Method for improving the solubility of spent tungsten base

Abstract

This invention belongs to the field of hydrometallurgy of tungsten, specifically relating to a method for improving the alkaline solubility of waste tungsten. By adding a catalyst to the waste tungsten material to solidify impurities and simultaneously catalytically oxidize the waste tungsten material, the oxidation of the waste tungsten material is more complete during the oxidative roasting process, thereby increasing the oxidation rate of tungsten, which in turn increases the subsequent alkaline solubility, reduces the residue rate, and reduces waste emissions, thereby improving the tungsten recovery rate and product quality.

Description

Technical Field

[0001] This invention belongs to the field of hydrometallurgy of tungsten, and specifically relates to a method for improving the alkali solubility of waste tungsten. Background Technology

[0002] In the recycling and processing of waste tungsten, improving the alkali dissolution rate is key to achieving efficient utilization of tungsten resources. Traditional waste tungsten treatment methods suffer from problems such as uneven oxidation, unstable phase structure, slow alkali dissolution reaction rate, and high residue rate, which affect the tungsten recovery rate and product quality.

[0003] Currently, the main methods for treating waste tungsten include direct high-temperature roasting and simple oxidation treatment. Although existing technologies have addressed some aspects of waste tungsten processing, several problems and shortcomings remain. First, existing roasting methods cannot precisely control the degree of oxidation and phase structure, resulting in slow alkali dissolution rates, incomplete reactions, and high residue rates. Second, the waste tungsten material needs to be recycled after roasting, leading to low tungsten recovery rates and increased waste emissions. Furthermore, the low alkali dissolution rate and high residue rate of waste oxidizers affect tungsten recovery rates and product quality. Simultaneously, traditional roasting processes are energy-intensive and polluting, hindering the green recycling of tungsten resources. Therefore, developing a novel roasting method to improve the alkali dissolution rate of waste tungsten is of significant practical importance and application value. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a method for improving the alkaline dissolution rate of waste tungsten. By adding a catalyst to the waste tungsten material to solidify impurities and simultaneously catalytically oxidize the waste tungsten, the waste tungsten material can achieve a more complete oxidation effect after a single roasting, thereby increasing the oxidation rate of tungsten and thus improving the subsequent alkaline dissolution rate.

[0005] This invention provides the following technical solution: A method for improving the alkaline solubility of waste tungsten includes the following steps: S1. Mix nitrate and carbonate evenly to obtain a catalyst; S2. Mix the waste tungsten material with the catalyst to obtain a mixture; S3. The mixture is oxidized and roasted to obtain roasted material.

[0006] In the above technical solution, in step S1, nitrate and carbonate are mixed to prepare a catalyst. The carbonate solidifies impurities during the subsequent oxidation roasting process, while the nitrate catalyzes the oxidation of waste tungsten during the same process, ensuring complete oxidation of the waste tungsten material in a single oxidation roasting step. In step S2, mixing ensures sufficient contact between the waste tungsten material and the catalyst, preparing for subsequent oxidation roasting. In step S3, the tungsten is oxidized during the oxidation roasting process, increasing the subsequent alkali solubility.

[0007] In a preferred embodiment of the method for improving the alkaline solubility of waste tungsten according to the present invention, the content of nitrate is 30wt%~40wt% and the content of carbonate is 60wt%~70wt%.

[0008] As a preferred embodiment of the method for improving the alkaline solubility of waste tungsten according to the present invention, the nitrate includes any one or more combinations of sodium nitrate, potassium nitrate and ammonium nitrate; the carbonate includes any one or more combinations of sodium carbonate, potassium carbonate and calcium carbonate.

[0009] As a preferred embodiment of the method for improving the alkaline solubility of waste tungsten according to the present invention, the waste tungsten material includes powdered material from tungsten alloy grinding and cutting or tungsten-containing contaminated material, wherein the tungsten-containing contaminated material includes contaminated material of tungsten powder or contaminated material of tungsten carbide.

[0010] As a preferred embodiment of the method for improving the alkaline solubility of waste tungsten according to the present invention, in step S2, the amount of catalyst added is 5wt% to 10wt% of the waste tungsten material.

