ROLE OF V.V.MINERAL IN REDUCING GLOBAL WARMING THROUGH GREEN MINING TECHNOLOGY OF GARNET

T Anitha and K Nithiya Kalyani^*

Beach Mineral Producers’Association V.V. Mineral Environmental Laboratory, Tamil Nadu, India.

*Corresponding Author:

K Nithiya Kalyani
Beach Mineral Producers’Association V.V. Mineral Environmental Laboratory, Tamil Nadu, India.
E-mail: nithya@vvmineral.com

Received date: 01 August, 2017; Accepted date: 23 May, 2018

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Abstract

According to EPA, 21% of global green house gas emission is from industry sector and mining sector is one of the major emitters of green house gases. Scientists are certain that climate change effects are expected to increase in the coming decades and urge nations to implement mitigation measures. Implementation of Green technology at industrial level aid in reduction of global warming, green house effect, pollution and climate change. Present study aims to explore the importance of green mining of garnet and garnet based abrasive water jet cutting in reducing green house gas emission and climate change effects. M/s V.V.Mineral implemented two common sense steps manual mining and solar drying to address the challenge of climate change in mining and beneficiation of garnet. The case study finding shows manual mining operation adopted by M/s V.V.Mineral for garnet sand mining is green and completely reduced the emission of 0.893 - 1.19 kg CO2/ton sand, normally emitted through mechanised mining process practiced in the area. Implementation of solar drying in the beneficiation process results in elimination of 29.67 - 32.36 Kg CO2 emission by every ton of sand dried in fossil fuel based driers. Garnet is the commonly used abrasive around the world. Garnet based abrasive water jet cutting is an environment friendly green process. Since it is a cold process, all materials can cut without fuel combustion and heat generation process. This paper highlights the advantages of replacing thermal cutting process by garnet based abrasive water jet cutting in mineral fabrication sector to reduce air pollution in the form of fumes and gases and reduces CO2 emission and global warming.

Keywords

Garnet, Green mining technology, Solar drying, Abrasive water jet cutting, Global warming, CO₂ emissions

Introduction

Global warming is responsible for climate change effects worldwide and is recognised as a significant man made global environmental challenge. This warming is cumulative and irreversible on a time scale of centuries (Solomon, et al., 2008). Intergovernmental Panel on Climate Change reported sustained and unequivocal rise in global temperatures are being caused by increasing concentrations of greenhouse gases produced by human activities (IPCC, 2013). To limit global warming to meet the internationally agreed 2°C target level would require major efforts to reduce greenhouse gas emissions. While many view the effects of global warming to be more substantial and more rapidly occurring than others do, the scientific consensus on climatic changes related to global warming is that the average temperature of the Earth has risen between 0.4 and 0.8°C over the past 100 years. The increased volumes of carbon dioxide and other greenhouse gases released by the burning of fossil fuels, land clearing, agriculture, mining and other human activities, are believed to be the primary sources of the global warming that has occurred over the past 50 years. Scientists from the Intergovernmental Panel on Climate Change carrying out global warming research have recently predicted that average global temperatures could increase between 1.4 and 5.8°C by the year 2100. Changes resulting from global warming may include rising sea levels due to the melting of the polar ice caps, as well as an increase in occurrence and severity of storms and other severe weather events. Think green act green is the potential option for reducing the effects of global warming. Green mining is implementation of technologies, best practices and mine process as a means to reduce the environmental impacts associated with the extraction and processing of metals and minerals (Kirkey, 2014). M/s V.V. Mineral, a pioneer in Garnet mining in Southern India contributing 60% of India’s Garnet production took pivotal steps to reduce carbon foot prints in garnet mining and mineral processing through replacing conventional mechanized mining method and fossil fuel-based dryers by manual mining and solar drying techniques. Beach Placer deposits are the major source of world’s garnet production where garnet occurs as sand or gravel either alone or along with other minerals such as ilmenite, rutile and zircon (V.V. Mineral, 2014) with a concentration of approximately 30% and can be easily mined ( Olson, 2000, NSW Department of Primary Industries, 2007). Worldwide mining of mineral sand deposits is generally undertaken by using heavy earth moving machineries (Force, 1991) and the principal sources of emissions are from consumption of energy such as diesel fuel in mining equipment results in direct emissions of greenhouse gases carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). To combat the environment impacts of mechanized mining, V.V.Mineral implemented scraping and scooping method of manual mining for collection of garnet sand. The associated minerals in the garnet sand are separated and 99% pure garnet is produced by physical methods in the Mineral Separation Plant. To increase the mineral response to the magnetic and electromagnetic separators elimination of moisture content in the mined placer garnet sand is prerequisite and it is normally done by the use of dryers. Operation of dryers require diesel fuel consumption and ultimately combustion of carbon bearing diesel fuel results in the emission of greenhouse gases viz., carbondioxide (CO₂), methane (CH₄) and Nitrous oxide (N₂O). V.V.Mineral utilized the environment friendly renewable energy source solar energy for drying mineral sand to limit the environmental problem and GHG emissions associated with fossil fuel based dryers. Renewable energy is central to climate change mitigation effects. India is endowed with rich solar energy resource and the average intensity of solar radiation received on India is 200 MW/km square (megawatt per kilometer square) with 250–300 sunny days in a year. The intensity of solar radiation received in the coastal districts of Tamil Nadu varies between 5.6 – 6.0 KW/m square (Garud and Purohit, 2009) which is utilized by V.V. Mineral in the drying process to eliminate moisture content of the mineral sand. In the present investigation carbon foot print or CO₂ equivalent emission from the mechanized mining events using Front end loader and dryer operations in the Mineral Separation Plant is studied based on fuel consumption estimates and applying default emission factors (EPA, 2016). The greenhouse gas (GHG) value is expressed in emissions per ton of garnet sand and simple calculation method is used to estimate the percentage saving of GHG emissions by green technology implemented by V.V. Mineral compared to the fossil fuel based operations.

