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Article

Economic and Environmental Aspects of Applying the Regeneration of Spent Moulding Sand

by
Mariusz Łucarz
Faculty of Foundry Engineering, AGH University of Krakow, Reymonta 23 St., 30-059 Krakow, Poland
Sustainability 2024, 16(19), 8462; https://doi.org/10.3390/su16198462
Submission received: 12 August 2024 / Revised: 26 September 2024 / Accepted: 27 September 2024 / Published: 28 September 2024
(This article belongs to the Section Waste and Recycling)
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Abstract

:
This article presents issues related to the rational management of foundry sand in the context of sustainable development. Attention was drawn to the need to take appropriate measures to protect available natural deposits of good foundry sands in terms of their depletion. The main objective of the analyses undertaken was to find out whether more expensive but more efficient thermal regeneration can compensate for the higher energy consumption in relation to mechanical regeneration of spent moulding sand with an organic binder. This aspect was considered from the point of view of the multiple operations performed to clean the grain matrix from the spent binder, taking into account the direct and indirect costs of the process. This paper presents a comparative analysis of the mechanical and thermal regeneration of spent moulding sand on equipment offered by an exemplary manufacturer. Attention was drawn to the successively increasing price of the regeneration process. When analysing the grain matrix recovery process for sustainability reasons, attention was drawn to an important factor in grain matrix management related to its yield in different regeneration methods. Based on an analysis of the costs of regenerating 1 tonne of spent moulding sand, it was concluded that, in the long term, thermal regeneration, which is more expensive due to the cost of equipment and energy consumption, can offset the outlay incurred. Sand consumption was found to be 4.6 times higher by mechanical regeneration in the case studied. At the same time, the grain matrix after thermal regeneration was found to have significantly better and more stable technological parameters in subsequent sand mould preparation cycles. The reproducibility and stability of the technological process can also be an important component of economic growth as part of sustainable development.