[0011] As a preferred embodiment of the method for improving the alkaline solubility of waste tungsten according to the present invention, in step S3, the oxidation roasting temperature is ≥650℃ and the holding time is 2.5~3.5h.

[0012] As a preferred embodiment of the method for improving the alkali solubility of waste tungsten according to the present invention, step S3 further includes: alkali dissolving the roasted material, followed by solid-liquid separation to obtain waste residue, wherein the waste residue contains WO3 < 9 wt%.

[0013] As a preferred embodiment of the method for improving the alkaline solubility of waste tungsten according to the present invention, step S3 includes: continuously introducing oxygen-containing gas into the mixture for oxidative roasting to obtain roasted material, wherein the oxygen-containing gas is a mixture of oxygen and nitrogen, and the oxygen content is not less than 5 vol%. The oxygen content in the oxygen-containing gas is 5 vol% to 10 vol%. In the above technical solution, continuously introducing oxygen-containing gas with an oxygen content of 5 vol% to 10 vol% allows the material to undergo oxidative roasting in an environment with stable oxygen content, thereby maintaining the stable progress of the oxidation reaction and enabling more complete oxidation of the waste tungsten material. The roasted material is then dissolved in alkali and subjected to solid-liquid separation to obtain waste residue, wherein the WO3 content in the waste residue is <4 wt%.

[0014] As a preferred embodiment of the method for improving the alkaline solubility of waste tungsten according to the present invention, in step S3, the oxidation roasting adopts a segmented roasting process, specifically: first, the temperature is raised from room temperature to a first temperature at a rate of 5℃ / min and held for 30~50min, and then the temperature is raised to a second temperature at a rate of 3℃ / min and held for 2~3h; wherein, the first temperature is 300~400℃, and the second temperature is ≥650℃.

[0015] In the above technical solution, a segmented roasting process is adopted. First, the temperature is raised from room temperature to a first temperature at a rate of 5℃ / min and held for 30-50 minutes to ensure uniform initial heating of WC in the waste tungsten material and improve its activity. Then, the temperature is raised to a second temperature at a slower rate of 3℃ / min, allowing the WC in the waste tungsten material to gradually oxidize and form a stable phase structure. A holding time of 2-3 hours is then used to fully oxidize the WC, forming a phase conducive to alkali solubility, thereby further improving the alkali solubility rate. The roasted material is then alkali-dissolved and subjected to solid-liquid separation to obtain a waste residue, wherein the WO3 content in the waste residue is <3.5 wt%.

[0016] As a preferred embodiment of the method for improving the alkali solubility of waste tungsten according to the present invention, step S3 includes: continuously introducing oxygen-containing gas into the mixture for oxidative roasting to obtain roasted material, wherein the oxygen-containing gas is a mixture of oxygen and nitrogen, and the oxygen content is not less than 5 vol%; the oxidative roasting adopts a segmented roasting process, specifically: first heating from room temperature to a first temperature at a rate of 5℃ / min and holding for 30~50 min, then heating to a second temperature at a rate of 3℃ / min and holding for 2~3 h; wherein the first temperature is 300~400℃, and the second temperature is ≥650℃; the roasted material is alkali-dissolved, and then solid-liquid separated to obtain waste residue, wherein the WO3 in the waste residue is <0.5 wt%.

[0017] This invention proposes a method to improve the alkaline dissolution rate of waste tungsten, which has the following beneficial effects: by adding a catalyst to the waste tungsten material to solidify impurities and simultaneously catalytically oxidize the waste tungsten material, the waste tungsten material can achieve a more complete oxidation effect through oxidation roasting, thereby improving the oxidation rate of tungsten, thus improving the subsequent alkaline dissolution rate, reducing the residue rate, and reducing waste emissions, thereby improving the tungsten recovery rate. Detailed Implementation

[0018] The technical solutions described below in conjunction with the embodiments will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] This invention provides the following technical solution: A method for improving the alkaline solubility of waste tungsten includes the following steps: S1. Mix nitrate and carbonate evenly to obtain a catalyst; Preferably, the nitrate content is 30wt%~40wt%, and the carbonate content is 60wt%~70wt%; the nitrate includes one or more combinations of sodium nitrate, potassium nitrate, and ammonium nitrate; the carbonate includes one or more combinations of sodium carbonate, potassium carbonate, and calcium carbonate.

