A low-phosphorus cold-bonded pellet for vanadium extraction, its preparation method and vanadium extraction method

Abstract

The present invention discloses a preparation method of low-phosphorus cold-bonded pellets for vanadium extraction. The method includes uniformly mixing iron scale, iron ore powder, dedusted ash, and binder, pelletizing the mixture to obtain green pellets, and then drying to obtain low-phosphorus cold-bonded pellets for vanadium extraction. Based on the total mass of the mixture, by mass percentage, the dosage of iron scale is 30-59%, the dosage of iron ore powder is 19-40%, the dosage of dedusted ash is 15-40%, and the dosage of binder is 1-5%. The dedusted ash is dry dedusted ash from a vanadium extraction converter, wherein the phosphorus content of the dedusted ash < 0.05 wt%, and the CaO content < 1 wt%. The present invention also discloses a low-phosphorus cold-bonded pellet for vanadium extraction and a method for vanadium extraction using the low-phosphorus cold-bonded pellet for vanadium extraction. The present invention solves the problems that in the production of vanadium extraction from converters, the phosphorus in vanadium slag is high, resulting in difficulties in the subsequent treatment of vanadium slag and the need to add a phosphorus removal process, and the degradation of products.

Description

Technical Field

[0001] The present invention belongs to the field of hot metal pretreatment in iron and steel smelting, and particularly relates to a low-phosphorus cold solid ball for vanadium extraction, a preparation method thereof, and a vanadium extraction method. Background Art

[0002] Vanadium is known as the "monosodium glutamate in steel". Adding a certain amount of ferrovanadium alloy or vanadium nitride alloy to steel can refine the grains and improve the strength and toughness of the steel. At present, ferrovanadium alloy or vanadium nitride alloy is mainly produced from vanadium-containing hot metal to produce vanadium slag, and the vanadium slag is obtained by oxidative roasting, precipitation reduction, and smelting to obtain ferrovanadium alloy or vanadium nitride alloy.

[0003] Since vanadium slag is smelted from hot metal containing iron, the hot metal inevitably contains a high amount of phosphorus, and a large amount of phosphorus is contained in the vanadium extraction coolant used. Usually, the phosphorus content in vanadium slag reaches more than 0.07%. In the subsequent sodium roasting-water leaching vanadium production process, it is necessary to remove phosphorus from the leaching solution to ensure that the phosphorus in the vanadium product meets the product requirements. In the calcification roasting-acid leaching process, there is no effective phosphorus removal measure, so it often leads to excessive phosphorus in the vanadium product. In some extreme cases, it will lead to difficulty in leaching precipitation and production interruption.

[0004] Therefore, controlling the phosphorus content in vanadium slag and producing low-phosphorus vanadium slag can not only effectively control the phosphorus content of the final vanadium product, but also reduce the phosphorus removal process link in the sodium roasting-water leaching vanadium production process and simplify the process flow.

[0005] In summary, how to produce low-phosphorus vanadium slag efficiently and simply is an urgent problem to be solved. Summary of the Invention

[0006] To solve the above technical problems, an embodiment of the present invention provides a low-phosphorus cold solid ball for vanadium extraction, a preparation method thereof, and a vanadium extraction method. This method solves the problems of high phosphorus in vanadium slag during converter vanadium extraction production, which leads to difficulties in vanadium slag treatment in subsequent processes and product degradation.

[0007] A preparation method of a low-phosphorus cold solid ball for vanadium extraction disclosed in an embodiment of the present invention includes mixing iron scale, iron ore powder, dedusting ash, and binder evenly, pressing the mixture to make green balls, and then drying to obtain the low-phosphorus cold solid ball for vanadium extraction;

[0008] Based on the total mass of the mixture, in terms of mass percentage, the dosage of the iron scale is 30-59%, the dosage of the iron ore powder is 19-40%, the dosage of the dedusting ash is 15-40%, and the dosage of the binder is 1-5%;

[0009] The dedusting ash is dry dedusting ash from a vanadium extraction converter. Among them, the phosphorus content of the dedusting ash <0.05wt%, and the CaO content of the dedusting ash <1wt%.

[0010] Furthermore, the phosphorus content of the mill scale is less than or equal to 0.02 wt%.

[0011] Furthermore, the mill scale is one or more of continuous casting mill scale, hot rolling mill scale and cold rolling mill scale.