1. Introduction

The process of casting metal alloys requires the preparation of a mould and cores from refractory materials. The basic material that guarantees high refractoriness is foundry sand [1]. Quartz sand is the most widely used for moulding sand preparation, but chromite sand, zircon sand, and olivine sand are also used. Binders of natural or synthetic origin are used to bind them and create the shape of the mould. The binding material present in the environment is clay, especially kaolinite and montmorillonite (bentonite), which are added in amounts of up to 8% in relation to the grain matrix and water. The moulding compound obtained in this way is called “green moulding sand”. The need to thicken such moulding sand to give it the required strength and sometimes dry it prolongs the manufacturing process and the cost of production.
Increasingly, organic binders—chemically bonded synthetic resins with hardeners or catalysts—are being used due to increased production efficiency. Their share in relation to the grain matrix currently does not exceed 2% [1,2,3]. After being set, the mould reproducing the shape of the component to be produced is flooded with liquid metal. Depending on the casting alloy, as a result of contact between the moulding sand and the liquid metal at temperatures ranging from 660 °C (aluminium alloys, silicas) to over 1500 °C (cast steels), the organic binder burns out to varying degrees. The temperature degradation of the organic compounds at the metal–mould interface facilitates, after the casting alloy has cooled, its breakout from the mould. In publications [4,5,6], the authors presented thermal analysis studies to determine the mechanism of the organic binder degradation process in relation to the mould conditions. The temperature of onset of binder degradation and, at the same time, the temperature at which it begins to lose its binding properties, which promotes the process of removing the casting from the mould, have been determined. Since, in most cases, the moulding sand in the mould does not burn out completely, the process of producing castings using moulding and core sand technology generates large amounts of wasted moulding sand. There are essentially three ways to handle it. The first, which is the least effective, is to divert the spent moulding sand to landfills. The second is to use it in other production sectors, such as construction. In particular, in the production of building materials, the possibility of using moulding sand waste has been recognised. An area of research in this field is presented in publications [7,8,9,10,11,12]. On the other hand, the most effective way to manage waste is to reuse it within the company after a regeneration procedure. This mode of treatment is influenced by the development of low- or zero-waste technologies, where materials are used in a closed loop.
Sustainability is a global challenge from the point of view of production and consumption of goods and services [13]. The simple production model of extracting resources, transforming them into products, and finally disposing of waste is becoming insufficient due to the growing needs of people [14,15], especially in terms of economic development [16,17] based on productive or financial capital [18]. The issue of innovation related to the digital economy is also not insignificant for sustainable development [19,20,21,22].
Reclamation of spent moulding sand is a fundamental challenge from the point of view of sustainability [23,24]. It is a way to reduce the consumption of nonrenewable resources such as good foundry sands and, indirectly, to eliminate hazardous and toxic substances in landfills. Considering the three basic pillars of sustainable development—society, environment, and economy, which are inextricably linked—the issue of regeneration of spent moulding sand in industrial production seems to be crucial.
Currently, the development of the regeneration of spent moulding sand is influenced by legal regulations related to waste management, its appropriate segregation, and environmental protection. The availability of energy sources and their costs should also not be overlooked in recent times.
At the same time, as the authors of the film Sand Wars present [25], quartz sand has become an essential commodity for modern economies: it is used in toothpaste, detergents, and cosmetics, and computers and mobile phones could not exist without it. Houses, skyscrapers, bridges, and airports are basically made of quartz sand, with the result that it has become the most consumed natural resource on the planet, right after fresh water. The global construction boom fuelled by emerging economies and increasing urbanisation and the production of many consumer goods has led to the intensive extraction of sand on land and in the oceans, which has a detrimental effect on the environment and a devastating effect on natural resources for future generations. This is why the use of regeneration of spent moulding sand in one of the industries that exploit deposits of good foundry sand so intensively, where at the same time its multicyclical use is possible, is so important for sustainable development.
