduced silver powder containing 15 ppm lead and 2 ppm tellurium was placed in an alumina crucible, and the crucible was placed in a crucible furnace. The inside of the crucible furnace was heated as an atmospheric atmosphere, and after reaching 1100 ° C., 5 g of calcium phosphate was charged into the crucible and held for 1 hour. At this time, when the partial pressure of oxygen was measured using an expendable oxygen probe, the partial pressure of oxygen (logPo ₂ ) was −5. Next, charcoal powder was added in the crucible to such an extent that the surface of the molten silver was covered and held for 15 minutes. Then, after remaining charcoal powder was separated, molten silver was cast into a heat-resistant cast iron mold. After casting, it was gradually cooled using a propane gas burner while pouring the molten metal surface. It should be noted that the generation of bubbles due to oxygen release during casting of the molten silver was very small, and the surface of the ingot thus obtained was in a very good state with no blistering and hole opening due to bubbles. Thereafter, lead and tellurium of the obtained silver ingot were analyzed. As a result, the lead and tellurium contents were both 1 ppm or less.

(Comparative Example 1)
The procedure was the same as in Example 1 except that calcium phosphate was not added, and the oxygen partial pressure was measured and lead and tellurium were analyzed. As a result, the oxygen partial pressure (log Po ₂ ) was −5. The contents of lead and tellurium were 15 ppm and 2 ppm, respectively.

(Comparative Example 2)
The procedure was the same as in Example 1 except that a graphite crucible was used, and the oxygen partial pressure was measured and lead and tellurium were analyzed. As a result, the oxygen partial pressure (log Po ₂ ) was −9. The contents of lead and tellurium were 12 ppm and 2 ppm, respectively.

  From the above, in Examples 1 to 4, since silver powder was heated and melted in the presence of calcium phosphate in an oxidizing atmosphere and carried out according to the method of the present invention, lead, tellurium, In addition, it can be seen that each of the selenium content and the selenium content is stably reduced to 10 ppm or less, further 1 ppm or less, and a high-purity silver ingot having a good surface state can be obtained by deoxidation treatment. On the other hand, in Comparative Example 1 or 2, it can be seen that either the addition of calcium phosphate or the atmosphere does not meet these conditions, so that satisfactory results cannot be obtained in the content of the impurity element.

  As is clear from the above, the method for producing a high purity silver ingot according to the present invention is based on lead, tellurium and selenium when a high purity silver ingot is obtained by casting by melting silver powder such as electrodeposited silver or reduced silver. It is useful as a method capable of removing an ingot and further obtaining an ingot having a smooth surface shape.

It is a figure showing the schematic structure of the crucible furnace used by the Example and the comparative example.

Explanation of symbols

1 crucible furnace 2 furnace body 3 lid 4 crucible

Claims (6)

Silver powder containing at least one impurity element selected from lead, tellurium or selenium was heated and melted in the presence of calcium phosphate in an oxidizing atmosphere in the following manner (a) or (b) to remove the impurity element. Then, the manufacturing method of the high purity silver ingot characterized by casting the obtained molten silver to a casting_mold | template.
(A) Calcium phosphate is added to the silver powder and then heated and melted in a crucible.
(B) After the silver powder is heated and melted in a crucible, calcium phosphate is added and further heated and melted.   2. The method for producing a high purity silver ingot according to claim 1, wherein the addition amount of the calcium phosphate is 0.5 to 15 wt% with respect to the silver powder. 2. The method for producing a high purity silver ingot according to claim 1, wherein the oxidizing atmosphere is formed while controlling an oxygen partial pressure of molten silver within a range represented by the following formula.
−0.7> log Po ₂ > −7
(However, in the formula, Po ₂ represents the oxygen partial pressure in the molten silver in atm units, and is converted to a temperature reference of 1200 ° C.)   The method for producing a high purity silver ingot according to claim 1, wherein the crucible is made of a non-reducing material.   The method for producing a high-purity silver ingot according to claim 1, wherein a temperature of the heat melting is 970 to 1300 ° C.   The method for producing a high-purity silver ingot according to claim 1, further comprising deoxidizing the molten silver prior to casting.

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