Study on The Test Methods of Heavy Metal in Exhaust Gas for a Copper Melting Factory

. The raw materials, auxiliary materials and production process of a typical copper smelting factory were investigated, the emission characteristics of exhaust gas pollutants produced by the production process were studied. To test the samples by graphite furnace, firstly in order to unify the digestion system in the experiment, the applicability of three different method was verified by experiments. According to the results, the recovery of nitric acid-perchloric acid digestion system is a relatively good, so it can be recommended. The testing results show linear range, detection limit and detectability of the method could meet the requirements of environmental monitoring.


Introduction
At present, the issue of environmental pollution in China has become a public focus, while heavy metal pollution among them has attracted more attention due to the characteristics of being difficult to degrade and hazardous to human health.Heavy metals refer to metal elements whose density is greater than 4.5g/cm3.In terms of environmental pollution, the heavy metals mainly refer to the biological toxic elements such as copper, lead, chromium and metal-like selenium and arsenic, which can have strong interaction with protein and enzyme in human body, can cause chronic poisoning.Copper in exhaust gas mainly comes from smelting, metal processing, iron and steel manufacturing, among which smelting is the main source of copper pollution in the atmosphere [1].At present, with the continuous strengthening of air pollution control in China, there are a number of pollutant emission standards in China that stipulate the emission limits of heavy metals.Relevant standards related to the emission of copper pollutants, such as:" emission standards of pollutants for inorganic chemical industry" (GB31573-2015) [2]."Standard for pollution control on the municipal solid waste incineration" (GB18485-2014) [3]."Standard for pollution control on hazardous waste incineration" (GB 18484-2020) [4].However, compared with the emission control elements in developed countries, the required monitoring methods lack uniformity and systematization.In addition to the uniform sample collection method in the monitoring process, the pretreatment, analysis and testing, quality control and other aspects need to be further improved and standardized.The monitoring ability of environmental monitoring departments varies greatly among regions, with western regions obviously lagging behind eastern regions.Even among monitoring stations at the same level, there are also great differences in personnel and technical level, instruments and equipment [5].Therefore, in the future, it is necessary to focus on the study and introduction of operational and high accuracy monitoring methods for heavy metals in exhaust gas to meet the increasing requirements of environmental management.

Typical production process
The main processes of this project are melting, casting, rolling and wire drawing, annual operation 8760h, annual production of 6000 tons of high frequency resistance welding wire.The production of the enterprise is equipped with a leading furnace, which puts the electrolytic copper raw materials into the furnace body in batches and electrically heats the furnace body to 1100~1200℃ for continuous heating and melting.During the melting process of electrolytic copper, a certain amount of copper-containing exhaust gas will be produced.At the same time, a small amount of flake graphite should be added as a covering agent to prevent copper oxidation.After melting into copper water, the mold is used to pull out 17 mm copper rod, the 17mm copper rod is rolled into 8mm copper rod through the mill, the 8mm copper rod is made into 3 mm copper wire through mechanical wire drawing, and then the 3 mm copper wire is made into 1.3 mm, 1.8mm and 1.5mm copper wire respectively.Raw and auxiliary materials used in the production process are shown in Table 1 Note: for electrolytic copper, the thick plate made of crude copper (including 99% copper) is used as anode, the pure copper is made into thin sheet as cathode, and the mixed solution of sulfuric acid and is used as electrolyte.After power on, copper dissolves from the anode into copper ions and moves to the cathode.

Pollutant emission analysis
The pollutant of the exhaust gas in the factory is copper.that produced in the process of copper melting.According to the"The First National Survey of Pollution Sources Manual on The Coefficient of Production and Discharge of Industrial Pollution Sources" (Part II).Using electrolytic copper raw material to produce copper wire products, the process is "casting + tying process" soot emission coefficient is 1.62 kg/t -product.The annual output of high frequency resistance welding wire of the enterprise is 6000t.According to the calculation, the soot production amount of melting process is 9.72t/a, and the annual working hours of melting process is 8760h, so the particle production rate is 1.11kg/h.An air collecting hood is set above the upper leading furnace, and the dust generated is collected by an induced draft fan into the pulse and bag dust collector for treatment, and the exhaust gas after treatment is discharged by a 15m high exhaust cylinder.The air volume of the air collecting hood fan is 20000m3/h, the air volume of the air collecting hood is close to the exhasut gas generation part, and the air volume can meet the collection efficiency requirements, so the exhaust gas collection efficiency of this project can reach 90%.The calculation process is only for particulate matter, and the emission of copper containing exhaust gas cannot be known.Therefore, on-site sampling and graphite furnace method should be used to monitor the concentration of copper emission in exhaust gas.

