Preparation method of polymeric cyanamide low-toxicity gold leaching agent

Abstract

The invention discloses a preparation method of a polymeric cyanamide low-toxicity gold leaching agent, and belongs to the field of hydrometallurgy. The preparation method comprises the following steps: uniformly mixing urea, sodium carbonate and ferrous chloride and finely grinding; then putting the fine grinding material into a high-temperature heating container, gradually raising the temperature, and preserving the heat; gradually cooling the reaction product to room temperature along with the furnace; and finally, a reaction product is crushed and ground into powder, and the polymeric cyanamide low-toxicity gold leaching agent is obtained. The synthetic raw materials of the gold leaching agent are simple, cheap and easy to obtain, the synthetic process is simple and easy to regulate and control, and the synthetic cost is low. The gold leaching agent obtained through the method is low in toxicity, the gold leaching speed is high, the gold leaching rate is high, and leaching tailings are common solid waste and are easier to treat.

Description

Technical Field

[0001] The present invention relates to a method for preparing a low-toxic gold leaching agent, in particular to a method for preparing a polycyanamide low-toxic gold leaching agent, and belongs to the field of hydrometallurgy. Background Art

[0002] Due to its advantages such as simple process, low production cost and high gold leaching rate, cyanide has always been the mainstream technology for gold extraction. However, cyanide is extremely toxic and its use poses huge environmental safety risks. Therefore, the development of green and environmentally friendly non-toxic / low-toxic gold leaching agents is of great significance to the green and sustainable development of the gold industry. However, the non-toxic gold leaching agents such as thiosulfate, thiourea, thiocyanate, and glycine, which are widely studied at home and abroad, have rarely been reported in industrial applications due to their low gold leaching rate, high reagent consumption, and difficulty in recovering gold from the leachate.

[0003] In recent years, a series of new low-toxic gold leaching agents have gradually emerged in my country's hydrometallurgical market, and some of them have been successfully applied in industry at home and abroad. According to literature reports, these new gold leaching agents are mainly obtained by high-temperature roasting of a variety of chemical reagents, and their toxicity is lower than that of sodium cyanide, but their specific formula and synthesis conditions have not yet been disclosed.

[0004] At the same time, several domestic invention patents have disclosed the synthesis and application of some new gold leaching agent products. For example, the invention patent application with publication number CN111304456A discloses "an environmentally friendly gold leaching agent and its preparation method". The patent uses sodium carbonate, urea, sodium chloride, sodium sulfide, iodine, potassium ferrocyanide, sodium thiocyanate, sodium bromide and sodium thiosulfate as raw materials, and heats for 1 to 5 hours at 700 to 800 ° C in a closed container to obtain a low-toxic gold leaching agent. However, the patent has too many types of synthetic raw materials, resulting in a complex synthesis reaction process that is difficult to control, and the synthesis temperature is high and the insulation time is long, resulting in excessively high synthesis costs.

[0005] For another example, the invention patent application with publication number CN116640922A discloses “an improved powdered gold leaching agent and its preparation method”, which uses urea, sodium carbonate, sodium sulfate, activated carbon, iron catalyst, sodium tetrapolyphosphate, sodium citrate and sodium lignin sulfonate as raw materials. First, urea and sodium carbonate are mixed and added to the reactor, and then the temperature is raised to 120°C, and activated carbon and iron catalyst are added. After reacting for 20 to 30 minutes, the temperature is raised to 150 to 200°C, and the reaction is continued for 20 to 30 minutes to obtain an intermediate product; then, sodium sulfate is added to the intermediate product after keeping it warm for 30-50 minutes, and the mixture is mixed and heated to 300 to 350°C, and the reaction is kept warm for 0.5 to 1 hour to obtain a first-order reactant; then, the first-order reactant is heated to 450 to 500°C, and the reaction is kept warm for 1 to 1.5 hours to obtain a second-order reactant; then, the second-order reactant is cooled to 200°C, kept warm for 20 to 30 minutes, and the reaction is continued for 1 hour. min, add sodium tetrapolyphosphate and sodium citrate, mix evenly, and cool to room temperature to obtain a semi-finished product; finally, crush the semi-finished product into powder, add sodium lignin sulfonate and mix evenly to obtain a gold leaching agent. Obviously, this method not only has too many types of synthetic raw materials, but also requires five-stage roasting, and the synthesis process is very complicated, which is difficult to promote and apply.

