According to the China Carbon Accounting Database (CEADs), the total carbon emissions from China's mineral resource mining and dressing industry were about 14.126 Mt in 1997, and about 21.735 Mt in 2019, an increase of 53.87%. In terms of the energy consumption distribution of the mining and dressing industry, 3% to 5% is used for blasting, 5% to 7% for crushing, and 80% to 90% for grinding. According to statistics, the global electricity consumption in the grinding process accounts for 3% to 4% of the global total power generation, and the unit electricity consumption accounts for more than 50% of the total cost of the dressing plant.
Carbon emission accounting is a measure to measure the direct and indirect emission of CO2 and its equivalent gases from industrial activities to the Earth's biosphere. It can directly quantify carbon emission data and identify potential emission reduction links and methods by analyzing carbon emission data at each link. At present, the main carbon emission accounting measurement standards are: GHG Protocol standard, PAS2050 standard, ISO14064 standard, ISO14040/14044 standard.
There are two main types of carbon emission accounting methods. The first type is the calculation method of the "2006 IPCC Guidelines for National Greenhouse Gas Inventories" (hereinafter referred to as the "IPCC Guidelines"), and the second type is the carbon footprint method. The "IPCC Guidelines" provide three calculation methods: emission coefficient method, material balance method, and actual measurement method. The emission coefficient method is currently the most widely used greenhouse gas emission accounting method. This method obtains carbon emissions by multiplying activity data with emission factors, which can reflect the correlation between energy intensity and carbon emission intensity. The material balance method calculates carbon emissions through the law of conservation of mass, which is highly targeted and accurate.
However, many parameters need to be collected during accounting, and the quality requirements are high. The measurement method refers to a statistical calculation method that actually measures the activity level of relevant emission sources on site and calculates carbon emissions.
Grinding operation is the most energy-intensive key process in the mineral processing, and its energy conservation and consumption reduction has always been the focus of mineral processing plants. The carbon emission accounting of different grinding media in the grinding process is an important way to study the energy conservation and consumption reduction, cost reduction and efficiency improvement of the grinding process. This study uses the emission coefficient method to calculate the carbon emissions of two types of grinding media, steel forging and nano-ceramic balls (hereinafter referred to as "ceramic balls"), in the grinding process. Combined with the grinding application case of a non-ferrous metal mine in Chenzhou City, Hunan Province, the carbon emissions and economic benefits before and after the application of ceramic balls instead of steel forgings in the two-stage grinding process are studied.
The grinding process of the metal mine is shown in Figure 2. In the grinding process, the overflow ball mill is used in the grinding-classification circuit.
MQGΦ3 600 mm×4 500 mm and 2FG-3000 mm high-lying double spiral classifier form a closed circuit; the second-stage grinding-classification circuit uses overflow ball mill MQYΦ3 600 mm×4 000 mm and CZQ300 mm hydrocyclone to form a closed circuit. After the raw ore is ground in a closed circuit, the overflow from the first stage classification enters the second stage mill for closed circuit grinding, and the overflow from the second stage classification enters the subsequent flotation operation as the final grinding product.
According to the production conditions of the mine, on the basis of laboratory research, industrial tests were carried out without affecting the normal production of the concentrator, and the steel forging media in the second-stage ball mill was replaced with ceramic ball media. The initial loading of steel forgings before replacement was 48.8 t, and the initial loading of ceramic balls after replacement was 32 t. The comparison of grinding efficiency and classification efficiency before and after replacement is shown in Table 1, and the comparison of particle size characteristics distribution of grinding products is shown in Table 2.
As can be seen from Table 1, the grinding-classification circuit has been significantly improved after replacement. The circulation load has been reduced by 21.28%, the classification quality efficiency has been increased by 7.32%, the classification volume efficiency has been increased by 0.13%, and the mill utilization factor has been increased by 9.48%.
As can be seen from Table 2, the particle size distribution of the sorted products was improved after replacement. The fineness of the discharge product increased by 11.84%, the yield of the intermediate particle size increased by 3.35%, and the distribution of tungsten metal increased by 8.07%. The fineness of the overflow product changed slightly, but the yield of the intermediate particle size and the distribution of tungsten metal increased significantly, by 12.87% and 28.61% respectively. The comparison before and after replacement shows that the particle size distribution of the product is relatively more concentrated when the ceramic ball is milled, which is more conducive to the enrichment of tungsten metal to the intermediate particle size.