Method and Principle
Acquisition stationary sources with the quartz fiber filter cylinder exhaust particulate matter, after nitric acidperchloric acid digestion preparation into the solution, the solution of copper in graphite furnace atomization device, via high temperature atomization, the ground state copper atoms of hollow cathode lamp emission characteristic spectral line (324.7 nm) absorption, within a certain range, the absorbance value has a linear relation with the mass concentration.

Main Equipment.
The main equipment includes: graphite furnace atomic absorption spectrophotometer, graphite tube, copper hollow cathode lamp, smoke sampler, quartz fiber filter cartridge, temperature regulating electric heating plate, suction filtration device, etc.
(5) Mixed solution of nitric acid and hydrochloric acid: add 55.5 ml nitric acid(ρ= 1.42 g/ml) and 167.5 ml hydrochloric acid(ρ= 1.19 g/ml) to 500 ml pure water, and then diluted to 1 L with pure water.

Sample Collection
The distribution and sampling of heavy metals in exhaust gas shall comply with the relevant provisions of GB/T 16157 and HJ/T 397.The filter cartridge shall collect at least 0.6m3 of exhaust gas sample (in standard dry state).
If the concentration of heavy metal copper is low, the sampling volume shall be increased appropriately.When the temperature of flue gas in the flue is higher than the melting point of copper, cooling measures should be taken.The temperature of flue gas before entering the filter cylinder is lower than the melting point of metal elements.During sampling, each batch of samples should carry at least two filters of the same batch, which are not connected with the sampler after being brought to the sampling site.After sampling, they should be brought back to the laboratory for testing.After sampling, take out the filter cartridge carefully, fold the seal inward, put it back into the filter cartridge box vertically, and take it back to the laboratory for analysis.

Sample Digestion
In order to unify the digestion system, the applicability of different digestion systems of nitric acid-perchloric acid, nitric acid-hydrofluoric acid-perchloric acid and nitric acid-hydrochloric acid was verified by experiments.
Electrothermal plate digestion was used to verify the digestion system for the determination of copper digestion effect.

Nitric acid-perchloric acid.
Cut the filter cartridge into pieces with ceramic scissors, place in a 150 mL conical flask and moisten with a little water.add 30ml nitric acid and 5ml perchloric acid (it is appropriate to dip the sample in acid, not enough nitric acid), Insert a short-necked glass funnel into the mouth of the bottle, heat it on the electric heating plate until boiling, steam it until nearly dry, remove it and cool it.Add another 10mL nitric acid and continue to heat until nearly dry and slightly cool.Add a small amount of water for extraction and filtration, transfer to a 50mL volumetric flask, and then dilute with water to scale.

Nitric acid-hydrofluoric acid-perchloric acid.
Cut the filter cartridge into pieces with ceramic scissors, put it into the digestion tank, add 20.0 mL nitric acid, shake the digestion tank, immerse the filter cartridge fragments, let stand for 2 min, heat to 140 °C and hold for 60 min, cool for 5 min, add 5.0 mL hydrofluoric acid, shake for 2 min, heat again to 140 °C, hold for 60 min, cool for 5 min and add 2.0 mL perchloric acid, shake for 2 min, heat to 160 °C to maintain 150 min to catch the acid to nearly dry.Cooled to a fixed capacity of 50 mL.

Nitric acid-hydrochloric acid.
Cut the filter cartridge into pieces with ceramic scissors, put it into the digestion tank, add 25.0 mL of nitric acidhydrochloric acid mixed solution (5.55% HNO3/16.75%HCl), immerse the filter cartridge fragments in it, cover the digestion lid, heat the reflux at 100 °C for 120 min, remove the digestion cap after cooling, add 10 mL of water, and let stand for 30 min for maceration.The extract was filtered into a 50 mL volumetric flask, volumeted with water, to be measured.

The data of contrast experiment for three methods.
The comparative experiments of the above three digestion methods were carried out on two blank filters and three blank standard addition filters.The data are shown in table 2: According to the results, the recovery of nitric acidperchloric acid digestion system is a relatively good.Therefore, nitric acid-perchloric acid system is recommended.