[0012] Furthermore, the iron ore powder is low-phosphorus iron ore powder, wherein the phosphorus content < 0.03 wt% and the CaO content < 2 wt%.

[0013] Furthermore, the mass percentage content of the chemical components of the low-phosphorus cold bonded pellets for vanadium extraction includes: TFe 55 - 75%, CaO 0.5 - 2%, SiO2 2 - 10%, P < 0.04%, H2O < 3%.

[0014] Furthermore, the binder is organic starch made from biomass.

[0015] An embodiment of the present invention also discloses a low-phosphorus cold bonded pellet for vanadium extraction, which is prepared by the above-mentioned preparation method, and the mass percentage of phosphorus in the low-phosphorus cold bonded pellet for vanadium extraction is < 0.04%.

[0016] An embodiment of the present invention also discloses a vanadium extraction method, which includes blowing oxygen for smelting the vanadium-bearing hot metal. During the smelting process, pig iron blocks and low-phosphorus cold bonded pellets for vanadium extraction are added to the vanadium-bearing hot metal to control the temperature of the vanadium extraction bath; wherein, the low-phosphorus cold bonded pellet for vanadium extraction is the above-mentioned low-phosphorus cold bonded pellet for vanadium extraction.

[0017] Furthermore, before blowing oxygen for smelting the vanadium-bearing hot metal, the blast furnace slag on the surface of the vanadium-bearing hot metal is removed.

[0018] Furthermore, the addition amount of pig iron is 5 - 30 kg / ton of iron, and the addition amount of low-phosphorus cold bonded pellets for vanadium extraction is 10 - 60 kg / ton of iron; the blowing oxygen smelting conditions include an oxygen supply flow rate of 18000 - 26000 m 3 / h, an oxygen supply pressure of 0.7 - 1 MPa, a blowing oxygen smelting time of 4 - 8 min, and the temperature is controlled at 1330 - 1400 °C.

[0019] Adopting the above technical solution, the present invention has at least the following beneficial effects:

[0020] The low-phosphorus cold bonded pellet for vanadium extraction of the present invention uses mill scale, iron ore powder, dust removal ash and binder as raw materials, and restricts the phosphorus content, calcium oxide content and relative proportion in the raw materials, so that the phosphorus content of the low-phosphorus cold bonded pellet for vanadium extraction is maintained at a relatively low value. Furthermore, when the low-phosphorus cold bonded pellet for vanadium extraction is used as a coolant for vanadium extraction subsequently, it will not introduce excessive phosphorus, solving the problems that the phosphorus in the vanadium slag is high during the production of vanadium extraction in the converter, resulting in difficulties in the treatment of vanadium slag in the subsequent process and the need to add a phosphorus removal process, and the downgrading of the product. Detailed implementation mode

[0021] To make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the following further elaborates on the embodiments of the present invention in detail in conjunction with specific embodiments.

[0022] A preparation method of a low-phosphorus cold-bound pellet for vanadium extraction provided by an embodiment of the present invention includes uniformly mixing mill scale, iron ore powder, dedusting ash and binder, pelletizing the mixture to obtain green pellets, and then drying to obtain the low-phosphorus cold-bound pellet for vanadium extraction; specifically, the mill scale, iron ore powder and dedusting ash are added to a mixer in proportion and a binder is added for mixing. After sufficient mixing, it enters a pair-roll pellet press for pelletizing to obtain green pellets. The green pellets are naturally air-dried and sun-dried for 24 - 48 hours to obtain finished pellets, which are then taken into a storage bin for standby. In another preferred embodiment, the drying method can also be heating and drying, with the conditions being: temperature 110°C - 180°C, time 6 - 8 hours.

[0023] Based on the total mass of the mixture, the dosage of mill scale is 30 - 59wt%, preferably 45 - 56wt%; the dosage of iron ore powder is 19 - 40wt%, preferably 25 - 30wt%; the dosage of dedusting ash is 15 - 40wt%, preferably 15 - 25wt%; the dosage of binder is 1 - 5wt%, preferably 2 - 4wt%.

[0024] Mill scale, iron ore powder and dedusting ash all use low-phosphorus components, and the content of CaO in iron ore powder and dedusting ash is strictly controlled. Excessive CaO will cause a dephosphorization reaction during the vanadium extraction process, and its reaction equation is 4CaO + 2[P] + 5(FeO) → 5[Fe] + 4CaO·P2O5.