The development of the regeneration process is stimulated by ecological factors [6], among which are:
⎯
a reduction in the consumption of fresh moulding sands, very important in view of the depletion of deposits of these raw materials,
⎯
a reduction in the devastation of the area by sand mines, which are growing in size,
⎯
a reduction in the area of landfill sites occupied by spent moulding sand dumps,
⎯
reducing the transfer of dust and toxic substances from landfills into the environment,
⎯
protection of landscape values of the environment.
A separate issue for the development of reclamation methods is the economic aspect mainly related to:
⎯
reduction of transport—reduction of operating costs (fuel or energy needed for the transportation of fresh sand and the removal of used sand to the dumps, labour costs, etc.),
⎯
reduction in the costs of purchasing fresh sand and eliminating its drying,
⎯
reduction of landfill fees,
⎯
reduction in the energy intensity of the process and availability of energy sources.
Depending on the type of bonding agent applied to the grain matrix, two methods are used to regenerate the spent moulding and the core sand for its removal:
⎯
wet method (for bonding materials dispersed in an aqueous environment),
⎯
dry method (pneumatic or mechanical at ambient temperature or thermal at elevated temperature).
The most popular method, due to cost, for recovering the grain matrix from spent moulding and core sand is mechanical regeneration. The work [26,27,28,29,30,31,32,33,34,35,36] presents issues related to mechanical regeneration. As studies show, it is not a fully waste-free process. Mechanical interactions produce significant amounts of dust, consisting of the binder used and the fine grain matrix fraction produced by its crushing if the equipment exerts an impact on the grain matrix that is too intensive [37]. This has the effect of reducing the yield of the processed material.
Another solution for grain matrix recovery is the thermal treatment of spent moulding and core sand in which an organic binder has been used. The thermal regeneration is described by the authors in [38,39,40,41,42,43,44,45,46]. Thermal regeneration is perceived as an expensive process in the evaluation because of the purchase of complex equipment with high operating costs associated in most cases with the consumption of gas to burn the deposit. However, all aspects related to grain matrix management in the foundry, as well as sustainability aspects, should be considered in this case.
The question of the cost of regeneration has been addressed in publications [6,33,47,48,49,50]. Comparing mechanical and thermal regeneration methods only for reasons of energy consumption in a broader context seems to be inappropriate. The predisposition of mechanical regeneration to be used in many cases seems to be unjustified, both from the point of view of sustainability and also from the point of view of technological factors. It should be emphasised that the problems of depletion of deposits of good foundry sands, nonrenewable fuel deposits for transporting silica sand to foundries, and waste to landfill sites, which is associated with increased CO2 emissions, are still not recognised. The prevailing view among foundry industry producers seems to be that there is an unlimited supply of silica sand, and they are not taking sustainability measures. There is a lack of comparative analyses of the various regeneration methods from an economic and environmental point of view. The issues of the impact of chemically active spent moulding sand placed in landfills and the infiltration of its contents into groundwater are not taken too seriously.
The main objective of the analyses undertaken was to verify whether the more efficient, yet more costly, thermal reclamation can compensate for the higher energy consumption with lower indirect costs with respect to the mechanical reclamation of the spent moulding sand with organic binder. This aspect has not been considered so far from the point of view of the multiple operations performed to clean the grain matrix from the binder used. As part of the research conducted, this study examined how the costs of the process develop over a longer period of time. On the other hand, it examines, from the point of view of sustainability, on what order savings can of natural quartz sand deposits be obtained.
This article provides a broader perspective on the costs incurred by the regeneration methods presented.