Strength and chemical inertness of garnet made it the preferred abrasive in water jet cutting industry. In fabrication sector metal working by thermal cutting process generate fumes and gases that can impact on the environment can contribute to climate change. Abrasive water jet cutting (AWJC) is a climate friendly metal cutting process which is a cold process uses environmentally benign water and abrasive as the cutting tool can cut through almost every material, including those that are too hard to brittle or too soft to be effectively cut with other technologies (Khan and Yeakub, 2011) without any negative impact on the environment. Garnet is the most acceptable abrasive worldwide. Garnet can be re-used after recycling. Recycling is simple and does not alter the chemical composition of abrasives and reduce costs by an estimated 1/3 of the total cost (Bajor, et al., 2012, Sobotova, et al., 2014, Melekhin, 2016). This paper focus on the importance of garnet based AWJ cutting process over other non contact thermal methods in reducing heat and smoke emission and products on the environment and climate change.

V.V. MINERAL

V.V. Mineral is a twenty five year old company dealing with Mining, Manufacturer and Exporter of Garnet, Ilmenite and associated heavy minerals have achieved significant market share all over the world. V.V. Mineral (VVM) is the only company in India with a 40 km stretch of beach area under an exclusive mining lease for 30 years in Tirunelveli, Tuticorin and Kanyakumari Districts, Tamilnadu, India (Fig. 1). The annual production of V.V. Mineral is 7,00,000 metric tonnes of heavy minerals which include Garnet Abrasive, Ilmenite, Zircon, Rutile, Sillimanite and Leucoxene. V.V. Mineral's product fetch huge overseas market due to its quality and high class management system implemented during every step of manufacturing process which was acknowledged by the ISO 9001: 2008, ISO 14001, OHSAS 18001 certificates issued by TUV Germany.

Materials and Methods

For cost and practicality reasons (IPCC, 2000), (World Resources Institute, 2005) WRI/WBCSD (World Business Council for Sustainable Development) GHG Protocol 2005, recommends the use of calculation based method of estimating direct Carbondioxide (CO₂), Methane (CH₄) and Nitrous oxide (N₂O) emissions from mobile and stationary sources. In the present study mining equipment front end loader GHG emissions are estimated based on EPA Greenhouse Gas Inventory Guidance for Direct Emissions from Mobile Combustion Sources (EPA, 2016). Mining equipment is categorised under non road vehicles. Based on the fuel consumption records and actual fuel heat content Table 1 CO₂ emission is calculated based on the follwing equation (Equation 2)

Table 1: Diesel oil characteristics based on fuel purchase records

Emissions = Fuel × HHV × EF₂

Where Emissions is the Mass of CO₂ emitted; Fuel represent Mass or volume of fuel combusted,

HHV is Fuel heat content, in units energy per mass or volume of fuel; EF₂ is CO₂ emission factor per energy unit.

CH₄ and N₂O emissions is calculated based on the equation (Equation 5)

Emissions = Fuel × EF₅

Where Emissions represent Mass of CH₄ or N₂O emitted, Fuel denotes Volume of fuel combusted

And EF₅ is CH₄ or N₂O emission factor per volume unit.