[0025] Specifically:

[0026] The mill scale is one or more of continuous casting mill scale, hot rolling mill scale and cold rolling mill scale. The phosphorus content of the mill scale is less than or equal to 0.02wt%.

[0027] The iron ore powder is a low-phosphorus iron ore powder with an iron grade of more than 50, wherein the phosphorus content < 0.03wt% and the CaO content < 2wt%.

[0028] The dedusting ash is dry dedusting ash from a vanadium extraction converter. Among them, the phosphorus content of the dedusting ash < 0.05wt% and the CaO content of the dedusting ash < 1wt%. The mass percentage content of other components of the dedusting ash is: TFe 55 - 70%, FeO 50 - 60%, SiO2 1 - 3%.

[0029] According to the preparation method provided by the present invention, the binder not only needs to bond the mill scale, iron ore powder and dedusted ash together for pelletizing, but also needs to use substances with low CaO or no CaO to minimize the introduction of CaO, so as to accurately control the CaO content.

[0030] Specifically, the binder is organic starch with biomass as the raw material. Preferably, it is organic starch with corn as the raw material.

[0031] According to the preparation method provided by the present invention, the mass percentage content of the low-phosphorus cold-bound pellets for vanadium extraction prepared includes: TFe 55 - 75%, CaO 0.5 - 2%, SiO2 2 - 10%, P < 0.04%, H2O < 3%. The average particle size of the low-phosphorus cold-bound pellets for vanadium extraction can be selected within a relatively wide range. Preferably, the average particle size of the low-phosphorus cold-bound pellets for vanadium extraction can be 10 - 50 mm. By controlling the average particle size of the pellets within the above range and not being too fine, it is to prevent the dedusting fan from blowing away or sucking away the cold-bound pellets.

[0032] The present invention also provides a vanadium extraction method in an embodiment. The method includes blowing oxygen for smelting the vanadium-bearing hot metal. During the smelting process, pig iron blocks and low-phosphorus cold-bound pellets for vanadium extraction are added to the vanadium-bearing hot metal to control the temperature of the vanadium extraction molten pool; wherein, the low-phosphorus cold-bound pellets for vanadium extraction are the above-mentioned low-phosphorus cold-bound pellets for vanadium extraction.

[0033] Since the present invention mainly relates to the improvement of the low-phosphorus cold-bound pellets for vanadium extraction, there are no special limitations on other steps and specific operations of the vanadium extraction method, and it can be referred to the prior art.

[0034] Preferably, before blowing oxygen for smelting the vanadium-bearing hot metal, the blast furnace slag on the surface of the vanadium-bearing hot metal is removed. Specifically, the vanadium-bearing hot metal is subjected to slag skimming treatment to remove the blast furnace slag on the surface of the hot metal ladle, because the blast furnace slag contains a high content of CaO (the CaO content reaches 20 - 30 wt%). When CaO enters the vanadium extraction converter, it will cause the CaO in the vanadium slag to rise and a dephosphorization reaction will occur. The vanadium-bearing hot metal with the blast furnace slag removed is poured into the vanadium extraction converter, and blowing oxygen for smelting is carried out on the vanadium extraction converter.

[0035] Preferably, the addition amount of pig iron is 5 - 30 kg / ton of iron, and the addition amount of the low-phosphorus cold-bound pellets for vanadium extraction is 10 - 60 kg / ton of iron; the blowing oxygen smelting conditions include an oxygen supply flow rate of 18000 - 26000 m 3 / h, an oxygen supply pressure of 0.7 - 1 MPa, a blowing oxygen smelting time of 4 - 8 min, and the temperature is controlled at 1330 - 1400 °C.

[0036] 1 - 2 minutes before the end of smelting, 200 - 500 kg of carburizer is added to adjust the slag state of vanadium slag and control the FeO content in the vanadium slag. After the vanadium extraction blowing is completed, the semi-steel tapping operation is carried out, and the semi-steel in the furnace is tapped into the semi-steel ladle. After the semi-steel is tapped out, the vanadium slag is poured out. The semi-steel is sent to the next process, the steelmaking converter, for smelting, and the vanadium slag is sent to the vanadium products process for roasting - leaching, precipitation reduction treatment, and low-phosphorus vanadium oxide can be produced. The phosphorus in the vanadium slag can be controlled within 0.04 wt%.