[0020] S2. Mix the waste tungsten material with the catalyst to obtain a mixture; Preferably, the waste tungsten material includes powdered material from tungsten alloy grinding and cutting or tungsten-containing contaminated material, wherein the tungsten-containing contaminated material includes contaminated material of tungsten powder or contaminated material of tungsten carbide; the catalyst addition amount is 5wt%~10wt% of the waste tungsten material; Specifically, the amount of catalyst added can be any one of 5wt%, 6wt%, 7wt%, 8wt%, 9wt%, or 10wt% of the waste tungsten material, or a range between any two.

[0021] S3. The mixture is subjected to oxidative calcination to obtain calcined material; Preferably, the oxidative roasting temperature is ≥650℃, the holding time is 2.5~3.5h, the roasted material is alkali dissolved, and then solid-liquid separation is performed to obtain waste residue, wherein the WO3 in the waste residue is <9wt%; Specifically, the oxidation calcination time can be any one of 2.5h, 2.7h, 2.9h, 3.1h, 3.3h, and 3.5h, or a range between any two.

[0022] Further preferably, an oxygen-containing gas is continuously introduced into the mixture for oxidative roasting, wherein the oxygen-containing gas is a mixture of oxygen and nitrogen, and the oxygen content is not less than 5 vol%. More preferably, the oxygen content in the oxygen-containing gas is 5 vol% to 10 vol%. The roasted material is then alkali-dissolved and subjected to solid-liquid separation to obtain waste residue, wherein the WO3 content in the waste residue is less than 4 wt%. Specifically, the oxygen content in the oxygen-containing gas can be any one of 5 vol%, 6 vol%, 7 vol%, 8 vol%, 9 vol%, or 10 vol%, or a range between any two of them.

[0023] More preferably, the oxidative roasting adopts a segmented roasting process, specifically: first, the temperature is raised from room temperature to a first temperature at a rate of 5℃ / min and held for 30-50 min; then, the temperature is raised to a second temperature at a rate of 3℃ / min and held for 2-3 h, wherein the first temperature is 300-400℃ and the second temperature is ≥650℃; the roasted material is then alkali-dissolved and subjected to solid-liquid separation to obtain waste residue, wherein the WO3 content in the waste residue is <3.5wt%. Specifically, the first temperature can be any one or a range between any two of 300℃, 320℃, 340℃, 360℃, 380℃, and 400℃; the holding time at the first temperature can be any one or a range between any two of 30min, 34min, 38min, 42min, 46min, and 50min; and the holding time at the second temperature can be any one or a range between any two of 2h, 2.2h, 2.4h, 2.6h, 2.8h, and 3h.

[0024] More preferably, oxygen-containing gas is continuously introduced into the mixture for oxidative roasting, wherein the oxygen-containing gas is a mixture of oxygen and nitrogen, and the oxygen content is not less than 5 vol%. More preferably, the oxygen content in the oxygen-containing gas is 5 vol% to 10 vol%. The oxidative roasting adopts a segmented roasting process, specifically: the temperature is first raised from room temperature to a first temperature at a rate of 5 °C / min and held for 30 to 50 min, and then raised to a second temperature at a rate of 3 °C / min and held for 2 to 3 h, wherein the first temperature is 300 to 400 °C and the second temperature is ≥ 650 °C. The roasted material is then alkali-dissolved and then subjected to solid-liquid separation to obtain waste residue, wherein the WO3 in the waste residue is < 0.5 wt%.

[0025] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0026] Example 1

[0027] A method for improving the alkaline solubility of waste tungsten includes the following steps: S1. Sodium nitrate and sodium carbonate are mixed evenly to obtain a catalyst; wherein the nitrate content is 30 wt% and the carbonate content is 70 wt%. S2. Mix the waste tungsten material with the catalyst to obtain a mixture; wherein the amount of catalyst added is 7 wt% of the waste tungsten material. S3. The mixture is subjected to oxidative roasting to obtain roasted material. The oxidative roasting temperature is 850℃ and the oxidative roasting time is 3h. After cooling the roasted material, it is alkali dissolved and then solid-liquid separated to obtain waste residue. The waste residue contains 8.66wt% WO3.

