KUYUM ATÖLYELERİNDE KULLANILAN ATIKGAZ YIKAMA EKİPMANLARININ VERİMLİLİKLERİNİN BELİRLENMESİ

Kuyum atolyelerinde asit ve siyanur kullanilan islemler esnasinda, insan ve cevre sagligi ile malzeme uzerinde zararli olacak nitelikte atik gazlar ortaya cikmaktadir. Olusan bu atik gaz akimlarinin, olustugu atolye icinde uygun yontem ve ekipmanlarla kontrol edilmesi ve zararsiz hale getirilmesi gerekmektedir. Bu amacla cesitli ureticiler tarafindan kuyum atolyelerinde kullanilabilecek atik gaz yikama ekipmanlari uretilmektedir. Yapilan calismada bu amacla uretilmis bir sistemin atik gaz akimi icerisindeki zararli emisyonlari giderme verimi incelenmis ve verimin surekliliginin saglanmasi icin alinmasi gereken onlemler belirtilmistir. Elde edilen sonuclar incelendiginde; kullanilan atik gaz yikama sistemi sayesinde demir/bakir indirme isleminden kaynaklanan nitrik asit emisyonlarinin %94,5, sulfurik asit emisyonlarinin ise %88,3 oraninda giderildigi belirlenmistir. Patlatma isleminden kaynaklanan atik gazlarin yikama sisteminde yikanmasi ile siyanur emisyonun ise %98,7 oraninda giderildigi tespit edilmistir. Elde edilen sonuclara gore aritilmis toksik gaz emisyonlari; Sanayi Kaynakli Hava Kirliliginin Kontrolu Yonetmeligi ve Hollanda Hava Emisyon Klavuzu’nda verilen limit degerlerin altinda kalmistir.