[0037] The present invention will be described in detail below through examples.

[0038] Example 1

[0039] The scale accounts for 45 wt%, the iron ore powder accounts for 30 wt%, the dedusted ash accounts for 23 wt%, and the binder is 2 wt% to produce low-phosphorus cold bonded pellets for vanadium extraction. Using these cold bonded pellets for vanadium extraction, the addition amount of the cold bonded pellets is 35 kg / ton of iron, 13 kg / ton of iron of pig iron is added, 260 kg of carburizer is added, the vanadium extraction temperature is controlled at 1352 °C, the phosphorus content of the obtained vanadium slag is 0.037 wt%, and the V2O5 content in the vanadium slag is 17.6 wt%. Using this vanadium slag for the production of vanadium products without the phosphorus removal process, the phosphorus content in the obtained 80FeV alloy is 0.040 wt%, meeting the requirements of grade A products of this alloy (phosphorus requirement < 0.05 wt%).

[0040] Since the phosphorus content of the vanadium slag in the comparison furnace is 0.076 wt% and the V2O5 of the vanadium slag is 16.8 wt%. The vanadium slag of this furnace is produced using conventional cold bonded pellets as coolants. The cold bonded pellets are produced using inorganic binders, and the P content in the cold bonded pellets is 0.067 wt%, and the CaO content is 3 wt% - 6 wt%. Using this vanadium slag for the production of vanadium products without the phosphorus removal process, the phosphorus content in the obtained 80FeV alloy is 0.065 wt%, not meeting the requirements of grade A products of this alloy (phosphorus requirement < 0.05 wt%).

[0041] Example 2

[0042] The scale accounts for 48 wt%, the iron ore powder accounts for 25 wt%, the dedusted ash accounts for 25 wt%, and the binder is 3 wt% to produce low-phosphorus cold bonded pellets for vanadium extraction. Using these cold bonded pellets for vanadium extraction, the addition amount of the cold bonded pellets is 31 kg / ton of iron, 16 kg / ton of iron of pig iron is added, 280 kg of carburizer is added, the vanadium extraction temperature is controlled at 1338 °C, and the phosphorus content of the obtained vanadium slag is 0.034 wt%. The V2O5 content in the vanadium slag is 17.1 wt%.

[0043] Using this vanadium slag for the production of vanadium products without the phosphorus removal process, the phosphorus content in the obtained 80FeV alloy is 0.041 wt%, meeting the requirements of grade A products of this alloy (phosphorus requirement < 0.05 wt%).

[0044] Example 3

[0045] The scale accounts for 56 wt%, the iron ore powder accounts for 25 wt%, the dedusted ash accounts for 15 wt%, and the binder is 4 wt% to produce low-phosphorus cold-bonded pellets for vanadium extraction. Using these cold-bonded pellets for vanadium extraction, the addition amount of the cold-bonded pellets is 38 kg per ton of iron, 15 kg of pig iron is added per ton of iron, 300 kg of recarburizer is added, the vanadium extraction temperature is controlled at 1347 °C, the phosphorus content of the obtained vanadium slag is 0.032 wt%, and the V2O5 content in the vanadium slag is 18.2 wt%.

[0046] Using this vanadium slag for the production of vanadium products without the phosphorus removal process, the phosphorus content in the obtained 80FeV alloy is 0.039 wt%, meeting the requirements of grade A products of this alloy (phosphorus requirement < 0.05 wt%).

[0047] Example 4

[0048] The scale accounts for 30 wt%, the iron ore powder accounts for 35 wt%, the dedusted ash accounts for 30 wt%, and the binder is 5 wt% to produce low-phosphorus cold-bonded pellets for vanadium extraction. Using these cold-bonded pellets for vanadium extraction, the addition amount of the cold-bonded pellets is 41 kg per ton of iron, 13 kg of pig iron is added per ton of iron, 280 kg of recarburizer is added, the vanadium extraction temperature is controlled at 1356 °C, the phosphorus content of the obtained vanadium slag is 0.035 wt%, and the V2O5 content in the vanadium slag is 17.7 wt%. Using this vanadium slag for the production of vanadium products without the phosphorus removal process, the phosphorus in the obtained 80FeV alloy is 0.037 wt%, meeting the requirements of grade A products of this alloy (phosphorus requirement < 0.05 wt%).