[0028] Example 2

[0029] A method for improving the alkaline solubility of waste tungsten includes the following steps: S1. Sodium nitrate and sodium carbonate are mixed evenly to obtain a catalyst; wherein the nitrate content is 30 wt% and the carbonate content is 70 wt%. S2. Mix the waste tungsten material with the catalyst to obtain a mixture; wherein the amount of catalyst added is 7 wt% of the waste tungsten material. S3. Oxygen-containing gas is continuously introduced into the mixture for oxidative roasting to obtain roasted material. The oxidative roasting temperature is 650℃, and the oxidative roasting time is 2 hours. The oxygen content in the oxygen-containing gas is 5 vol%. After cooling, the roasted material is alkali-dissolved and then subjected to solid-liquid separation to obtain waste residue. The waste residue is found to contain 3.52 wt% WO3. Comparing this example with Example 1, it can be seen that, under the condition of adding a catalyst, introducing oxygen-containing gas to maintain a stable oxygen content in the roasting environment can improve the oxidation rate of tungsten, thereby improving the subsequent alkali dissolution rate.

[0030] Example 3

[0031] A method for improving the alkaline solubility of waste tungsten includes the following steps: S1. Sodium nitrate and sodium carbonate are mixed evenly to obtain a catalyst; wherein the nitrate content is 30 wt% and the carbonate content is 70 wt%. S2. Mix the waste tungsten material with the catalyst to obtain a mixture; wherein the amount of catalyst added is 7 wt% of the waste tungsten material. S3. The mixture is subjected to oxidative roasting to obtain roasted material. The oxidative roasting adopts a segmented roasting process. Specifically, the oxidative roasting is carried out by first raising the temperature from room temperature to a first temperature at a rate of 5°C / min and holding it at that temperature for 30 minutes, and then raising the temperature to a second temperature at a rate of 3°C / min and holding it at that temperature. The first temperature is 350°C, the second temperature is 650°C, and the holding time at the second temperature is 2 hours. After cooling the roasted material, it is subjected to alkali dissolution and then solid-liquid separation to obtain waste residue. The waste residue is found to contain 3.32 wt% WO3. Comparing this example with Example 1, it can be seen that under the condition of adding a catalyst, segmented roasting of waste tungsten material can improve the oxidation rate of tungsten, thereby improving the subsequent alkali dissolution rate.

[0032] Example 4

[0033] A method for improving the alkaline solubility of waste tungsten includes the following steps: S1. Sodium nitrate and sodium carbonate are mixed evenly to obtain a catalyst; wherein the nitrate content is 30 wt% and the carbonate content is 70 wt%. S2. Mix the waste tungsten material with the catalyst to obtain a mixture; wherein the amount of catalyst added is 5 wt% of the waste tungsten material. S3. Oxygen-containing gas is continuously introduced into the mixture for oxidative roasting to obtain roasted material. The oxidative roasting adopts a segmented roasting process. Specifically, the oxidative roasting is as follows: the temperature is first raised from room temperature to a first temperature at a rate of 5°C / min and held for 30 min, then raised to a second temperature at a rate of 3°C / min and held. The first temperature is 350°C, the second temperature is 650°C, the holding time at the second temperature is 2 h, and the oxygen content in the oxygen-containing gas is 5 vol%. After cooling, the roasted material is alkali-dissolved and then subjected to solid-liquid separation to obtain waste residue. The waste residue is found to contain 0.25 wt% WO3. Comparing this example with Example 2, it can be seen that under the conditions of adding a catalyst and maintaining a stable oxygen content in the roasting environment by introducing oxygen-containing gas, segmented roasting of waste tungsten material can further improve the oxidation rate of tungsten, thereby improving the subsequent alkali dissolution rate.