[0006] For example, the invention patent application with publication number CN106399712A discloses "a low-toxic and environmentally friendly gold dressing agent and its preparation method", which uses urea, sodium carbonate, ferrous salt, catalyst (activated carbon, nickel catalyst, iron catalyst), sodium cyanate, sodium bicarbonate, sodium citrate, thiourea and sodium thiosulfate as raw materials. First, urea, sodium carbonate, ferrous salt, catalyst and sodium cyanate are mixed and added to the reaction container. After the temperature is raised to the melting point of all the materials in the reaction container, sodium cyanate is added and kept at 700-850°C for 3-5 hours. Then, the step roasting product is filtered to remove impurities, introduced into the receiving tray, cooled, and then sodium bicarbonate, sodium citrate, thiourea and sodium thiosulfate are added to obtain the gold leaching agent. Similarly, the method has complex types of synthetic raw materials, which makes the roasting process difficult to control, and the insulation temperature is high and the time is long, so the synthesis cost is too high. In addition, the gold leaching agent is used in a high amount when used for stirring and leaching of gold concentrate.

[0007] For example, the patent application with publication number CN102121067A discloses a "harmless mineral processing additive and its preparation method and application", which uses sodium hydroxide, sodium carbonate, urea, and sodium ferrocyanide as raw materials, and heats for 1 to 2 hours at 600 to 1000 ° C to obtain the gold leaching agent. The raw materials and synthesis process used in this method are relatively simple, but the ferrocyanide in the raw material is expensive, and it will decompose to produce highly toxic sodium cyanide at the heat preservation temperature, resulting in the prepared gold leaching agent with high toxicity. In addition, the roasting temperature can reach up to 1000 ° C, so the synthesis cost is relatively high.

[0008] In summary, the gold leaching agent synthesis methods reported in the prior art have technical problems such as a wide variety of raw materials, complex synthesis process, high cost, high toxicity of the synthesized gold leaching agent, and large dosage. Summary of the invention

[0009] The technical problem to be solved by the present invention is to provide a method for preparing a polycyanamide low-toxic gold leaching agent. The prepared gold leaching agent has low toxicity, small dosage, good gold leaching effect, and the raw materials used in the preparation are simple in type, cheap and easy to obtain, the synthesis process is simple, and the cost is low.

[0010] In order to achieve the above object, the present invention provides the following technical solutions: A method for preparing a polycyanamide low-toxic gold leaching agent is prepared according to the following steps: (1) Mixing and fine grinding: Mix the raw materials urea, sodium carbonate and ferrous chloride evenly and finely grind them; (2) Calcination: Place the fine abrasive in a high-temperature heating container, gradually increase the temperature, and then keep it warm; (3) Furnace cooling: After the roasting is completed, the reaction product is gradually cooled to room temperature in the furnace; (4) The reaction product is crushed and ground into powder to obtain a polycyanamide low-toxic gold leaching agent.

[0011] In step (1), the mass ratio of the urea, sodium carbonate and ferrous chloride is preferably 100: (33-135): (16-67), and more preferably 3:2:1.

[0012] In step (2), the calcination process conditions are preferably as follows: the fine abrasive is heated along with the furnace, the heating rate is 5-20°C / min, the holding temperature is 300-500°C, the holding time is 5-30 min, and the calcination atmosphere is N2:O2 volume ratio (4-10):1.

[0013] The heating rate is more preferably 5 to 10°C / min; the holding temperature is more preferably 400°C; and the holding time is more preferably 10 min.

[0014] In step (3), the furnace cooling process conditions are preferably: a cooling rate of 5 to 20 °C / min, and a cooling atmosphere of N2:O2 volume ratio of 4:1 to 8:1.

[0015] The cooling atmosphere is further preferably N2:O2 volume ratio of 4:1.

[0016] Preferably, the fine grinding particle size of the mixture in step (1) and the grinding particle size in step (4) are both -200 mesh, accounting for 70 to 90%.