CO₂ – equivalent emission calculation

Carbon foot print "the total set of greenhouse gas (GHG) emissions caused by an event (Thurwachter, et al., 1998) is often expressed in terms of the amount of carbon dioxide, or its equivalent of other emitted GHGs, calculated as carbon dioxide equivalent (CO₂e) using the relevant 100-year global warming potential (GWP100) (IPCC, 2014). CH₄ and N₂O emissions are multiplyed by the respective global warming potential (GWP) to calculate CO₂ – equivalent emissions. The GWP are 28 for CH₄ and 265 for N₂O from the Intergovernmental Panel on Climate Change, Fifth Assessment Report (AR5), 2014. Sum the CO₂ equivalent emissions from CH₄ and N₂O with the emissions of CO₂ to calculate the total CO₂ equivalent ( CO₂ e) emissions.

GHG emissions from Dryer is calculated based on EPA Greenhouse Gas Inventory Guidance for Direct Emissions from Stationary Combustion Sources (EPA, 2016). CO₂, CH₄ and N₂O emissions from dryer is estimated using fuel analysis approach. The following equation is used to calculate CO₂, CH₄ and N₂O using appropriate emission factors.

Emission = Fuel × HHV × EF₂

Where Emissions is Mass of CO₂, CH₄ or N₂O emitted; Fuel is Mass or volume of fuel combusted;

HHV represents Fuel heat content in units of energy per mass or volume of fuel; and EF₂ is CO₂, CH₄ or N₂O emission factor per energy unit

GHG saving potential of the green technique adopted by VVMineral is calculated by the equation

GHG saving potential (%) = GHG emissions from

In the present study importance of garnet based abrasive water jet cutting in reducing greenhouse gas emission and climate change effects in metal fabrication sector over the non traditional thermal cutting process is presented based on open literature published reports and scientific findings.

Results and Discussion

Analysis of energy savings in green mining technology

In order to explain the importance of green mining technology of garnet in saving fossil fuel-based energy in mining operation, analysis on energy profile of mechanized and manual mining process were undertaken. Table 2 compares the amount of energy interms of diesel oil input and their energy equivalent for one-hour mechanized mining and manual mining event for garnet sand. It was observed that front end loader based mechanical garnet sand mining activity (Fig. 2) utilized nine litres diesel oil to expend 331.430 MJ/hr energy required to produce 25-ton garnet sand per hour. On the other hand V.V.Mineral achieved the same task by manual mining through field works of 100 man –hr. From the study it is evident that V.V.Mineral intelligently replaced this fuel based operation which is the greatest part of total energy input of the mining operation by manual mining. Since garnet sand is present on the surface requires less effort to acces. V.V.Mineral collect the garnet sand using hand showls and bucket (Fig. 3) results in a human power usage of 87.864 MJ/hr. From Table 2 it is clear that in manual mining process to achieve the same tonnage garnet sand production per hour as mechanized mining event a total of 87.864 MJ manual energy is expended which is 3.77 times lesser than the energy utilized in mechanized mining. This makes an overwhelming choice for manual mining by V.V. Mineral to replace the diesel fuel based mechanized operation and made the garnet mining operation green.

Table 2: Input Energy profile of Mechanized mining and manual mining operations

Similar studies for bauxite mining reported after topsoil and overburden removal the loosened bauxite ore excavation process using surface mining equiment requires 54.34 MJ energy/ton bauxite excavation (Griffing and Overcash, 2010) which is 4.1% higher than garnet sand mechanized mining process suggest that garnet sand mining is very simple process and requires less energy when compared to other surface mining activities and pave way for manual mining idea of V.V.Mineral.

Analysis of greenhouse gas emissions mitigation potential of green mining technology

Based on the energy input data of mechanized and manual mining events Table 2 the corresponding GHG emissions was estimated and presented as Table 3. Results showed every ton garnet sand produced in the diesel fuel based mining equipment resulted in 0.9293 Kg CO₂, 0.0542 g CH₄ and 0.0247 g N₂O emissions whereas no such emissions are accounted in manual mining operations. Manual mining operation is having 100% GHG saving potential and significantly contribute to mitigate the carbon foot print of mechanized mining operations.

Table 3: Greenhouse gas emission profile of mechanized and manual mining operation

It is estimated Carbon foot print of mechanized mining event is 0.93737 Kg CO₂ e/ton garnet sand and yearly garnet sand mechanized mining activity will emit an average of 46.8685 tonnes CO₂e into the atmosphere.