Ceramic ball grinding can not only optimize the grinding effect, but also significantly reduce grinding energy consumption and medium material consumption. The comparison of production indicators before and after replacement is shown in Table 3.
As can be seen from Table 3, ceramic ball grinding does not affect the mill processing capacity. Although the filling rate of ceramic balls is higher than that of steel forging media after replacement, the overall weight of ceramic balls in the mill is lighter than that of steel forging due to the smaller specific gravity of ceramic balls, which effectively reduces the mill operation load. The unit power consumption decreased by 39.32%, and the medium operation cost decreased by 19.93%.
The raw materials for steel forging come from steel bars, so they should be traced back to steelmaking production. There are two main processes for steel production: the converter steelmaking process (long process) and the electric arc furnace steelmaking process (short process). China's crude steel production is mainly based on the long process, which accounts for more than 80% of crude steel output, so the main focus is on carbon emissions in the long process of steel production. Long process production mainly includes coking, sintering, pelletizing, steelmaking, ironmaking, steel rolling and other production processes. The "Guidelines for the Calculation and Reporting of Greenhouse Gas Emissions from Chinese Steel Production Enterprises (Trial)" (hereinafter referred to as the "Steel Guidelines") defines that carbon emissions from the steel industry are mainly divided into: carbon emissions from fuel combustion, carbon emissions from industrial production processes, and carbon emissions implied by net purchased electricity and heat. Usually in steel, during production, 30% of the coal is consumed for combustion and the remaining 70% is used for coking; 75% of the coke is consumed for combustion and the remaining 25% is used as a reducing agent.
The default values of relevant parameters listed in the "Steel Guidelines" are shown in Table 4.
(1) Carbon emissions from fuel combustion
Carbon emissions from the combustion of net fossil fuels, including emissions from stationary sources (such as coke ovens, sintering machines, blast furnaces and other stationary combustion equipment) and emissions from mobile sources used in production.
The carbon emission accounting method is used for calculation, and the calculation formula is as follows:
Formula (1):
ADi is the activity level of the ith fossil fuel, GJ; EFi is the CO2 emission factor of the ith fossil fuel, tCO2/GJ.
(2) Carbon emissions from industrial production processes
Carbon emissions from industrial production processes include carbon emissions from the decomposition and oxidation of other purchased carbon-containing raw materials (such as electrodes, pig iron, ferroalloys, direct reduced iron, etc.) and fluxes during sintering, ironmaking, steelmaking and other processes. Refer to the "Steel Guidelines" and the calculation formula is as follows:
Formula (2):
P is the net consumption of flux, t. EF is the carbon emission factor of flux, tCO2/t flux.
(3) Carbon emissions embodied in net purchased electricity
The carbon emissions implicit in the net purchase of electricity are calculated using formula (3), taking the latest electricity carbon emission factor as 0.5810 tCO2/MWh.
Formula(3)
AD is the net electricity purchase, MWh; EF is the CO2 emission factor of electricity, tCO2/MWh.
(4) Total carbon emissions from crude steel production
The total carbon emissions from crude steel production are the carbon emissions from fuel combustion, carbon emissions from industrial production processes, and carbon emissions implicit in net purchased electricity.
Based on the energy consumption data of China's steel industry provided by the China Statistical Yearbook (see Table 5), a comprehensive calculation and analysis can be performed to obtain the total carbon emissions of the steel industry from 2011 to 2019, as shown in Figure 3.
As can be seen from Figure 3, with the continuous development of the steel industry, crude steel production and total carbon emissions are increasing. However, carbon emissions per ton of steel are decreasing, indicating that my country has made positive progress in energy conservation and emission reduction in the steel industry. In 2019, crude steel production was 995,418,900 tons, total carbon emissions were 1,688,571,300 tons, and carbon emissions per ton of steel were 1.70 tons. Compared with 2011, crude steel production increased by 45.26%, total carbon emissions increased by 16.54%, and carbon emissions per ton of steel decreased by 19.43%.