[0049] Example 5

[0050] The scale accounts for 59 wt%, the iron ore powder accounts for 19 wt%, the dedusted ash accounts for 19 wt%, and the binder is 3 wt% to produce low-phosphorus cold-bonded pellets for vanadium extraction. Using these cold-bonded pellets for vanadium extraction, the addition amount of the cold-bonded pellets is 43 kg per ton of iron, 16 kg of pig iron is added per ton of iron, 260 kg of recarburizer is added, the vanadium extraction temperature is controlled at 1338 °C, the phosphorus content of the obtained vanadium slag is 0.030 wt%, and the V2O5 content in the vanadium slag is 18.9 wt%. Using this vanadium slag for the production of vanadium products without the phosphorus removal process, the phosphorus in the obtained 80FeV alloy is 0.032 wt%, meeting the requirements of grade A products of this alloy (phosphorus requirement < 0.05 wt%).

[0051] Comparative Example 1

[0052] The scale accounts for 22 wt%, the iron ore powder accounts for 25 wt%, the dedusted ash accounts for 45 wt%, and the clay binder is 7 wt% to produce a cold bonded pellet. The mass percentage content of the components of the cold bonded pellet includes: TFe 51.8%, CaO 4.1%, SiO2 8.3%, P < 0.067%, the H2O content is 1.3%. Using this cold bonded pellet for vanadium extraction, the addition amount of the cold bonded pellet is 37 kg per ton of iron, 16 kg of pig iron is added per ton of iron, 290 kg of carburant is added, the vanadium extraction temperature is controlled at 1342 °C, the phosphorus content of the obtained vanadium slag is 0.076 wt%, and the V2O5 content in the vanadium slag is 16.3 wt%. Using this vanadium slag for the production of vanadium products, without the phosphorus removal process, the phosphorus in the obtained 80FeV alloy is 0.065 wt%, which does not meet the requirements of the A-grade product of this alloy (the phosphorus requirement < 0.05 wt%).

[0053] Comparative Example 2

[0054] The scale accounts for 26 wt%, the iron ore powder accounts for 24 wt%, the dedusted ash accounts for 44 wt%, and the clay binder is 6 wt% to produce a cold bonded pellet. The mass percentage content of the components of the cold bonded pellet is: TFe 52.6%, CaO 3.8%, SiO2 8.1%, P < 0.064%, H2O 1.3%. Using this cold bonded pellet for vanadium extraction, the addition amount of the cold bonded pellet is 36 kg per ton of iron, 17 kg of pig iron is added per ton of iron, 310 kg of carburant is added, the vanadium extraction temperature is controlled at 1351 °C, the phosphorus content of the obtained vanadium slag is 0.078 wt%, and the V2O5 content in the vanadium slag is 16.8 wt%. Using this vanadium slag for the production of vanadium products, without the phosphorus removal process, the phosphorus in the obtained 80FeV alloy is 0.066 wt%, which does not meet the requirements of the A-grade product of this alloy (the phosphorus requirement < 0.05 wt%).

[0055] Comparing the examples with the comparative examples, it can be seen that the phosphorus content of the vanadium slag prepared in Examples 1-5 is much lower than that of the vanadium slag in the comparative example. In this way, in the subsequent production process without the phosphorus removal process, the phosphorus content in the 80FeV alloy < 0.05%, meeting the requirements of the A-grade product of this alloy. Moreover, the present invention makes more effective use of secondary resources such as dedusted ash and scale generated in the iron and steel industry, reduces the preparation cost of the low-phosphorus cold bonded pellet for vanadium extraction, and also solves the problem of difficult reuse of dedusted ash (especially dry dedusted ash).

[0056] Restricting the phosphorus content, calcium oxide content and relative ratio in the raw materials makes the phosphorus content of the low-phosphorus cold bonded pellet for vanadium extraction maintain at a relatively low value. Furthermore, when the low-phosphorus cold bonded pellet for vanadium extraction is used as a coolant for vanadium extraction subsequently, it will not introduce excessive phosphorus, solving the problems of high phosphorus in the vanadium slag during converter vanadium extraction production, difficult vanadium slag treatment in the subsequent process and the need to add a phosphorus removal process, resulting in product degradation.