From the study it is evident that V.V.Mineral manual mining operation is environmentally sustainable and contribute a solution to climate risks by reducing carbon foot print. Based on the GHG emissions results of mechanized mining event it is concluded that the 25 ton/hr capacity mine of V.V.Mineral by utilizing manual energy for garnet sand mining has contributed to a total estimated GHG emission reductions of 234.3425 tonnes CO₂e over the past five years.

Contribution of v.v.mineral Manual mining operations to Sustainable development

Social and economical benefits

Manual mining operation of the 25 ton/hr capacity mine has provided direct employment to 100 persons and indirect employment opportunities to about 250 persons in and around the project area where seasonal fishing and palmyrah climbing has been the avocations. Since V.V.Mineral implemented the scraping and scooping method of manual mining the operation requires less effort and 75% of employment opportunity is given to women to address poverty reduction and aid in achieving the Millennium Development Goal of Promote gender equality and empower woman. In addition V.V.Mineral has established an Engineering college for the upliftment of Education among rural village people and a free Health Centre for the benefit of surrounding villagers.

Environmental benefits

The project utilizes manual energy for mining which otherwise would have been generated through fossil fuels thereby contributing to the reduction in greenhouse gas emissions. Over the past five years the estimated amount of GHG emission reductions by the 25 ton/hr capacity mine through manual mining activity is 234.3425 tonnes CO₂e. In addition the intense effort undertaken by V.V.Mineral to grow green belt (Fig. 4) in the periphery of the mine area improves the environmental conditions and aesthetic beauty of the area.

Analysis of energy savings in garnet sand green drying process

The energy profile study of rotary dryer and solar drying process illustrated the importance of solar drying technique adopted by V.V.Mineral in saving fossil fuel based energy. The results of the present study showed Table 4 1841.2345 KW.h energy is required for uninterrupted operation of dryer to eliminate moisture content and to produce 15 ton of dry garnet sand/hr, which is derived by combustion of 180 litres diesel oil/hr ultimately resulted in environmental pollution and GHG emissions. The inexpensive method of solar energy utilizationnatural open air solar drying method adopted by V.V.Mineral has the potential to replace the fossil fuel based dryer operation. Natural solar energy of 5.8 KW/sq.m available in the rainshadow region 300 days in a year is brilliantly utilized by V.V.Mineral and produced the same tonnage output as dryer without any environmental impact. The present study exhibit that V.V.Mineral drying yard represents ambient air drying technique using atmospheric heat. The natural daily average solar radiation of 20.88 MJ/m² available in the drying yards is utilized to remove moisture from garnet sand. The ambient air temperature in the drying yards ranged between 26 – 45°C and the mean daily maximum temperature recorded is 37.1°C. For even drying in the yards wet surface of garnet sand is exposed to the drying medium (sun) by mixing up together every now and then using hand scooper. V.V.Mineral achieved 100% moisture removal in every 2.54 cm height layer of sand in the yard within 5 hours.

Table 4: Input Energy profile of Rotary dryer and Natural Solar dring operations in 15 ton/hr capacity plant

Present study inferred that V.V.Mineral drying yards performance is similar to fixed type bed drying process. (Bengtsson, 2008) reported experimental fixed bed drying tests of biomass layer with a depth of 0.4 to 0.6 m requires a drying time of 5 hours for saw dust and ten hours for wood chips at an operating temperature 80 -110°C suggest performance of V.V.Mineral drying yard of garnet sand layer is superior than fixed bed drying process because in fixed bed drying the operating temperature is high and derived from fossil fuels combustion process which ultimately results in greenhouse gas emissions but V.V.Mineral achieved moisture removal within 5 hours with green renewable solar thermal energy due to the unique characteristics of garnet sand. Since garnet sand is inert and non porous material moisture is not drawn inside the sand hence it is very easy to remove the moisture adhere on the surface of garnet sand using natural solar radiation at a maximum avarage daily temperature 37.1°C. V.V.Mineral replaced the fossil fuel based dryer operation with solar drying and achieved 100% moisture removal without any environmental impacts. Another advantage of solar drying process is it minimizes the surface coating on minerals therby increase the mineral response efficiency by 5 to 10% compared to dryer produced garnet sand in the Mineral Separation Process.

Analysis of green house gas emissions mitigation potential of green solar drying technology

The study illustrated dryer operation is extremely energy intensive and contributes to the emission of greenhouse gases. Based on the input energy data of rotary dryer events Table 4 the corresponding GHG emissions was calculated. The emissions from diesel combustion from the dryer equipment is calculated to be 30.6797 Kg CO₂, 1.2565 g CH₄ and 0.2513 g N₂O/ ton garnet sand Table 5 whereas no such emissions are accounted in natural solar drying operations. Solar drying operation is having 100% GHG saving potential.