[0034] Example 5

[0035] A method for improving the alkaline solubility of waste tungsten includes the following steps: S1. Sodium nitrate and sodium carbonate are mixed evenly to obtain a catalyst; wherein the nitrate content is 40 wt% and the carbonate content is 60 wt%. S2. Mix the waste tungsten material with the catalyst to obtain a mixture; wherein the amount of catalyst added is 7 wt% of the waste tungsten material. S3. Oxygen-containing gas is continuously introduced into the mixture for oxidative roasting to obtain roasted material. The oxidative roasting adopts a segmented roasting process. Specifically, the oxidative roasting is carried out by first raising the temperature from room temperature to a first temperature at a rate of 5°C / min and holding it for 30 min, and then raising the temperature to a second temperature at a rate of 3°C / min and holding it for 3 min. The first temperature is 350°C, the second temperature is 650°C, the holding time at the second temperature is 3 h, and the oxygen content in the oxygen-containing gas is 10 vol%. After cooling the roasted material, it is alkali dissolved and then solid-liquid separated to obtain waste residue. The waste residue is found to contain 0.13 wt% WO3.

[0036] Example 6

[0037] A method for improving the alkaline solubility of waste tungsten includes the following steps: S1. Sodium nitrate and sodium carbonate are mixed evenly to obtain a catalyst; wherein the nitrate content is 35 wt% and the carbonate content is 65 wt%. S2. Mix the waste tungsten material with the catalyst to obtain a mixture; wherein the amount of catalyst added is 10 wt% of the waste tungsten material. S3. Oxygen-containing gas is continuously introduced into the mixture for oxidative roasting to obtain roasted material. The oxidative roasting adopts a segmented roasting process. Specifically, the oxidative roasting is carried out by first raising the temperature from room temperature to a first temperature at a rate of 5℃ / min and holding it for 30min, and then raising the temperature to a second temperature at a rate of 3℃ / min and holding it for 2.5h. The oxygen content in the oxygen-containing gas is 8 vol%. After cooling the roasted material, it is alkali dissolved and then solid-liquid separated to obtain waste residue. The waste residue is found to contain 0.22 wt% WO3.

[0038] Example 7

[0039] A method for improving the alkaline solubility of waste tungsten includes the following steps: S1. Sodium nitrate and sodium carbonate are mixed evenly to obtain a catalyst; wherein the nitrate content is 30 wt% and the carbonate content is 70 wt%. S2. Mix the waste tungsten material with the catalyst to obtain a mixture; wherein the amount of catalyst added is 7 wt% of the waste tungsten material. S3. Oxygen-containing gas is continuously introduced into the mixture for oxidative roasting to obtain roasted material. The oxidative roasting adopts a segmented roasting process. Specifically, the oxidative roasting is carried out by first raising the temperature from room temperature to a first temperature at a rate of 5°C / min and holding it for 30 min, and then raising the temperature to a second temperature at a rate of 3°C / min and holding it for 2 h. The first temperature is 450°C, the second temperature is 850°C, the holding time at the second temperature is 2 h, and the oxygen content in the oxygen-containing gas is 5 vol%. After cooling the roasted material, it is alkali dissolved and then solid-liquid separated to obtain waste residue. The waste residue is found to contain 0.24 wt% WO3.

[0040] Example 8

[0041] A method for improving the alkaline solubility of waste tungsten includes the following steps: S1. Sodium nitrate and sodium carbonate are mixed evenly to obtain a catalyst; wherein the nitrate content is 30 wt% and the carbonate content is 70 wt%. S2. Mix the waste tungsten material with the catalyst to obtain a mixture; wherein the amount of catalyst added is 7 wt% of the waste tungsten material. S3. Oxygen-containing gas is continuously introduced into the mixture for oxidative roasting to obtain roasted material. The oxidative roasting adopts a segmented roasting process. Specifically, the oxidative roasting is carried out by first raising the temperature from room temperature to a first temperature at a rate of 5°C / min and holding it for 30 min, and then raising the temperature to a second temperature at a rate of 3°C / min and holding it for 2 h. The first temperature is 350°C, the second temperature is 650°C, the holding time at the second temperature is 2 h, and the oxygen content in the oxygen-containing gas is 15 vol%. After cooling the roasted material, it is alkali dissolved and then solid-liquid separated to obtain waste residue. The waste residue is found to contain 0.1 wt% WO3.

[0042] Comparative Example 1 The difference from Example 1 is that steps S1 and S2 are omitted. The waste residue was found to contain 10.26 wt% WO3. Comparing this comparative example with Example 1 shows that without the addition of a catalyst, the oxidation rate and alkali solubility of tungsten decrease.