The carbon emissions in the steel forging production process are calculated mainly based on the annual data provided by a new material company in Henan Province. The raw materials of the steel forging products processed by the company are purchased steel, the annual output of steel forging is 120,000 tons, and a total consumption of 4.8 million m3 of natural gas and 7,200 MWh of electricity. According to the carbon emission coefficient of natural gas of 2.162 kgCO2/m3 and the carbon emission factor of electricity of 0.581 0 tCO2/MWh, the annual carbon emissions of natural gas combustion in the steel forging production process are 10377.60 t, the net carbon emissions of purchased production electricity are 4183.20 t, and the total carbon emissions are 14560.8 t. Therefore, the carbon emissions of producing 1 t of steel forging are 0.12 t (excluding the front-end crude steel production process).
Combined with the calculated carbon emissions per ton of steel from 2011 to 2019 and taking the average value of 1.95 tCO2/t, the company's carbon emissions per ton of steel forging production process is 2.07 t.
As an emerging product, ceramic balls have been widely used in cement dry grinding systems and metal ore secondary grinding processes. They have the advantages of high wear resistance, high hardness, and no iron pollution. According to the "Guidelines for the Calculation Methods and Reporting of Greenhouse Gas Emissions of Chinese Ceramic Production Enterprises (Trial)", the accounting carbon emission sources include: carbon emissions from fossil fuel combustion, carbon emissions from industrial production processes, and carbon emissions embodied in net purchased production electricity. Carbon emissions from industrial production processes mainly refer to the release of CO2 by high-temperature decomposition of carbonates (such as CaCO3 and MgCO3) in calcite, magnesite, and dolomite during the firing process. The energy consumption of this process accounts for more than 50% of the total energy consumption. The embodied carbon emissions of net purchased production electricity mainly refer to the carbon emissions embodied in the consumption of net purchased electricity by electrical equipment, such as raw material crushing, ball milling, stirring, molding, and forming processes.
The carbon emissions from the production process of nano ceramic balls were verified mainly based on the data provided by SANXIN ceramic ball manufacturers in 2017.
The annual natural gas consumption of the plant is 1.1895 million m3, the annual consumption of alumina and kaolin clay raw materials is 3,600 t and 384 t respectively, the annual electricity consumption is 4,017.71 MWh, and the annual production of ceramic balls is 4,500 t. According to the natural gas emission coefficient of 2.162 kgCO2/m3 and the electricity carbon emission factor of 0.581 0 tCO2/MWh, the carbon emissions from fuel combustion are 2,571.70 t, and the implicit carbon emissions from net purchased production electricity are 2,334.29 t. The carbon emission coefficient of clay raw materials is
145.32 kgCO2/t, the carbon emission of raw materials in the production process is calculated to be 578.95 t. Therefore, the total annual carbon emission of the factory's ceramic ball production is 5484.94 t, and the carbon emission of producing 1 t of ceramic balls is 1.22 t.
For the production of steel forging media, it is mainly made by rolling steel bars, so iron ore concentrate → blast furnace ironmaking → converter steelmaking → steel bars are defined as the raw material end process during steel forging production, and steel bars → steel forging are defined as the processing end process. For the production of ceramic ball media, the main raw materials are alumina and kaolin clay raw materials. In order to compare the carbon emission intensity of steel forging and ceramic ball production processes, the above calculation results are summarized in Table 6.
It can be seen from Table 6 that the carbon emission intensity of the steel forging production process is 2.07 tCO2/t, and its carbon emissions mainly come from the raw material end, that is, the crude steel production end. The carbon emission intensity of the ceramic ball production process is 1.22 tCO2/t. Compared with steel forging, the production of 1 t of ceramic balls emits 0.85 tCO2 less than the production of 1 t of steel forging.
From the perspective of power consumption, it can be seen from Table 3 that the unit power consumption of steel forging is 5.62 kWh/t, and the unit hour is 90 t/h. If the average working days of the concentrator are 330 days per year, the annual power consumption of steel forging as grinding medium is 5.62×90×24×330×10−3=4 005.94 MWh, and the indirect carbon emissions are 4 005.94×0.581 0=2 327.45 t.
From the perspective of consumables, Table 3 shows that the unit consumption of steel forging medium is 0.46 kg/t, and the unit-hour is 90 t/h. If the average working days of the concentrator are 330 days per year, the annual consumption of steel forging is 0.46×90×24×330 × 10−3=327.90 t, and the indirect implied carbon emissions are 327.90×2.07=678.75 t.