INTRODUCTION
Gold is an element present in the periodic table.The symbol is Au, which is the abbreviation of Aurum in Latin.Atom number of gold is 79.Gold does not react with most chemicals, but it interacts with chlorine, fluoride, royal water and cyanide and is soluble in mercury.In particular, nitric acid dissolves many metals but gold does not dissolve in nitric acid.Because of this property, nitric acid has been used for a long time to confirm the presence of gold in materials (Zhang et. al., 1995, Müezzinoğlu, 2003, Kaspin, 2013, Klotz, 2010).
The annual use of gold which is a valuable metal, is around 25,000 tons per year.Jewelery production is the most important use of gold with an 80% share.This is followed by industrial use with 12% and other uses with 8% (Corti, 2002, Corti, 2004, Yue et. al., 2017).
When the industrial use of gold is examined, it is seen that the most use is in the electronics sector, followed by dentistry and other industrial and decorative uses.Due to the chemical and physical properties of gold, such as corrosion resistance, industrial importance is increasing day by day (Corti, 2004).
When the processes applied and the chemicals used in processing the metals are considered from the environmental point of view, metal finishing processes are known as one of the most important causes of environmental pollution.Because of this feature, the sector is monitored worldwide and subject to legal regulations.Therefore, the commercial risks and operating costs of the industry are high.While this impacts on the reduction of environmental pollutants, firms also face a difficult problem of finding effective and cost-effective solutions (Giannetti et. al., 2008, Thammaraksa et. al. 2017).
Jewelery is one of the world's oldest manufacturing processes and contains many harmful operations.Today, harmful chemicals used in jewelery production are the most important problem in the industry.These chemicals are; cyanides, acids, solvents, compressed gases, powder of materials, polishing compounds and solders (Müezzinoğlu, 2003, https://www.dtsc.ca.gov/.../Jewelry/.../HWM_FS_Jewelry_P2_Recommendations.pdf).
Free cyanide (HCN or CN -) emissions are important problem in terms of gold processing.It is a respiratory poison and has high acute toxicity.The dose-effect curve of the acute effects in humans is steep by step.120-150 mg/m 3 is dangerous to life and may lead to death after 0.5-1 h, and 300 mg/m 3 is immediately fatal (http://www.who.int/ipcs/publications/cicad/en/cicad61.pdf).
Sulphuric acid and nitric acid mists are another problem originated from gold processing.Sulfuric acid is a highly corrosive and irritating.It cause direct local effects on the skin eyes, and respiratory and gastrointestinal tracts when there is direct exposure to sufficient concentrations.Breathing sulfuric acid mists can result in tooth erosion and respiratory tract irritation.US Occupational Safety and Health Administration (OSHA) limits the amount of sulfuric acid that can be present in workroom air to 1 mg/m³.The National Institute for Occupational Safety and Health (NIOSH) also recommends a time-weighted average limit of 1 mg/m³ (https://www.atsdr.cdc.gov/ToxProfiles/tp117-c1-b.pdf).
The toxicity of HNO 3 is predominately associated with the extremely corrosive nature of this strong acid.In addition, it is an excellent oxidizing agent and reacts immediately with any tissue to cause such effects as skin burns, eye irritation, coughing, dyspnea, and pulmonary edema.inhalation exposure limits for HNO 3 .US Association Advancing Occupational and Environmental Health (ACGIH) set the Threshold Limit Value (TLV) time-weighted average (TWA) for HNO 3 at 2 ppm (5.16 mg/m 3 at 25 0 C) The TLV-short-term exposure limit is twice the TLV-TWA value (https://www.ncbi.nlm.nih.gov/books/NBK230439/).
In the cyanide/peroxide bombing process, a significant amount of gold is often lost as a valuable end product because of the dissolution of gold in the bombing chemical.In this context, electrostripping systems with potassium or sodium can be preferred due to the fact that it is environmentally safer than the cyanide system.However, if the operator cannot provide good control of this process, there may be intense corrosion on the surface of the jewel (https://www.dtsc.ca.gov/.../Jewelry/.../HWM_FS_Jewelry_P2_Recommendations.pdf).
For safety in bombing with cyanide, the use of water-diluted 4% cyanide solution is recommended.It is important that the acids do not interfere with cyanide in the process, and if the acids mistakenly mix with cyanide, cyanide gas, which causes sudden deaths, appears.In the closed bombing system with cyanide, jewel fragments can be cleaned with minimal human interaction, the cyanide solution can be rendered harmless, and dissolved precious metals can be recovered from the waste (Yannopoulos, 1991, https://www.dtsc.ca.gov/.../Jewelry/.../HWM_FS_Jewelry_P2_Recommendations.pdf).
Nitrogen dioxide (NO 2 ) from the recovery process of gold also causes significant air pollution.This key problem is related to the quality control process of all recovery methods performed in small scale enterprises (Kaspin, 2013).
Significant use of acid and cyanide are present in the processes applied in jewelery and ramming shops.The acidic and cyanide wastes generated from these processes cause damage to the material as well as human health and environment.These cyanide and sulphuric acid mists are defined as inorganic vapors and gaseous substances Class II and nitrogen oxides (NO x ) and sulphure oxides (SO x ) are defined as inorganic gaseous substances Class IV according to Turkish Industrial Air Pollution Control Regulation (TIAPCR) Annex 7 (http://sgb.csb.gov.tr/mevzuat/dosyalar/r_20130929233746779_7ef68822-c2b0-41fc-b125-916a43b480a2.pdf)].Emission levels and limitations can be significant depending on the operation size (http://eippcb.jrc.ec.europa.eu/reference/BREF/stm_bref_0806.pdf).Limit values of emissions are tabulated in Table 1 below.In terms of Europen Union same limitations with the TIAPCR can be seen in the Netherlands Emission Guidelines for Air (NER), Section 3.2.4.Emission standards for inorganic substances (www.infomil.nl/publish/pages/63235/lvocannexviii.pdf).In places such as industrial sites, it is inevitable that these gas flows will be adversely affected if they are given to the ventilation systems which are open to the use of other businesses.For this reason, these waste gas flows must be controlled and rendered harmless in the workshop with suitable treatment methods and equipment.For this purpose, waste gas washing equipment which can be used by various producers in jewelery workshops are produced.In gold processing, possible waste management routes include new production methods like selective ion exchange with subsequent refining of gold.But classical methods described above and precipitation of the air contaminants via waste gas washing equipment is more common in the industry.Also gas washing systems are prefered in terms of easy and low cost operation (http://eippcb.jrc.ec.europa.eu/reference/BREF/stm_bref_0806.pdf).
In this study, the efficiency of removing the harmful emissions in the waste gas stream of two different systems manufactured by Permaksan Machine / İstanbul firm have been investigated.In the first phase of the study, removal efficiency of the acid mists originated from nitric and sulfuric acid used in the iron copper solvation process were investigated by means of PRM001 Iron / copper solvation and waste gas washing equipment.In the second phase of the study, removal efficiency of the cyanide emissions in the waste gas resulting from the treatment of gold with cyanide was investigated by means of PRM002 bombing and waste gas washing equipment.In this context, pollutant parameters were measured in waste gas inlet and washed gas outlet streams of both devices and the removal efficiencies of the devices were determined and compared with literature and limit values given by Turkish Industrial Air Pollution Control Regulation Annex 7. Also the measures to be taken for the continuity of the removing efficiency of devices are given as conclusion.