2. Materials and Methods

2.1. Devices to Be Compared

The spent moulding sand reclamation process consists of two stages: primary and secondary reclamation. Primary reclamation is carried out in almost all foundries and comprises shaking the casting out of the mould, preliminary crushing of lumps of spent moulding sand, magnetic separation, and pneumatic classification, whose task is to remove the so-called technologically unsuitable fractions (particle size below 0.1 mm). Secondary reclamation has the task, regardless of the implementation method, of restoring grain matrix properties similar to those of fresh sand.
A comparison of process costs for the two methods of mechanical and thermal regeneration was made using data from Omega Ltd. [49,50], which presented the energy requirement costs of 1 tonne of spent moulding sand with organic binders in equipment offered by the company. Regeneration included the use of the following secondary regeneration equipment:
⎯
mechanical—regenerator (USR 5-1 or USR 5-2),
⎯
thermal—thermal regeneration system with fluidized bed furnace.
Primary regeneration is used before both secondary mechanical and thermal regeneration, both of which consume the same amount of energy, so its costs are not included in future considerations.
A schematic diagram of a second-stage mechanical regenerator (secondary regeneration) is shown in Figure 1.
During operation of the presented machine, the following elementary operations are carried out to remove the used binder from the grain matrix: abrasion of the spent moulding sand against the structural surfaces of the machine, mutual rubbing of particles against each other during its movement, and crushing as a result of pressing the regenerated mass particles between the machine’s structural elements. All these operations, in addition to the removal of the binder, have a destructive effect on the grain matrix, causing its abrasion, crushing, and, as a result, fragmentation (change of granulometric composition). This is a factor causing a reduction in the grain matrix yield [51,52,53].
Figure 2 shows the idea of the operation of a thermal regenerator. Secondary regeneration in a thermal regenerator creates the conditions for the following actions:
⎯
aeration of the bed with oxygen enables the combustion of organic compounds on the surface of the matrix grains, and fluidisation mixing promotes uniform burning of the layer of thermally treated material,
⎯
reducing the process of lifting fine particles from the regenerator chamber into the chimney,
⎯
post-combustion of gases generated after the decomposition process of organic binders.
Realisation of these three stages of thermal regeneration requires a specific design of the regenerator chamber, which is characterised by a large height relative to the bed layer of the regenerated spent moulding sand or core [53].
An appropriately high combustion chamber height is needed because of the fluidised granular material of varying granulations. The correct chamber height, which is often also extended at the top, is intended to slow the velocity of particles of varying granulation. Along with fine matrix grains and dust, gases from the thermal decomposition of organic binders also enter the flue system. The requirement is that the flue gases be free of substances that have a negative impact on the environment. In the case of the thermal regeneration system, the neutralisation of the resulting flue gases is considered (to achieve clean emissions), which requires a temperature of 800 °C in the upper part of the chamber, with a gas supply time of 3 s [54,55]. After thermal regeneration, it is also necessary to cool the recovered grain matrix.