Table 5: Greenhouse gas emission details from Rotary dryer and Natural solar drying operation for producing one-ton dry garnet sand

The carbon foot print from fossil fuel diesel based drying process is 30.7815 Kg CO₂e/ton garnet sand and yearly fossil fuel based drying activity will emit an average of 923.445 tonnes CO₂e into the atmosphere. The study highlighted that V.V.Mineral solar drying technique is climate friendly and contribute a solution to climate risks by reducing carbon foot print. Based on the GHG emissions results of rotary dryer event it is concluded that V.V.Mineral 15 ton/hr capacity Mineral Separation Plant by utilizing solar energy for garnet sand drying has contributed to a total estimated GHG emission reductions of 4617.225 tonnes CO₂e over the past five years. It is inferred from the present study that solar drying technique make significant contribution in the mitigation of CO₂ emissions as a result of fuel switching.

Contribution of v.v.mineral solar Drying operations to sustainable Development

Environment benefits

V.V.Mineral Solar drying activity is in consistent with sustainable development policies of Government of India. Solar drying reduces the stress on Environment by eliminating the demand for fossil fuel based dryers and associated greenhouse gas emissions. Solar drying process produce no GHG emission and V.V.Mineral 15 ton/hr capacity Mineral Separation Plant has contributed to a total estimated GHG emission reductions of 4617.225 tonnes CO₂e over the past five years. Being a renewable resource, using solar energy for drying process contributes to dwindling fossil fuel resource conservation.

Garnet – Climate friendly green mineral

Garnet is a climate friendly mineral that donot harm environment during its usage and disposal in industrial process. Climate-friendly goods refer to those the utilization of which reduce climate risks to a greater extent than alternative products that serve the same purpose (Zhang, 2011). Globally 35% of refined almandine garnet is used by Abrasive Water Jet Cutting (AWJC) industry (Fig. 5) and blasting, water filtration and abrasive powders are the other end uses of garnet (Olson, 2014). In all such industries garned based operations are found to be green and climate friendly due to its natural inertness and perfect hardness provides excellent process efficiency without any environmental emissions. Another green property of garnet is the spent garnet can be disposed as land fill and this inert mineral does not pose bioaccumulation or waterway and food chain contamination problems. Natural inertness of garnet provide the advantage of reuse of garnet 5 to 8 times in industrial process and reduces resource usage. Hence usage of garnet in industrial process is green which is cost effective, curb environment impacts and reduces the impact on climate change.

Importance of Garnet in Abrasive Water Jet Cutting (AWJC)

Garnet is the extensively used abrasive in all renowned water jet cutting machines around the world. Almandine garnet, a naturally inert mineral, with hardness between 6.5 and 7.5 on the Mohs scale and a density of 3.9 to 4.1 g/cm3 is the single specific abrasive dominate the abrasive market (Mort, 1995) (Fig. 6).

Performance of AWJC operations are influenced by the abrasive characteristics such as hardness, density, particle form, degree of purity and size distribution (Ohman, 1993; Waterjets.Org, 2014). Even though a large number of materials represent all the aforementioned characteristics (Fig. 7), the abrasive market seems to be dominated by a single, specific abrasive: Almandine Garnet. The right combination of hardness, density, toughness and particle size in the garnet maximizes water jet cutting performances and its preference around the world (Fig. 8).

Profitability of AWJC relies on selecting the best abrasives which impart good cutting speed, edge quality and component life. Soft abrasives silica sand, quartz etc provide better component life but poor cutting speed and hard abrasives aluminum oxide and silicon carbide offers fast cutting but cause an increased wear of the nozzle and mixing chamber leads to poor accuracy and frequent nozzle replacement (Kulecki, 2002). Almandine garnet falls between 6.5 and 7.5 on the Mohs scale, which effectively balances the need to cut quickly and provide reasonable cutting tool life. Millions of years of weathering, the hard and tough sub-angular garnet grains provide the perfect balance between cutting speed and edge quality in water jet cutting process and made garnet as a very good and popular abrasive. Garnet – a hard brittle crystalline mineral is recognized as the world standard abrasive for water jet cutting (Carborundum Abrasives – Div. Saint – Gobain Abrasives).