[0017] The mechanism of the present invention is as follows:

[0018] A low-toxic polycyanamide gold leaching agent is prepared in a short process at low cost using urea, sodium carbonate and ferrous chloride as raw materials. Urea and sodium carbonate are reactants, and ferrous chloride is a catalyst. Urea and sodium carbonate react at a relatively low temperature to generate sodium cyanate under the catalytic action of ferrous chloride. As the insulation temperature increases, sodium cyanate undergoes a polymerization reaction under the catalytic action of ferrous chloride to generate a polycyanamide gold leaching agent with a triazine structure. In this structure, the cyanide group is fixed in a stable six-membered ring molecular skeleton, and it is difficult to dissociate into free cyanide ions, so its toxicity is low. The lone pair of electrons of the active atom N in the cyanide group on the skeleton can form a bond with gold ions, thereby effectively leaching gold. During the roasting process, part of the divalent iron ions are oxidized to trivalent iron ions. During gold leaching, the low-concentration free cyanide ions dissociated from the polycyanamide gold leaching agent will coordinate with the divalent and trivalent iron ions to form a stable and reversible redox couple Fe(CN)6 3- / Fe(CN)6 4- , catalyzed the dissolution reaction of gold, significantly shortened the leaching time, and increased the gold leaching rate.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] (1) The raw materials urea, sodium carbonate and ferrous chloride used in the present invention are all common agents on the market, with wide sources, non-toxic and cheap. Moreover, the polycyanamide low-toxic gold leaching agent can be synthesized by a simple low-temperature roasting step, the process is simple, the process is easy to control, and it is easy to promote and apply.

[0021] (2) The present invention uses ferrous chloride as a catalyst to catalyze the formation process of the intermediate product sodium cyanate and the final product polycyanamide low-toxic gold leaching agent, effectively reducing the reaction temperature and shortening the roasting time, thereby significantly reducing energy consumption and significantly reducing the synthesis cost. In addition, by controlling the heating / cooling rate and roasting atmosphere, it is avoided that the temperature and atmosphere change too much to cause the reaction to be too intense, causing the product to agglomerate and decompose and deteriorate, and destroying the microstructure of the material, thereby effectively ensuring the product yield and performance.

[0022] (3) The toxicity of the polycyanamide gold leaching agent obtained by the present invention is far lower than that of sodium cyanide, and the leaching tailings are general industrial solid waste, and the disposal cost will be greatly reduced. Moreover, in the leaching process, Fe(CN)6 3- / Fe(CN)6 4- The catalytic effect on gold leaching significantly increases the gold leaching rate. Under the condition of ensuring the gold leaching rate, the leaching time and the amount of reagents used are significantly reduced, thereby reducing the gold leaching cost. DETAILED DESCRIPTION

[0023] The present invention is further described below in conjunction with examples and experimental data.

[0024] The chemical composition (wt.%) of a gold-containing oxide ore is shown in Table 1. The following gold leaching experiments were all conducted using this gold ore.

[0025] Table 1 Chemical element analysis of gold-containing oxide ore / %

[0026] Note: *Unit: g / t.

[0027] Example 1 Weigh urea, sodium carbonate and ferrous chloride according to Table 2, mix them evenly and grind them with a mortar to -200 mesh, accounting for 80%. Place the finely ground material in a porcelain boat, then place it in a muffle furnace and heat it up with the furnace. The heating rate is 10 ℃ / min. When the temperature rises to 400 ℃, keep it at this temperature for 10 min. The atmosphere during the heating and insulation process is N2: O2 volume ratio 4:1. After the reaction is completed, the synthesized gold leaching agent is gradually cooled to room temperature at a cooling rate of 10 ℃ / min and an atmosphere of N2: O2 volume ratio 4:1. Finally, take it out and crush it to -200 mesh, accounting for 80%. The synthetic gold leaching agent was used to leach the gold ore in Table 1. The leaching test conditions are: liquid-solid ratio 2:1, low-toxic gold leaching agent dosage 0.05 wt.%, pH 12, and leaching time 24 h.

[0028] The influence of raw material ratio on the gold leaching effect of the synthetic low-toxic agent is shown in Table 2.

[0029] Table 2 Effect of raw material ratio on gold leaching effect of synthetic low-toxic reagent

[0030] As shown in Table 2, the raw material ratio has a great influence on the gold leaching rate. When the dosage of urea, sodium carbonate and ferrous chloride is 1g, 2g and 1g respectively, the gold leaching rate is only 76.5%, and the free cyanide concentration in the leachate is as low as 8.6 mg / L. With the increase of urea dosage, the gold leaching rate gradually increases. After the urea dosage exceeds 3g, the gold leaching rate begins to decrease. When the urea dosage is constant, with the increase of sodium carbonate and ferrous chloride dosage, the gold leaching rate and free cyanide concentration both show a pattern of first increasing and then decreasing. The most suitable mass ratio of urea, sodium carbonate and ferrous chloride is 3:2:1. Under this condition, the gold leaching rate reaches 97.3%, and the free cyanide concentration in the leachate is 18.2mg / L.