In summary, the annual embodied carbon emissions when steel forging is used as fine grinding medium is 2327.45+678.75=3 006.20 t.
From the perspective of electricity consumption, if calculated at 0.69 yuan per kilowatt-hour, the annual electricity cost when steel forging is used as grinding medium is 2.7641 million yuan.
From the perspective of consumables, if calculated at RMB 6,000 per ton of steel forgings, the annual purchase cost of steel forgings as grinding media is RMB 1.9674 million.
Considering the power consumption and consumables, the annual grinding cost when steel forging is used as fine grinding medium is 276.41+196.74=473.15 million yuan.
From the perspective of power consumption, it can be seen from Table 3 that the unit power consumption of ceramic balls is 3.41 kWh/t, and the unit hour is 90 t/h. If the average working days of the concentrator are 330 days per year, the annual power consumption of ceramic balls as grinding media is 3.41×90×24×330×10−3=2 430.65 MWh, and the indirect carbon emissions are 2 430.65×0.581 0=1 412.21 t.
From the perspective of consumables, Table 3 shows that the unit consumption of ceramic balls is 0.13 kg/t, and the unit hour is 90 t/h. If the average annual working days of the concentrator are 330 days, the annual consumption of ceramic balls is 0.13×90×24×330×10−3=92.70 t, and the indirect carbon emissions are 92.7×1.22=113.09 t.
In summary, the annual embodied carbon emissions when ceramic balls are used as fine grinding media are 1 412.21+113.09=1 525.30 t.
From the perspective of electricity consumption, if calculated at 0.69 yuan per kilowatt-hour, the annual electricity cost when ceramic balls are used as grinding media is 1.6771 million yuan.
From the perspective of consumables, if calculated at RMB 17,000 per ton of ceramic balls, the annual purchase cost of ceramic balls as grinding media is RMB 1.5759 million.
Considering the power consumption and consumables, the annual grinding cost when ceramic balls are used as fine grinding media is 167.71+157.59=3.253 million yuan.
In order to compare the annual indicators and carbon emissions during the grinding process of the two grinding media, the above research results are summarized in Table 7.
As shown in Table 7, the embodied carbon emissions during ceramic ball grinding are significantly lower than those of steel balls, which can reduce carbon emissions by about 1,480.90 tons per year, a decrease of 49.26%.
In addition, although the price of ceramic balls is higher than that of steel balls, the medium consumption and power consumption during ceramic ball grinding are greatly reduced, of which the medium consumption decreases by 71.73% and the power consumption decreases by 39.25%.
Therefore, the overall cost of ceramic ball grinding is lower than that of steel ball grinding, which can save 1.4785 million yuan in grinding costs each year. It can be seen that ceramic ball grinding, as a new type of grinding process technology, can not only bring economic benefits to the concentrator, but also reduce CO2 emissions and contribute to the goal of carbon neutrality.
(1)As an energy-intensive industry, the steel industry has a carbon emission intensity of 2.07 tCO2/t. Ceramic companies that produce nano-ceramic balls have the characteristics of an emerging materials industry, and the carbon emission intensity of their products is 1.22 tCO2/t.
(2)When steel forging is used as fine grinding medium, the annual embodied carbon emissions are 3,006.20 t, and when ceramic balls are used as fine grinding medium, the annual embodied carbon emissions are 1,525.30 t. After replacement, carbon emissions can be reduced by about 1,480.90 t per year.
(3) When steel forging is used as fine grinding medium, the annual grinding cost is RMB 4.7315 million. When ceramic balls are used as fine grinding medium, the annual grinding cost can be reduced to RMB 1.33 million.
The lowest cost is 3.253 million yuan. After using ceramic balls instead of steel forgings as fine grinding media, the company can save about 1.4785 million yuan in costs each year.
(4) Ceramic ball grinding technology has broken the technical barriers of traditional steel ball grinding and led the technological innovation in the field of fine grinding in metal mines. In the future, if nano ceramic balls are promoted in the field of fine grinding in metal mines, it can reduce the production costs of mining companies, reduce carbon emissions in mines.
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