Merits of replacing thermal cutting methods by garnet based abrasive water jet technology

All over the world manufacturing industries demand metal cutting technology for faster and precise cutting to withstand business competition. Garnet based abrasive water jet cutting came at first to fulfill the demand. Extensive studies were carried out regarding performance comparison of Garnet based abrasive water jet cutting with other cutting methods (Zeng, et al., 1991, Ohlsson, et al., 1994, Powell, et al., 1995; Akkurt, 2002, Akkurt, 2009; Akkurt, 2014; Alsoufi, 2017) and all the research approved garnet based abrasive water jet cutting system to be the most efficient method when compared to Oxy fuel cutting, Plasma arc cutting and laser cutting which are said to be competitors and the merits are presented here under.

Garnet based abrasive water jet cutting – A versatile technology

Garnet based abrasive water jet cutting is a versatile process than the other thermal cutting methods because it has no restriction on cutting material and shapes. This method can cut any type of material thin or thick, soft or hard viz., steel, aluminum, glass, wool, plastics, laminates, ceramics, composites, metals, stones, marbles etc. with good accuracy. But thermal cutting process has restrictions on type of material and material thickness as provided in Table 6. From the comparison table it is obvious that Garnet based abrasive Water jet cutting is the most positive method which cut any type of material upto 350 mm thickness with good accuracy and can effectively replace other thermal cutting process.

Table 6: Cutting characteristics of garnet based abrasive water jet cutting compared with thermal cutting methods

 

Garnet based abrasive water jet cutting – Environment friendly cold process

Thermal cutting processes release intense heat and substantial quantity of fumes and smoke due to vaporizing metals in the kerfs which has direct environment concern Table 7. Effects of these chemical pollutants can be completely reduced by replacing thermal cutting processes with garnet based abrasive water jet cutting technique. It is an alternate continuous cold cutting process where abrasive garnet particles are dragged into the water stream and impelled at high speed against the target to get desired shapes. As the process is provided by water, heat and smoke emission is completely reduced and it does not create any grinding dust or chemical pollutants (Hascalik, et al., 2007) and it can be stated that abrasive water jet cutting is the only nonpolluting method available for machining materials to precision details.

Table 7: Environment friendly characteristics of garnet based abrasive water jet cutting compared with thermal cutting methods

Garnet based abrasive water jet cutting – provides perfect surface quality

In metal cutting sector the effectiveness of different cutting process is evaluated by measuring the surface roughness of cutting section. Thermal cutting process affects the surface quality of the cut material where edges exhibit a rough surface caused by the action of cutting jets. All thermal cutting process showed heated affected zone (HAZ). Structural deformation is obvious in thermal cutting process and the changes in microstructure and hardness can be attributed to heat input during cutting process (Akkurt, 2009). On the other hand garnet based abrasive water jet cutting is a unique process that has no adverse effect on the microstructure of the cut material. As the process location in the water which accelerates the removal of heat from the treatment zone, this cutting technology release no heat effect and structural deformation problem on the cut material and generates satin smooth surface (Ojmertz, 1994, Hlavacek, et al., 2012). In garnet based abrasive water jet cutting process the original structure of cut material is preserved (Akkurt, 2009). Thermal cutting methods generate melted edges and require secondary process Table 8 to get clean surface which increases the production cost suggesting abrasive water jet cutting is superior in industrial application for producing perfect quality cutting than other thermal cutting process.

Table 8: Surface quality comparison of garnet based abrasive water jet cutting compared with other cutting methods

Garnet based abrasive water jet cutting – An ecological process

Different cutting process requires specific tools to perform effective cutting. In thermal cutting process cutting tools are constricted by utilizing either electric or fuel gas energy sourced from nonrenewable sources. Hence all thermal cutting process utilizes dwindling fossil fuel resources for generating the cutting tools Table 9 which is ecologically unsafe and huge quantity of fossil fuel usage in thermal cutting process will end up in resource depletion. But in garnet based abrasive water jet cutting the cutting "tools," are water and garnet abrasive which are the resources abundantly available in Earth hence Garnet based abrasive water jet cutting process is ecologically safe never results in resource depletion.

Table 9: Comparison of garnet based abrasive water jet cutting compared with other cutting methods based on ecological effects

Garnet based abrasive water jet cutting – A climate friendly process

Among the different cutting methods garnet based abrasive water jet cutting is climate friendly technology does not release harmful gases, UV radiation other hazardous substances harmful to environment and machine operators. Mixing of water and garnet abrasive do not emit any toxic vapor or unpleasant odor. The abrasive waste generally is inert garnet with a small fraction of particles from the material being cut. Unless the material being cut is a highly hazardous material, such as lead, the waste abrasive can be disposed of safely at a dry landfill, with no processing and no environmental risk (Arleo, 2010). The classification of the spent abrasive as inert waste makes its land filling an economically attractive option. Hence garnet based abrasive water jet cutting is the best method for processing engineering materials.