2.2. Data Adopted for the Analysis

A comparison of the thermal and mechanical reclamation was made for the prices of the individual components that affected its implementation in Polish conditions. In addition to the costs incurred during the operation of the compared facilities, expenses related to the purchase of fresh sand, landfill charges, and the costs of transporting fresh sand to the foundry and waste to the landfill were also included. The cost analysis also took into account the grain matrix yield after the application of the compared reclamation methods. Taking all these elements into account, it is only possible to estimate the actual cost to the company after applying the different methods.
The price of electricity per 1 MWh and the price of natural gas per 1 m3 are the components that are most often taken into account when assessing the cost of regeneration. Table 1 shows the change in the price of electricity over several years and Table 2 shows the price per 1 m3 of natural gas, converted to 1 MWh, for an effective comparison between the two regeneration methods. Due to the negotiated electricity rates for companies, the cost of energy is assumed to be 3 to 4 times higher than for individual consumers. For gas, the rate is 2 times higher. Approximate price estimates for the spot market in November of a given year and, for 2023, in July [56] have been adopted for analysis. In the analysis, a conversion rate of EUR 1 equal to PLN 4.30 was assumed.
Table 3 shows how the prices of landfilling in Poland have changed over the years. Individual landfills in the municipalities concerned make individual resolutions on the cost of landfilling. Additionally, not all landfills accept casting waste classified as 10 09 08 (casting cores and moulds after the casting process) [57].
The amount of fees for using the landfill depends on the type of waste and the form of ownership of the landfill (municipal, company, or foundry-owned landfill). The most costly use of municipal landfills is for the deposition of waste classified as a high risk. If pulp waste with bentonite is used for waste transfer and as insulating layers in municipal landfills, then the amount of landfill use charges is the lowest. The greatest rigour is imposed on dusty regeneration products, in which harmful substances accumulate in much higher concentrations than in the waste mass. Due to individual pricing of the landfills, the official prices announced by the Minister of Climate and Environment in the respective year were used. These are the upper unit rates of charges for the use of the environment and the placement of waste in a landfill [58,59,60,61,62].
Another element of the analysis was the price of moulding sand from a Polish supplier. The changing price of dried moulding sand over the last 5 years is shown in Table 4. The prices presented are for loose material, without packaging. The average prices shown in the table were obtained from a SIBELCO sales representative.
An important element in the development of reclamation costs is the price of transporting sand from the mine to the foundry and taking the waste to the landfill. In the analysis carried out, the cost of transport by dump truck with a 25-tonne semi-trailer was assumed. The corresponding rates per 1 km are presented in Table 5. It should be emphasised that transport prices depend on the type of material transported, the distance, and whether it is national or international transport [63,64]. The prices presented in the overview are estimates because they are mostly negotiated, and their values are the result of the analysis of Internet offers. It has been assumed that there is inflation and labour costs are rising, which translates into the price of 1 km of transport.
The data presented in Table 1, Table 2, Table 3, Table 4 and Table 5 (Figure 3) are approximate values resulting from the individual policies of the individual companies that: supply foundry sands, accept waste at landfills, provide transport services, and also supply electricity or gas. In many cases, these are individually negotiated prices for each component of the reclamation costs. The values presented in the tables determine the level of the various regeneration cost components and provide a starting point for the analysis of the evaluation of sustainability aspects.
Figure 4 shows a plan for the assumed cost analysis of the regeneration process and the consumption of fresh sand.