[0043] Comparative Example 2 The difference from Example 2 is that steps S1 and S2 are omitted. The waste residue was found to contain 8.24 wt% WO3. Comparing this comparative example with Example 2 shows that without adding a catalyst, simply introducing oxygen-containing gas to maintain a stable oxygen content in the roasting environment reduces the oxidation rate and alkali solubility of tungsten.

[0044] Comparative Example 3 The difference from Example 3 is that steps S1 and S2 are omitted. The waste residue was found to contain 8.91 wt% WO3. Comparing this comparative example with Example 3 shows that without adding a catalyst, simply performing staged roasting of the waste tungsten material reduces both the tungsten oxidation rate and the alkali solubility.

[0045] Comparative Example 4 The difference from Example 4 is that steps S1 and S2 are omitted. The waste residue was found to contain 3.27 wt% WO3. Comparing this comparative example with Example 4 shows that without adding a catalyst, maintaining a stable oxygen content in the roasting environment by introducing oxygen-containing gas, and by performing segmented roasting of the waste tungsten material, the oxidation rate and alkali solubility of tungsten are reduced.

[0046] Comparative Example 5 The difference from Example 4 is that in step S1, sulfate and phosphate are mixed evenly, wherein the sodium sulfate content is 35 wt% and the sodium phosphate content is 65 wt%. The waste residue was found to contain 11.38 wt% WO3. Comparing this comparative example with Example 4 shows that using a catalyst composed of non-nitrate and non-carbonate compounds reduces the oxidation rate and alkali solubility of tungsten.

[0047] The above description is merely a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural transformations made using the contents of the present invention under the inventive concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A method for improving the alkaline solubility of waste tungsten, characterized in that, Includes the following steps: S1. Mix nitrate and carbonate evenly to obtain a catalyst; S2. Mix the waste tungsten material with the catalyst to obtain a mixture; S3. The mixture is oxidized and roasted to obtain roasted material.

2. The method for improving the alkaline solubility of waste tungsten according to claim 1, characterized in that, In the catalyst, the nitrate content is 30wt%~40wt%, and the carbonate content is 60wt%~70wt%.

3. The method for improving the alkaline solubility of waste tungsten according to claim 1, characterized in that, The nitrates include any one or more combinations of sodium nitrate, potassium nitrate, and ammonium nitrate; the carbonates include any one or more combinations of sodium carbonate, potassium carbonate, and calcium carbonate.

4. The method for improving the alkaline solubility of waste tungsten according to claim 1, characterized in that, The waste tungsten material includes powdered material from tungsten alloy grinding and cutting or tungsten-containing contaminated material, wherein the tungsten-containing contaminated material includes contaminated material of tungsten powder or contaminated material of tungsten carbide.

5. The method for improving the alkaline solubility of waste tungsten according to claim 1, characterized in that, In step S2, the amount of catalyst added is 5wt% to 10wt% of the waste tungsten material.

6. The method for improving the alkaline solubility of waste tungsten according to claim 1, characterized in that, In step S3, the oxidation calcination temperature is ≥650℃ and the holding time is 2.5~3.5h.

7. The method for improving the alkaline solubility of waste tungsten according to claim 1, characterized in that, Step S3 further includes alkali dissolving the calcined material and then performing solid-liquid separation to obtain waste residue, wherein the waste residue contains WO3 < 9 wt%.

8. The method for improving the alkaline solubility of waste tungsten according to claim 1, characterized in that, Step S3 includes: continuously introducing oxygen-containing gas into the mixture for oxidative roasting, wherein the oxygen-containing gas is a mixture of oxygen and nitrogen, and the oxygen content is not less than 5 vol.

9. The method for improving the alkaline solubility of waste tungsten according to claim 1, characterized in that, In step S3, the oxidation calcination adopts a segmented calcination process, specifically: first, the temperature is raised from room temperature to a first temperature at a rate of 5℃ / min and held for 30~50min, and then the temperature is raised to a second temperature at a rate of 3℃ / min and held for 2~3h.

10. A method for improving the alkaline solubility of waste tungsten according to claim 9, characterized in that, In step S3, the first temperature is 300~400℃, and the second temperature is ≥650℃.