Measuring instrument condition
(1) Drying stage.The purpose of drying is to remove the water and acid in the solution.If the temperature is too low or the holding time is not enough, the water and acid in the graphite tube will damage the structure of the graphite tube in the high temperature ashing and atomization stage.If the temperature is too high or the temperature rises too fast, the solution bursts into boiling and a double absorption peak may occur during atomization.The effect of low temperature and long time drying is better.
(2) Ashing stage.The purpose of ashing is to remove low boiling compounds before atomization, reduce matrix interference, reduce background absorption, and increase the ashing temperature as much as possible on the premise of ensuring the non-volatile loss of the elements to be measured.If the matrix is complex and the background absorption is high, the ashing time can be appropriately prolonged.
(3) Atomization stage.The selection of atomization time and temperature is to maximize the atomization degree of the element to be measured, which depends on the properties of the element to be measured.Too low temperature, insufficient atomization time and incomplete atomization will reduce the signal, widen the peak shape and produce memory effect; If it is too high, the atomization is too fast, the response of the detection system can not keep up with the atomization signal, the reproducibility and sensitivity are reduced, and the service life of the graphite tube is shortened.( 4) Disability removal stage.The residue removal stage is to remove the matrix and nonatomically elements to be tested and remove the memory effect.The atomization temperature of chromium and nickel is high.On the premise that the instrument allows and ensures the service life of graphite tube, try to increase the residue removal temperature to ensure that all residual substances volatilize.
According to the curve of copper absorbance value, the absorbance value is the largest when the ashing temperature is 1100 ℃ and the ashing time is 20 s; The absorbance is stable when the atomization temperature is 2000 -2100 ℃, 2000 ℃ is recommended, and the atomization time is recommended to be 5s.Absorb 100 μg/L copper solution 0,2.50, 5.00, 7.50, 10.00, 15.00, 20.00 ml in a 50 ml volumetric flask, shake well with 1% nitric acid solution at constant volume.This standard series contains 0,5.0, 10.0, 15.0, 20.0, 25.0, 30.0, 40.0 μg/L copper.Then the absorbance of the standard solution series was measured in sequence from low to high according to the instrument reference measurement conditions.4.6 Detection limit and detectability of the method The method detection limit of each element in this standard revision is determined in accordance with the requirements of technical guidelines for the preparation and revision of environmental monitoring and analysis methods (HJ 168-2010).Since the used filter cartridge contains various target elements, according to the requirements of (HJ 168-2010), the blank filter cartridge can be detected.Directly use the blank filter cartridge to determine the detection limit, determine it according to the pretreatment method of fixed source exhaust gas filter cartridge, measure it in parallel for 7 times, and calculate the detection limit (MDL) according to formula (1).
-Determination times of parallel samples;  ��������� --T-distribution with N-1 degree of freedom and 99% confidence (one side) -Standard deviation of n parallel measurements Technical Guidelines for The Formulation and Revision of Environmental Monitoring and Analysis Method Standards (HJ 168) stipulates that the detection limit of 4 times is the detectability.When the sampling volume is 0.6m3 and the volume is constant to 50 mL, the method detection limit and determination limit of copper are shown in the table below.

Test Results
By sampling and testing the exhaust gas containing copper collected at the melting process, the monitoring point is at the outlet of the purification facility.A total of 7 samples and 2 blank filter cartridges were collected on site.The emission concentration of copper in the exhaust gas of 7 samples was respectively 0.047.0.042.0.050.0.041.0.039.0.045.0.037 mg/m 3 ,RSD value is 10.6%, the test method have good stability.

Conclusions
(1) In order to unify the digestion system, the applicability of three different method was verified by experiments.
According to the results, the recovery of nitric acidperchloric acid digestion system is a relatively good.Therefore, nitric acid-perchloric acid system is recommended.
(2) To test the samples by graphite furnace, the results show the linear range, detection limit and detectability of the method could meet the requirements of environmental monitoring.The testing results of samples that collected in the factory show that the method has a good stability.
(3) The exhaust gas containing copper produced by the melting process is monitored, and the emission concentration of copper meets the requirements of relevant standard limits.

Table 1 .
. List of raw and auxiliary materials used in the factory

Table 2 .
Test value in the different digestion system

Table 4 .
The of data calibration curve

Table 5 .
Detection limit and Detectability by nitric acidperchloric acid electrothermal plate digestion method