3. Results

The most significant difference between mechanical and thermal regeneration from the point of view of conservation of natural resources is the yield (recovery of the amount of grain matrix) after the implementation of treatment procedures. According to the equipment manufacturer of the presented equipment (Figure 1 and Figure 2), the reduction in the use of fresh sand (the proportion of regenerate) after mechanical regeneration of the spent moulding sand with organic binder is from 50 to 80%, that is, in each subsequent production cycle the regenerate must be replenished with fresh sand in the amount of 20 to 50%. In the case of thermal regeneration, the reduction in the use of fresh sand (regenerated share) is from 90 to 95%, which means that the next production cycle requires replenishment with fresh sand in an amount of 5 to 10%.
In the analysis carried out, the average values were adopted from the range adopted for the equipment analysed:
⎯
mechanical—after reclamation of 1 tonne of spent moulding sand, the loss of grain matrix is 35% on average, that is, this is the amount of fresh sand to be added and the same amount of waste to be transported to the landfill,
⎯
thermal—after reclamation of 1 tonne of spent moulding sand, the loss of matrix is 7.5% on average, that is, less fresh sand must be added and less waste needs to be deposited in a landfill in relation to mechanical reclamation.
Table 6 summarises the data used to compare the two reclamation methods. In addition to the assumed yields, a distance of 50 km from the sand supplier and the same distance to be covered when the waste to the landfill were assumed for the calculations. A truck transport with a 25-tonne trailer was assumed. Table 6 shows the energy requirement to regenerate 1 tonne of spent moulding sand for each solution [49,50]. Figure 5 is a graphical illustration of the collated data.
Taking the estimated prices of the individual components of grain matrix management in the foundry (according to Table 6), a calculation was made of the value of the expenditure incurred in regenerating 1 tonne of spent moulding sand. Table 7, Table 8, Table 9, Table 10 and Table 11 show how the costs of regeneration changed over several years.
The total costs presented in the tables after the first cycle of thermal reclamation Tcrt(first), or mechanical reclamation Tcrm(firt) were calculated from relationships (1) and (2) assuming that W is equal to 1 tonne of reclaimed spent moulding sand, Cs is the price of 1 tonne of fresh sand, Ct is the price of transporting 1 tonne of fresh sand or waste, Crt is the cost of thermal reclamation of 1 tonne of spent moulding sand, Crm is the cost of mechanical reclamation of 1 tonne of spent moulding sand, Cw is the landfill fee of 1 tonne of waste, Yrt is the percentage of fresh sand or waste after thermal reclamation (7.5%) and, respectively, and Yrm the percentage of fresh sand or waste after mechanical reclamation (35%). The cost of successive cycles of thermal regeneration Tcrt(next) and mechanical regeneration Tcrm(next) were calculated from relationships (3) and (4), respectively.
Tcrt(first) = W × Cs + W × Ct + W × Crt + Yrt × W × Ct + Yrt × W × Cw,
Tcrm(first) = W × Cs + W × Ct + W × Crm + Yrm × W × Ct + Yrm × W × Cw,
Tcrt(next) = Yrt × W × Cs + Yrt × W × Ct + W × Crt + Yrt × W × Ct + Yrt × W × Cw,
Tcrm(next) = Yrm × W × Cs + Yrm × W × Ct + W × Crm + Yrm × W × Ct + Yrm × W × Cw,
Based on the data summarised in Table 7, Table 8, Table 9, Table 10 and Table 11, estimates of the regeneration costs for 320 cycles were made, which corresponds to the consumption by thermal regeneration consumption of a single delivery of 25 tonnes of fresh sand in the foundry, assuming an equipment output of 1 t/h. The results are shown for individual years in Figure 6.
The summary presented in Figure 6 shows that for the same number of regeneration cycles, the consumption of sand is more than 4.6 times higher for mechanical regeneration than for thermal regeneration. For mechanical regeneration, the need for sand is after 70, 142, 214, and 285 cycles. In Figure 6a,b, for the assumed number of 320 process cycles, the indirect costs forming the total cost were found to be significantly higher for mechanical regeneration. According to the widely held opinion that thermal regeneration is expensive due to the consumption of utilities, as a process it turned out to be slightly cheaper but importantly with many times lower consumption of fresh quartz sand. During the period of energy price turbulence related to the geopolitical situation falling in 2021 and 2022, unfortunately, the costs of thermal regeneration were more than two times higher (Figure 6c,d), which does not change the fact that the amount of fresh grain matrix consumed is still many times lower. From the point of view of sustainability, saving natural resources for future generations still makes the thermal regeneration process more relevant. After the regulation of the energy price market in 2023 (Figure 6e), the costs of thermal regeneration were slightly higher than those of mechanical regeneration, but were nevertheless at a higher level.
Depending on the constituent costs of regeneration, the relationship between the two ways of cleaning the grain matrix is also presented. The following statements explain why thermal regeneration taking into account all component costs can be cheaper than mechanical regeneration.
Figure 7 shows the costs of the different regeneration methods for 1 tonne of spent moulding sand for 320 cycles. In 2019 and 2020, the cost of thermal regeneration was less than the cost of mechanical regeneration. In the following years, as a result of the turbulence in gas supply and thus the increase in gas prices, in 2021 the cost of thermal regeneration was more than 2.2 times that of mechanical regeneration, and in the following year even about 2.7 times. After the stabilisation of prices in the energy utility markets and the normalisation of gas supplies, the relationship between the costs of thermal and mechanical regeneration decreased to 1.1, which is very close.
Figure 8 shows the sand purchase costs for 320 cycles of the analysed regeneration methods. As can be observed, the higher consumption of sand for mechanical regeneration, which is the result of a lower yield, definitely increases the costs incurred by the foundry. At the same time, a change in the price of dried sand can be observed following changes in energy prices.
The transport of fresh sand and waste to landfill is another component that affects the costs of regenerating spent moulding sand. Figure 9 shows a comparison of the outlay generated by each of the regeneration methods analysed. In the case of this component of the total cost of remanufacturing, the price of transport is gradually increasing over a period of 5 years. Again, the increased amount of material delivered to the foundry as well as exported to the landfill causes a higher cost associated with mechanical reclamation.
An important component of the total cost of regeneration is the landfill fee, as illustrated in Figure 10. Again, this component of the total cost of regeneration is less favoured by the deposition of waste after the mechanical treatment of the spent moulding sand. At the same time, an increase in the cost of sending waste to landfill is observed.