[0057] The above are the exemplary embodiments disclosed by the present invention. However, it should be noted that various changes and modifications can be made without departing from the scope of the embodiments disclosed by the present invention as defined by the claims. Although the elements disclosed by the embodiments of the present invention can be described or claimed in individual form, they can also be understood as plural unless explicitly limited to the singular.

[0058] Those of ordinary skill in the art should understand that the discussion of any of the above embodiments is only exemplary and is not intended to imply that the scope of the embodiments disclosed by the present invention (including the claims) is limited to these examples; under the concept of the embodiments of the present invention, the technical features in the above embodiments or different embodiments can also be combined, and there are many other variations in different aspects of the embodiments of the present invention as described above, which are not provided in detail for the sake of brevity. Therefore, any omission, modification, equivalent replacement, improvement, etc. made within the spirit and principle of the embodiments of the present invention shall be included within the protection scope of the embodiments of the present invention.

Claims

1. A preparation method of low-phosphorus cold-bonded pellets for vanadium extraction, characterized in that, The method includes mixing scale, iron ore powder, dedusted ash and binder evenly, pelletizing the mixture to obtain green pellets, and then drying to obtain low-phosphorus cold-bonded pellets for vanadium extraction. Based on the total mass of the mixture, by mass percentage, the dosage of the scale is 30 - 59%, the dosage of the iron ore powder is 19 - 40%, the dosage of the dedusted ash is 15 - 40%, and the dosage of the binder is 1 - 5%. The dedusted ash is dry dedusted ash from a vanadium extraction converter. Among them, the phosphorus content of the dedusted ash < 0.05 wt%, and the CaO content of the dedusted ash < 1 wt%. The mass percentage content of the chemical components of the low-phosphorus cold-bonded pellets for vanadium extraction includes: TFe 55 - 75%, CaO 0.5 - 2%, SiO2 2 - 10%, P < 0.04%, H2O < 3%.

2. The preparation method of the low-phosphorus cold-bound pellets for vanadium extraction according to claim 1, wherein, The phosphorus content of the scale is less than or equal to 0.02 wt%.

3. The preparation method of the low-phosphorus cold-bonded pellets for vanadium extraction according to claim 2, characterized in that The scale is one or more of continuous casting scale, hot rolling scale and cold rolling scale.

4. The preparation method of the low-phosphorus cold-bound pellets for vanadium extraction according to claim 1, characterized in that, The iron ore powder is low-phosphorus iron ore powder. Among them, the phosphorus content < 0.03 wt%, and the CaO content < 2 wt%.

5. The preparation method of the low-phosphorus cold-bonded pellets for vanadium extraction according to claim 1, characterized in that The binder is organic starch made from biomass.

6. A low-phosphorus cold-bound pellet for vanadium extraction, characterized in that, The low-phosphorus cold-bonded pellets for vanadium extraction are prepared by the preparation method described in any one of claims 1 - 5, and the mass percentage content of phosphorus in the low-phosphorus cold-bonded pellets for vanadium extraction is < 0.04%.

7. A vanadium extraction method, characterized in that, The method includes blowing oxygen for smelting the vanadium-bearing hot metal. During the smelting process, pig iron blocks and low-phosphorus cold-bonded pellets for vanadium extraction are added to the vanadium-bearing hot metal to control the temperature of the vanadium extraction bath; among them, the low-phosphorus cold-bonded pellets for vanadium extraction are the low-phosphorus cold-bonded pellets for vanadium extraction described in claim 6.

8. The vanadium extraction method according to claim 7, characterized in that, Before blowing oxygen for smelting the vanadium-bearing hot metal, the blast furnace slag on the surface of the vanadium-bearing hot metal is removed.

9. The vanadium extraction method according to claim 7, characterized in that, The addition amount of pig iron is 5 - 30 kg per ton of iron, and the addition amount of low - phosphorus cold - solid balls for vanadium extraction is 10 - 60 kg per ton of iron; the oxygen - blowing smelting conditions include an oxygen supply flow rate of 18000 - 26000 m 3 / h, an oxygen supply pressure of 0.7 - 1 MPa, an oxygen - blowing smelting time of 4 - 8 min, and the temperature is controlled at 1330 - 1400 °C.