[0031] Example 2 Weigh urea, sodium carbonate and ferrous chloride in a mass ratio of 3:2:1, mix them evenly and grind them with a mortar to -200 mesh, accounting for 80%. Place the finely ground material in a porcelain boat, then place it in a muffle furnace and heat it up with the furnace. The heating rate is 10 ℃ / min. When the furnace temperature rises to the temperature shown in Table 3, keep it at this temperature for 10 min. The atmosphere during the heating and insulation process is N2: O2 volume ratio 4:1. After the reaction is completed, the synthesized gold leaching agent is gradually cooled to room temperature at a cooling rate of 10 ℃ / min and an atmosphere of N2: O2 volume ratio 4:1. Finally, take it out and crush it to -200 mesh, accounting for 80%. The synthetic gold leaching agent was used to leach the gold ore in Table 1. The leaching test conditions are: liquid-solid ratio 2:1, low-toxic gold leaching agent dosage 0.05 wt.%, pH 12, and leaching time 24 h.

[0032] The effect of holding temperature on the gold leaching effect of the synthetic low-toxic agent is shown in Table 3.

[0033] Table 3 Effect of holding temperature on the gold leaching effect of synthetic low-toxic reagent

[0034] As shown in Table 3, the holding temperature has a great influence on the gold leaching rate. When the holding temperature is 200 °C, the gold leaching rate is only 66.2%, and the free cyanide concentration in the leaching solution is only 5.3 mg / L. As the holding temperature increases, the gold leaching rate and the free cyanide concentration in the leaching solution increase rapidly. When the holding temperature is 400 °C, the gold leaching rate and the free cyanide concentration reach the maximum values ​​of 97.3% and 18.2 mg / L, respectively. When the holding temperature is further increased, the gold leaching rate and the free cyanide concentration gradually decrease.

[0035] Example 3 Weigh urea, sodium carbonate and ferrous chloride in a mass ratio of 3:2:1, mix them evenly and grind them with a mortar to -200 mesh, accounting for 80%. Place the finely ground material in a porcelain boat, then place it in a muffle furnace and heat it up with the furnace. The heating rate is 10 ℃ / min. When the furnace temperature rises to 400 ℃, keep it at this temperature for a period of time (as shown in Table 4). The atmosphere during the heating and insulation process is N2: O2 volume ratio 4:1. After the reaction is completed, the synthesized gold leaching agent is gradually cooled to room temperature at a cooling rate of 10 ℃ / min and an atmosphere of N2: O2 volume ratio 4:1. Finally, take it out and crush it to -200 mesh, accounting for 80%. The synthetic gold leaching agent was used to leach the gold ore in Table 1. The leaching test conditions are: liquid-solid ratio 2:1, low-toxic gold leaching agent dosage 0.05wt.%, pH 12, and leaching time 24 h.

[0036] The effect of holding time on the gold leaching effect of the synthetic low-toxic agent is shown in Table 4.

[0037] Table 4 Effect of holding time on gold leaching effect of synthetic low-toxic reagent

[0038] As shown in Table 4, the holding time has little effect on the gold leaching rate. At 5 min of roasting, the gold leaching rate reached 92.6%. As the holding time increased to 10 min, the gold leaching rate increased to 97.3%. Therefore, the synthesis reaction proceeded very quickly. As the holding time continued to be extended, the gold leaching rate showed a downward trend.

[0039] Example 4

[0040] Weigh urea, sodium carbonate and ferrous chloride in a mass ratio of 3:2:1, mix them evenly and grind them with a mortar to -200 mesh, accounting for 80%. Place the finely ground material in a porcelain boat, then place it in a muffle furnace and heat it up with the furnace. The heating rate is 10 ℃ / min. When the furnace temperature rises to 400 ℃, keep it at this temperature for 10 min. The atmosphere during the heating and insulation process is shown in Table 5. After the reaction is completed, the synthesized gold leaching agent is gradually cooled to room temperature at a cooling rate of 10 ℃ / min and the roasting atmosphere shown in Table 5. Finally, take it out and crush it to -200 mesh, accounting for 80%. The synthetic gold leaching agent was used to leach the gold ore in Table 1. The leaching test conditions are: liquid-solid ratio 2:1, low-toxic gold leaching agent dosage 0.05 wt.%, pH 12, and leaching time 24 h.