4. Discussion

The choice of the appropriate method for the regeneration of the spent moulding sand is taken into account [6]:
⎯
physical and chemical parameters of the mass to be remanufactured (crystalline structure of the matrix, quality and quantity of the binder),
⎯
impurities arising during the pouring of metal into moulds,
⎯
manner of further use of regenerated sand,
⎯
maximum allowed amount of impurities in the mould sand.
Considering the crystalline structure of the grain matrix, one should bear in mind during regeneration:
⎯
sand consisting of round grains has a small specific surface area and requires relatively less binder, and it is easier and less energy consuming to clean,
⎯
the specific surface area of nonspherical, cracked, and sharp-edged grains is greater, which increases the consumption of binder materials for the same strength and increases the time for cleaning and regeneration of the matrix, in which case cleaning is associated with abrasion of the sharp edges of the grains, leading to a significant reduction in regeneration yield,
⎯
it is not advisable to regenerate multicrystalline sand, because it is very difficult and often impossible to remove impurities from the cracks located at the contact surfaces of the crystals.
The sand selection issues outlined above predetermine the use of a grain matrix with specific characteristics in foundry engineering: the most round shape, a specific fraction, and a homogeneous crystalline structure. This means that these sands are selected sands that require special care in the exploitation of their deposits to provide future generations with the opportunity to benefit from their resources. Therefore, it is important to consider whether, in the larger context, only the economic aspect is important when choosing a reclamation method. When considering the solutions compared, the energy intensity of mechanical reclamation does not make it attractive considering all the cost components of reclamation. If we look at the cost components individually, apart from the consumption of energy media, which fluctuate periodically, thermal regeneration is the better solution. It seems that technical solutions for thermal regeneration with lower energy consumption should be sought, e.g., by selecting the required regeneration temperature for a given spent binder [6,65], by determining an appropriate degree of matrix cleaning that will not affect the decrease in quality of the moulding sand made in subsequent cycles. An example of such a solution is presented in [66], in which the authors demonstrated that it is possible to reduce the cost of thermal regeneration by selecting an appropriate regeneration temperature and using a different solution for post-combustion of gases resulting from the degradation of the burnt spent organic binder.
If we look at it from an environmental point of view, it is necessary to assess the solution from the point of view of CO2 emissions. In Poland, most of the electricity used to power equipment, including mechanical regenerators, comes from the combustion of hard coal, which is an element of significant CO2 emissions into the atmosphere. Thermal regeneration based on the combustion of natural gas is a more environmentally friendly solution, as less harmful substances are emitted during its combustion. This raw material is a safer solution both for the environment and for people, as there are fewer emissions not only of CO2, but also of other harmful substances, including mercury, sulphur, and nitrogen dioxide [67].
The increased demand for sand and, thus, the increased need to use road transport is also a factor in the increase in CO2 emissions into the atmosphere. According to the European Environment Agency, approximately one-quarter of total CO2 emissions in 2019 came from the transport sector, 71.7% of which came from road transport [68]. As shown in the analysis presented here, mechanical reclamation represents more than 4.6 times the environmental burden of road transport than thermal reclamation. The greater the distance from the sand mine and the landfill, the greater the impact on transport. The analysis presented here assumes small distances from the sand source and the landfill. The authors in publication [24] presented that transport distances could be distant, and at the same time smaller amounts of material (up to 15 tonnes) were transported at one time, further increasing the transport costs, especially of mechanical reclamation, and causing a higher environmental load of harmful exhaust emissions.
The technological value of the grain matrix after thermal regeneration is also very important. Quartz sand undergoes a polymorphic transformation at temperature 573 °C (β-quartz into α-quartz), which is associated with a change in the volume of the grain matrix to a larger volume. This is the cause of many casting defects [1]. Thermal regeneration carried out at a temperature above 600 °C results in thermal stabilisation of the entire grain matrix of spent moulding sand with different degrees of burn-in in the mould due to its distance from the liquid metal.
Another important component supporting the use of thermal reconditioning of organic binder masses is the surface morphology of the matrix grains. Example microscopic observations are presented to illustrate the surface of the changes that occur on the matrix after individual regeneration treatments. In the case of the presented scanning images of the grain matrix, mechanical regeneration was performed on the equipment described in publication [69] and thermal regeneration on the laboratory equipment described in publications [6,66,69]. Scanning images were taken of the materials (regenerates) obtained as a result of the treatment processes using a Hitachi SU-70 scanning microscope.
Figure 11a shows a scan photo of what the bound moulding sand with the furan binder looks like. After making the mould, casting the liquid metal and breaking the casting out of the mould, a sample of the spent moulding sand was taken, as shown in Figure 11b. A portion of the spent moulding sand was mechanically regenerated, and the resulting regenerate is shown in Figure 11c. Another portion of the spent moulding sand was subjected to thermal regeneration, and the resulting grain matrix is shown in Figure 11d.
When comparing photos of the grain matrix at different stages of processing, it is easy to see that the spent moulding sand is partially cleaned after mechanical regeneration, and the remains of partially degraded or unburned binder are smoothed after the technological process (Figure 11c). On the contrary, after thermal regeneration, the grain matrix, regardless of the amount and nature of the remaining binder in the spent moulding sand, is much better cleaned (Figure 11d). This state of affairs translates into subsequent technological cycles of moulding sand, which, based on thermal regeneration, is characterised by lower roasting losses, less gas formation, and a pH close to that of fresh sand, allowing technological cycles to be carried out under more reproducible conditions, without adjusting the amount of resin, hardener, or catalyst. Overall, this has a stabilising effect on the production process.
The question of the amount of waste generated is also not insignificant. In the case of waste spent on moulding sand with an organic binder, this is a fundamental problem. Not all landfills are designed to receive this type of chemically harmful pollutants, which can penetrate the soil and groundwater as a result of rainfall. Furthermore, the number of landfills is limited. As an example, the existing municipal waste in Poland has limited capacity, and it is assumed that the space will run out in 13 years [70]. This is another element in favour of using reclamation methods that generate as little waste as possible and are reasonably ecologically indifferent.
The issues presented are in line with the stated sustainability objectives. The use of thermal reclamation appears to be fully in line with the stated Goal 12, which is to reduce the emission of harmful substances into the environment that can negatively affect humans. As demonstrated, the use of thermal regeneration significantly reduces the level of waste generated. The analysis presented should also be part of the promotion of sustainability practices for companies, especially when it translates into the bottom line and economic growth. At the same time, the reduction of CO2 emissions generated from the generation of electricity from coal or the burning of oil during transport should work against climate change, as defined in Sustainable Development Goal 13.
The possibility of saving the grain matrix (quartz sand deposits with specific characteristics), demonstrated on the basis of the analysis presented, should not affect the needs of the current generation, and the action taken can be satisfied in a sustainable manner. This will be carried out with respect to the environment and taking into account the needs of future generations.