Iron / Copper Solvation And Waste Gas Washing Equipment
The process of iron and/or copper solvation in jewelery workshops is based on heating the materials which are covered with gold in different setting and contains iron or copper inside as well in the acid environment.Thus, iron or copper is absorbed into the solution, so that only the outer gold part of the materials are obtained and used to form ornamental products.In this process, nitric acid (HNO 3 ) is used as an acid for solving copper, and sulfuric acid (H 2 SO 4 ) is used for iron solvation.In practice, 150 mL technical grade nitric acid is added to 1 liter of water for copper solvation, and an acid solution is prepared by using technical grade sulfuric acid at the same rate in iron solvation.In this way, the acid prepared in the desired volume with iron/copper solvation material in it is heated with electrical resistors to 80-90 °C without boiling (https://www.dtsc.ca.gov/.../Jewelry/.../HWM_FS_Jewelry_P2_Recommendations.pdf,http://www.who.int/ipcs/publications/cicad/en/cicad61.pdf,Alp et. al., 2007).
Solvation and waste gas washing process at the workshop where the test is performed have been done by the equipment named PRM 001 manufactured by Permaksan Machine/Istanbul.This equipment is made from polypropylene material and consists of two basic parts which are acid boiling unit and waste gas treatment unit.Acid boiling unit is a closed system that can be opened at the front, with a transparent cover, two 20 liters capacity acid tanks in the side and one washing tank in the middle.Diluted HNO 3 and H 2 SO 4 solutions are obtained by adding 15 liters of water to 1.5 liters of acid in accordance with the above rates.In the process of iron solvation, about 5 kg of ferrous gold material can be processed with the acid mixture in the tank and 2.5 kg gold can be obtained.In other words, 2.5 kg of iron are dissolved.As a result of this operation, the acid solution in the tank is changed because of its strength loss.Off duty acids and washing waters are collected in a waste acid storage tank to which this unit is connected.The flow chart of the system is given in Figure 1.PRM 001 does not carry gold recovery from dross, which is carried out with acid mixture of HCl which is known as goldwater or royal water and HNO 3 (Alp et. al., 2007).
Sulfur dioxide (SO 2 ) gas and sulphate mists, which can be formed from sulfuric acid during the process; nitrogen monoxide (NO), nitrogen dioxide (NO 2 ) gases, nitric acid vapor, and mists that can come from nitric acid are transferred to the waste gas treatment unit which is the second unit, through the fan.This unit consists of a 150 liters capacity tank with acidic gas washing solution (Caustic soda (NaOH)/ Sodium carbonate (Na 2 CO 3 ) solution with pH = 13.8) at the bottom and two washing columns on it.Raschig rings are located inside the columns to increase the interaction of acid emissions with the washing solution by expanding the surface.With a pump located between the two columns, the absorbing solution is sprayed from the tops of both columns and absorbed by acidic gases in the opposite direction, thereby removing the acidic components.Caustic soda solution, which completes the washing process in the columns, returns to the storage tank again.The pH of the washing solution can be continuously controlled by means of a pH meter located inside the tank.Accordingly, necessary alkali additions can be made.Acidic gases are brought to the washing columns which is in connected in series to the first acid boiling section through a polypropylene fan and washed.After this process, the gases are treated and emitted to the air through the waste gas stack (Alp et. al., 2007).