[0041] The effect of roasting atmosphere on the gold leaching effect of the synthesized low-toxic agent is shown in Table 5.

[0042] Table 5 Effect of roasting atmosphere on the gold leaching effect of synthetic low-toxic reagent

[0043] As shown in Table 5, the roasting atmosphere has a great influence on the gold leaching rate. When the volume ratio of N2: O2 is 2:1, the gold leaching rate is only 42.5%, and the free cyanide concentration in the leachate is also very low, only 2.1 mg / L. As the volume ratio of N2: O2 increases to 4:1, the gold leaching rate increases rapidly to 97.3%, and the free cyanide concentration in the leachate also increases to 18.2 mg / L. Further increasing the volume ratio of N2: O2 does not significantly increase the gold leaching rate.

[0044] Example 5 Weigh urea, sodium carbonate and ferrous chloride in a mass ratio of 3:2:1, mix them evenly and grind them with a mortar to -200 mesh, accounting for 80%. Place the finely ground material in a porcelain boat, then place it in a muffle furnace and heat it up with the furnace. Adjust the heating rate (see Table 6). When the furnace temperature rises to 400 ℃, keep it at this temperature for 10 min. The atmosphere during the heating and insulation process is N2: O2 volume ratio 4:1. After the reaction is completed, the synthesized gold leaching agent is gradually cooled to room temperature at a cooling rate of 10 ℃ / min and an atmosphere of N2: O2 volume ratio 4:1. Finally, take it out and crush it to -200 mesh, accounting for 80%. The synthetic gold leaching agent was used to leach the gold ore in Table 1. The leaching test conditions are: liquid-solid ratio 2:1, low-toxic gold leaching agent dosage 0.05 wt.%, pH 12, and leaching time 24 h.

[0045] The effect of heating rate on the gold leaching effect of the synthesized low-toxic agent is shown in Table 6.

[0046] Table 6 Effect of heating rate on gold leaching effect of synthetic low-toxic reagent

[0047] As shown in Table 6, when the heating rate is 5 ℃ / min, the gold leaching rate quickly reaches 98.2%, and the free cyanide concentration in the leachate also reaches 20.1 mg / L. When the heating rate increases to 10 ℃ / min, the gold leaching rate slightly decreases to 97.3%, and the free cyanide concentration in the leachate is 18.2 mg / L. As the heating rate increases to 15 ℃ / min and 20 ℃ / min, the gold leaching rate decreases to 89.1% and 86.3%, respectively, and the free cyanide concentration in the leachate also decreases rapidly. Therefore, a too fast heating rate is not conducive to the synthesis of low-toxicity agents.

[0048] Example 6 Weigh urea, sodium carbonate and ferrous chloride in a mass ratio of 3:2:1, mix them evenly and grind them with a mortar to -200 mesh, accounting for 80%. Place the finely ground material in a porcelain boat, then place it in a muffle furnace and heat it up with the furnace. The heating rate is 10 ℃ / min. When the furnace temperature rises to 400 ℃, keep it at this temperature for 10 min. The atmosphere during the heating and insulation process is N2: O2 volume ratio 4:1. After the reaction is completed, the synthesized gold leaching agent is cooled to room temperature by placing it in the air for natural cooling and gradually cooling under controlled cooling rate conditions (see Table 6). Finally, take it out and crush it to -200 mesh, accounting for 80%. The synthetic gold leaching agent was used to leach the gold ore in Table 1. The leaching test conditions are: liquid-solid ratio 2:1, low-toxic gold leaching agent dosage 0.05 wt.%, pH 12, and leaching time 24 h.

[0049] The effect of the cooling method of the roasted product on the gold leaching effect of the synthesized low-toxic agent is shown in Table 7.

[0050] Table 7 Effect of cooling method of roasted product on gold leaching effect of synthetic low-toxic reagent

[0051] As shown in Table 7, the cooling method of the roasted product has a great influence on the gold leaching effect of the synthesized low-toxic agent. The low-toxic agent gold leaching rate obtained by directly taking out the roasted product and cooling it naturally in the air is only 78.5%. However, by gradually cooling with the furnace, the low-toxic agent gold leaching rate is as high as 98.2% at a cooling rate of 5 ℃ / min. As the cooling rate gradually increases, the gold leaching rate decreases, but it is more than 90%. Therefore, the gradual cooling with the furnace is conducive to obtaining a good gold leaching effect. This may be because the gradual cooling with the furnace avoids the destruction of the microstructure of the product during rapid cooling, thereby ensuring its yield and performance.