Garnet based abrasive water jet cutting curbs GHG emissions

Among the thermal cutting processes oxy fuel cutting is more hazardous and impacts the environment by emissions of green house gases during fuel combustion process. In this process a mixture of oxygen and the fuel gas is used to preheat the metal to its 'ignition' temperature which, for steel, is 700°C - 900°C (bright red heat) but well below its melting point. A jet of pure oxygen is then directed into the preheated area instigating a vigorous exothermic chemical reaction between the oxygen and the metal to form iron oxide or slag. The oxygen jet blows away the slag enabling the jet to pierce through the material and continue to cut through the material (DeGarmo, et al., 1999). The common fuel gases used for preheating the metal in oxy fuel cutting process are Acetylene, Propane, Propylene, natural gas which are hydrocarbons, and their combustion in oxy fuel cutting process leads to CO₂ emissions. Incomplete combustion of this fuel gas leads to other greenhouse gases methane and nitrous oxide emissions.

In order to explain the role of garnet based abrasive water jet cutting process in reducing green house emissions comparison of GHG emissions from oxy fuel cutting and garnet based abrasive water jet cutting process in cutting per foot of length steel pieces were undertaken based on the Fuel gas characteristics and the GHG emission factors of different gases Table 10 and the results are presented as Table 11.

Table 10: Fuel gas charcterisitcs and combustion emission factors

Table 11: Comparison of GHGs emissions from steel cutting by garnet based abrasive water jet cutting and oxyfuel cutting with different gases

From the Table 11 it is evident that fuel gas is not a cutting tool in garnet based abrasive water jet cutting process but in oxyfuel cutting fuel gas play a vital role in cutting process and the findings showed all the fuel gas used for oxy fuel cutting operation results in GHG emissions and the CO₂ equivalent emission of propane gas is the highest (9.978 Kg CO₂ e/hr). The environmental effects of oxy fuel cutting can be mitigated using abrasive water jet cutting. From an environmental perspective, Abrasive water jet cutting is a cold process, uses environmentally benign water and abrasive as the cutting tool shall not impose any negative impact on the environment. Hence AWJ can be an ideal climate friendly alternative to oxy fuel cutting operations in metal cutting industry.

Garnet based abrasive water jet cutting curbs environment pollution

Thermal cutting processes separate materials by applying heat, with or without a stream of cutting oxygen. All cutting operations releases intense heat, fumes, smoke and atmosphere contaminants. The most common gases emitted during thermal cutting process are ozone, nitrous gases and carbon monoxide. According to the Fifth Assessment Report of the IPCC (IPCC,2014) nearly all the non - CO₂ climate - altering pollutants are health damaging, either directly or by contributing to secondary pollutants in the atmosphere’. The hazards associated with different cutting process are presented as Table 12.

Table 12: Hazards associated with different cutting process

Table 12 illustrated garnet based abrasive water jet cutting is a reliable process due to the cold nature of the process does not vaporize the cut section and avoid fumes and smoke emission and associated hazards. Due to high temperature during the thermal cutting process, different substances in the cut material are vaporized. Then, the vapor condenses and oxidizes in contact with the air, leading to the formation of fumes. Fumes and gases generated during cutting materials consist primarily of metal oxides from cut material and other contaminations present in the cutting consumables, surface coatings and within the atmosphere. Cutting of stainless steel is potentially the most hazardous as the fumes will contain chromium and nickel. Copper and its alloys are also commonly cut and can also produce a significant fume hazard. Oxides of nitrogen are formed during thermal cutting and could accumulate in areas of poor ventilation. These are likely to be most significant during plasma cutting where air or nitrogen is used as the plasma gas. Ozone is most likely to be formed during cutting of aluminium or stainless steel. Where inert gases are used they may accumulate in confined spaces causing an asphyxiation risk. This is most likely to occur when the gas is significantly heavier than air, eg argon/nitrogen mixtures. The metal fume emitted from plasma cutting consist of carcinogen hexavalent chromium (Cr6+), and other toxic heavy metals Table 13. Overexposure to thermal cutting fume may cause pulmonary toxicity and other health effects (Wang, et al., 2017). Metal oxides and gas contaminants released by different thermal cutting process is illustrated in Table 13.