PRM 002 Bombing and Waste Gas Washing Equipment
Bombing is the process of removing impurities on silver or gold material using potassium cyanide (KCN) and hydrogen peroxide (H 2 O 2 ).At this stage 30-60 g of potassium cyanide is dissolved in 30 liters of water to obtain cyanide solution.Hydrogen peroxide solution is prepared to 30-50 mL in 30 liters of water.Proper volumes are processed from cyanide and hydrogen peroxide solutions proportional to the amount of substance to be bombed.These quantities are related to the gold setting and surface size of the material to be bombed.Generally, 0.1 liter of hydrogen peroxide solution with 1 liter of cyanide solution is used for 0.5 kg of material with large surface.Reaction is stated below and is finished in a few minutes.
PRM 002 bombing and wastewater treatment equipment consists of two parts.In the bombing unit made of polypropylene material, there are cyanide and hydrogen peroxide solution storage tanks in the side; and in the middle, there is a bombing part covered with a transparent cover that opens and closes on the front side.Solution can be taken from the tanks in the desired volume and delivered to the bombing tank.The wastewater resulting from the bombing reaction is gathered in a collection tank located under this unit.At the top of the bombing tank there is a stack like connection for transporting the generated waste gas.
The second unit is the part where cyanide wastes are washed away.This part is also made from polypropylene material and is as described in PRM 001.Caustic or sodium carbonate solutions are also used in the washings in columns with raschig rings to increase the liquid / gas contact surface.The flow chart of the system is given in Figure 2 (Alp et. al., 2007).

Process of Iron/Copper Solvation
At the jewelery workshop, the iron solvation waste gases mainly contain nitrogen compounds as well as sulphate mists.Nitrogen compounds include NO x gases (NO + NO 2 ) and nitric acid mists.Two different methods have been used to determine the refining efficiency of PRM 001 equipment: • Firstly, NO x gases were measured using the electrochemical cell method in the gas stream before and after treatment process.
• Secondly, gas is removed by means of an appropriate pump from the air stream containing acid gases before and after the treatment stage.Contaminants in the gas phase were absorbed by two interconnected gas absorbing bottles filled with 0.1 N sodium hydroxide solution.Then samples were analysed in the laboratory using standard analysis methods.

Bombing Process
In order to determine the efficiency of the PRM 002 unit, cyanide gas resulting from the bombing process was sampled from the gas stream at two points.First point was the inlet to the treatment system and second point was the outlet of the treated gas.For this purpose; the gas stream drawn by the pump means was passed through two interconnected washing flasks with 0.1 N sodium hydroxide absorption solution and analysed in the laboratory using standard analytical methods (colorimetric method).Descriptions of the methods used in the measurements are given in Table 2 below.

Measurements Made in PRM 001 Equipment Gas Phase
At the sampling points opened in the inlet and outlet pipes of the gas absorption device, waste gas velocities, gas temperature and flue gas flow rate were measured and given in Table 3.
Gas measurements were made according to the electrochemical cell method and the results are given in Table 4 below.In this context, the oxygen values in the waste gas have a saturation value of 21%, so the measured values are taken as they are.

Measurements Made with Absorption Mechanism
At the input and output of the device, the gas streams were separately adsorbed by passing them through an alkali solution (NaOH), and SO 4 -2 and NO 3 -were measured in the obtained input and output absorption solutions.In the experiment, absorption was carried out using 250 mL of 0.1 N NaOH solution on two successive gas absorbing bottles.In this case, the total amount of absorption solution is 0.5 liters.These substances were also measured in the original solution as a blank used in the absorption to control.The value found is subtracted from the value found for the samples to avoid interferences.For each experiment, flow rates of the gases that passed through the gas absorption bottles were measured (Eaton et. al. 2005 ).