[0052] Example 7 Weigh urea, sodium carbonate and ferrous chloride in a mass ratio of 3:2:1, mix them evenly and grind them with a mortar to -200 mesh, accounting for 80%. Place the finely ground material in a porcelain boat, then place it in a muffle furnace and heat it up with the furnace. The heating rate is 10 ℃ / min. When the furnace temperature rises to 400 ℃, keep it at this temperature for 10 min. The atmosphere during the heating and insulation process is N2: O2 volume ratio 4:1. After the reaction is completed, the synthesized gold leaching agent is gradually cooled to room temperature at a cooling rate of 10 ℃ / min and a roasting atmosphere of N2: O2 volume ratio 4:1. Finally, take it out and crush it to -200 mesh, accounting for 80%. The synthetic gold leaching agent and sodium cyanide were used to leach the gold ore in Table 1, and the differences in gold leaching rate and free cyanide concentration under different agent dosages and leaching time conditions were investigated. The other leaching test conditions are: liquid-solid ratio 2:1, pH 12.

[0053] The gold leaching rate and free cyanide concentration under different low-toxic gold leaching agents, sodium cyanide dosages and leaching time conditions are shown in Table 8.

[0054] Table 8 Comparison of gold leaching effects of synthetic low-toxic reagents and sodium cyanide

[0055] It can be seen from Table 8 that when the amount of the leaching agent is 0.01%, the leaching rate of the low-toxic leaching agent reaches 83.2% after leaching for 6 hours, which is significantly higher than the leaching rate of 72.9% of sodium cyanide. With the increase of leaching time, the leaching rates of both leaching agents increase. After leaching for 24 hours, the leaching rate of the low-toxic leaching agent reaches 90.8%, while the leaching rate of sodium cyanide is only 85.6%, which is basically equivalent to the leaching rate of 86.5% of the low-toxic leaching agent for 12 hours. Similarly, under the same leaching agent dosage and leaching time conditions, the free cyanide concentration in the leaching solution of the low-toxic leaching agent is significantly lower. The leaching rate and free cyanide concentration under other leaching agent dosage and leaching time conditions also basically follow the above rules. Therefore, compared with sodium cyanide, the low-toxic leaching agent synthesized by the present invention has lower toxicity, faster leaching rate and higher leaching rate.

Claims

1. A method for preparing a polycyanamide low-toxic gold leaching agent, characterized in that Prepare according to the following steps: (1) Mixing and fine grinding: Mix the raw materials urea, sodium carbonate and ferrous chloride evenly and finely grind them; (2) Calcination: Place the fine abrasive in a high-temperature heating container, gradually increase the temperature, and then keep it warm; (3) Furnace cooling: After the roasting is completed, the reaction product is gradually cooled to room temperature in the furnace; (4) The reaction product is crushed and ground into powder to obtain a polycyanamide low-toxic gold leaching agent.

2. The preparation method according to claim 1, characterized in that: In step (1), the mass ratio of the materials urea, sodium carbonate and ferrous chloride is 100: (33-135): (16-67).

3. The preparation method according to claim 2, characterized in that: In step (1), the mass ratio of urea, sodium carbonate and ferrous chloride is 3:2:

1.

4. The preparation method according to claim 1, characterized in that In step (2), the calcination process conditions are as follows: the fine abrasive is heated along with the furnace, the heating rate is 5-20°C / min, the holding temperature is 300-500°C, the holding time is 5-30 min, and the calcination atmosphere is N2:O2 volume ratio (4-10):

1.

5. The preparation method according to claim 4, characterized in that: The heating rate is 5-10 ℃ / min; the holding temperature is 400 ℃ and the holding time is 10 min.

6. The preparation method according to claim 1, characterized in that: In step (3), the furnace cooling process conditions are: cooling rate is 5 to 20 °C / min, and cooling atmosphere is N2: O2 volume ratio of 4:1 to 8:

1.

7. The preparation method according to claim 6, characterized in that: The cooling atmosphere was N2:O2 in a volume ratio of 4:

1.

8. The preparation method according to any one of claims 1 to 7, characterized in that: The fine grinding particle size of the mixture in step (1) and the grinding particle size in step (4) are both -200 mesh, accounting for 70 to 90%.

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