Table 13: Potential air emissions from different cutting process

Many of these contaminants fall within the scope of the Control of Substances Hazardous to Health Regulations (COSHH) 2002 (Amendment) 2004 and have permissible exposure limits (PEL) set by the Occupational Safety & Health Administration (OSHA) Table 14.

Table 14: DNA/methyl green IC₅₀μg/ml of H₂L and its metal complexes.

Air pollution is considered as a threat to human health as well as to the Earth's ecosystems. Based on WHO report, around 7 million people worldwide died due to the air pollution in 2012 (WHO, 2014). Replacing thermal cutting process with garnet based abrasive water jet cutting totally curbs the problem of hazardous fumes and air pollution. Climate-friendly aspects of water jet cutting process is illustrated in Table 13 which highlighted no direct air emissions from Abrasive water jet machining operations. The advantage of this method is that it is a cold process. All materials can cut without any heat generation. Hence, unwanted poisonous gas formations are avoided. Having no negative effect on environment, Abrasive water jet cutting can be used as alternate to thermal cutting process in metal and other industries to avoid global warming.

Conclusion

From the present study it is concluded that green mining of garnet undertaken by V.V.Mineral in the coastal districts of Tamil Nadu, India through offering employment to local community has great potential to mitigate greenhouse gas emissions from mechanized mining process. Manual mining carried out by scrapping and scooping method using hand showls and bucket is efficient in garnet sand collection and save 13.257 MJ fossil fuel energy and mitigate 0.9293 Kg CO₂, 0.0542 g CH₄ and 0.0247 g N₂O emissions per ton garnet sand . The calculated global warming potential of mechanized mining operation expressed in CO₂ equivalent unit is 0.93737 Kg/ton of garnet sand using diesel as fuel for a frontend loader equipment which is completely mitigated by green mining technology. Over the past five years the estimated amount of GHG emission reductions by the 25 ton/hr capacity mine through manual mining activity is 234.3425 tonnes CO₂e.Solar drying yards of V.V.Mineral is a brilliant idea which effectively utilized the atmospheric heat for moisture removal from garnet sand. The beach area registered a mean daily maximum temperature of 37.1ºC for 300 days and this heat energy is effectively utilized to replace dryer-based moisture removal process and save 30.6797 Kg CO₂, 1.2565 g CH4 and 0.2513 g N₂O emissions/ton garnet sand. The calculated global warming potential expressed in CO₂ equivalent unit is 30.7815 Kg/ton of garnet sand using diesel fuel for rotary dryer equipment which is potentially mitigated by solar drying technique and V.V.Mineral 15 ton/hr capacity Mineral Separation Plant has contributed to total estimated GHG emission reductions of 4617.225 tonnes CO₂e over the past five years. Garnet is the world standard abrasive. Unique hardness and chemical inertness of garnet provides perfect balance between cutting speed and edge quality in abrasive water jet cutting process hence among the various natural and synthetic abrasives garnet is the most preferred abrasive in Abrasive water jet cutting process. Information on climate friendly aspects of garnet based abrasive water jet cutting process over the other non-conventional metal cutting process is presented based on literature review from which it is understood oxy fuel cutting is more hazardous and release GHGs during fuel combustion process. Among all common fuel gases, CO₂ equivalent emission of propane gasbased cutting is the highest which is 9.978 Kg CO₂ e/hr. The environmental effects of oxy fuel cutting can be mitigated by replacing this thermal process with garnet based abrasive water jet cutting. More over all thermal cutting operations releases intense heat, fumes, smoke and atmosphere contaminants leads to air pollution. Gases emitted during thermal cutting process are ozone, nitrous gases and carbon monoxide which have intense health effects apart from contributing to secondary pollutants in the atmosphere’. The metal fume emitted from plasma cutting consists of carcinogen hexavalent chromium (Cr6+), and other toxic heavy metals. Overexposure to thermal cutting fume may cause pulmonary toxicity and other health effects. Environment pollution and work place contamination hazards related to thermal cutting process will be curtailed by replacing the thermal cutting process with garnet based abrasive water jet cutting which is a climate friendly green process. The advantage of this method is that it is a cold process; produce most accurate, fast and superior surface quality without any environmental hazards. As the process is provided by water heat and smoke emission is completely reduced and it does not create any grinding dust or chemical pollutants and it can be stated that replacing thermal cutting process by garnet based abrasive water jet cutting will reduce air pollution in the form of fumes and gases and reduces CO₂ emission and global warming.

Acknowledgement

The authors would like to acknowledge the support and interest afford by Thir.S.Vaikundarajan, The Founder, Chairman and Managing Director of V.V.Mineral. His generous help enable us to complete this paper work.

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