Efficiency Calculations in Absorption Process
The calculation of treatment efficiency of HNO 3 and H 2 SO 4 acid emissions obtained on PRM 001 device is given in Table 5.

Measurements Made in The Gas Phase of PRM 002 Equipment
Flow rate measurements of PRM 002 device used for treatment of waste gases are given in Table 6 and the calculation of treatment efficiency is given in Table 7.In terms of wet scrubber systems, removal efficiencies vary for each pollutant, solvent and type of gas absorber used.While the most absorbers have removal efficiencies in higher than 90%, the packed tower absorbers may achieve efficiencies as high as 99.9% for some pollutantsolvent systems.According to literature, removal efficiencies of sulphuric acid mists varied between 76.8-89.9 in packed bed towers related with the gas flow rate and liquid/gas mass ratio (Jafari et. al., 2002).According to another literature, spray dryer absorber systems have efficiency much more than 98% on SO 2 , SO 3 , HCI and HF removal from waste gas streams (https://www.babcock.com/-/media/documents/products/pollution-control/so2-acid-gases/e101-3221_so2-and-acid-gascontrol.ashx?la=en&hash=C7A3B2F80858E0759EFE65ED21392328F0E0742B).
A bench-scale study was conducted on the simultaneous removal of SO 2 , NO X , and mercury from a simulated coal flue gas using a wet calcium carbonate scrubber.The results showed a maximum scrubbing of 100% for SO 2 and Hg species and near complete NO oxidation with about 60% scrubbing of the resulting NO X species (Hutson et. al., 2008).When achievements of this study compared with literature findings, it can be seen that the removal efficiencies of the wet scrubbing systems were enough and output pollutant concentrations were below the limit values given in the Table 1.

EVALUATION AND CONCLUSION
Iron/copper solvation and waste gas washing equipment of PRM 001 device that belongs to Permaksan Company and bombing and wastewater washing equipment of PRM 002 which belongs to the same company have been examined for treatment efficiency.In this context, waste gas flow is determined for both equipment and samples are taken from the inputs and outputs of the treatment unit.
HNO 3 and H 2 SO 4 acids are used in iron and copper solvation in PRM 001 equipment.The acids are heated to 80-90 0 C with an electric heater without boiling.It was accepted that NO x gases (NO + NO 2 ) and nitric acid mists as well as SO 2 / SO 3 and SO 4 mists were generated as gas emission.Especially NO x gases are measured by electrochemical method and 75% efficiency value is obtained.It has been determined that only NO x emissions can be measured with this method, nitric acid mist can not be measured.Therefore this measurement does not show the actual efficiency value.In the measurement test made with 0.1 N NaOH absorption solution, treatment efficiency was found as 94.5%.In the removal of SO 2 /SO 3 and SO 4 mists, over 88% treatment efficiency was determined.For PRM 001 equipment this efficiency is in the expected range and can be regarded as an average value that can be achieved.The structure of the system is very compact and it has the potential to meet the needs of the sector and ensure the safety of work.With simple modifications to be made, efficiency can be further increased.PRM 002 equipment, which is used to control the bombing process emissions, also treat HCN gas emissions with an efficiency rate of 98.7%.This value is also an average and representative efficiency value for PRM 002.The volume control of the KCN and H 2 O 2 solutions is an advantage in terms of study design.
Removal efficiency results of the two systems were suitable with literature and output pollutant emissions were below the TIAPCR and NER limit values.It is important that the equipment has the necessary control opportunities to maintain the supply they have at the beginning of the process.This may be possible if the absorption capacity of the absorption solution (sodium hydroxide or carbonate) is always maintained.This requires the continuous monitoring of the pH values of the solutions in the equipment and addition of fresh wash solution without allowing the pH to fall below a certain value.In this way, it is possible to maintain the above-mentioned efficiencies during operation.In this scope; PRM 001 has this advantage which is having a pH meter on it that allows continuous control of the effectiveness of the washing solution.On the other hand, PRM 001 and PRM 002 also provide an advantage in terms of control and refining of wastes since they have wastewater collecting structures for weakened washing solutions and accumulating wastewater resulted from washed substances.