5. Conclusions

Based on a cost analysis for two methods of regenerating spent moulding sand with an organic binder, it was found that:
⎯
the costs of purchasing fresh sand are lower in the case of thermal reclamation, due to the lower loss of grain matrix during the process;
⎯
the costs of transporting fresh sand to the foundry and waste to the landfill are lower when the foundry uses thermal reclamation;
⎯
costs associated with landfill waste are lower when thermal reclamation is used;
⎯
the unit cost of the thermal reclamation process is higher than mechanical reclamation, but the total cost of thermal reclamation may be lower than mechanical reclamation if other process cost components are taken into account,
⎯
the consumption of quartz sand is many times lower when using thermal regeneration, which is the main factor in favour of this method of grain matrix recovery from a sustainability point of view,
⎯
the quality of the grain matrix after thermal reclamation is better for technological reasons than after mechanical reclamation.
Thermal reclamation of spent organic binder masses evaluated in the broader context of cost components, under stable market conditions, is a process comparable to mechanical reclamation in terms of implementation costs. On the other hand, in ecological, technological terms, it has a definite advantage. Therefore, looking from the perspective of sustainable development, it is necessary to support those foundries that decide to bear the cost of purchasing more expensive equipment at the beginning. This will be an important ingredient for our environment to keep the air, or water, clean. Protecting the environment is also about caring for a healthy society.
The production of castings without defects, with adequate productivity, and with lower costs for raw materials promotes improved economic performance. Stable casting production, which is guaranteed by the grain matrix after thermal regeneration of spent moulding sand with organic binder, is also an important component of sustainable development in the area of economic growth.
Technical thought and research should focus on the search for such solutions of thermal regeneration equipment, so that the process of recovering the grain matrix from spent moulding sand with an organic binder is as low energy as possible, since the other components of the overall cost, for this type of regeneration, are unequivocally favourable.
The use of thermal regeneration should aim to select the minimum required regeneration temperature at which the binder in question degrades, and at the same time the quality of the resulting regenerate guarantees good production quality. The use of a lower regeneration temperature than commonly used reduces the cost of the process. However, the afterburning of gases arising from the decomposition of chemical binders must be taken into account so that the process does not burden the environment.
It seems that the only right direction to preserve good foundry sands and reducing harmful emissions is to look for technical solutions that meet this challenge. From the point of view of sustainable development, this guarantees respect for the environment (natural resources, air, water). In turn, the creation of new, more specialised equipment solutions for the regeneration of spent moulding sand is a factor that generates social progress, which in the end should translate into economic growth.

Funding

This research was funded by the Polish Ministry of Science and Higher Education (Grant Number 16.16.170.7998/B407).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic diagram of the operation of a mechanical regenerator.
Figure 1. Schematic diagram of the operation of a mechanical regenerator.
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Figure 2. Schematic diagram of the operation of a thermal regenerator.
Figure 2. Schematic diagram of the operation of a thermal regenerator.
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Figure 3. Component costs of the spent moulding sand reclamation process: (a) price of 1 MWh of electricity, (b) conversion price from 1 m3 of natural gas to 1 MWh, (c) landfill fees for 1 tonne of waste, (d) price of 1 tonne of foundry sand, and (e) price for transporting 25 tonnes of loose material per km.
Figure 3. Component costs of the spent moulding sand reclamation process: (a) price of 1 MWh of electricity, (b) conversion price from 1 m3 of natural gas to 1 MWh, (c) landfill fees for 1 tonne of waste, (d) price of 1 tonne of foundry sand, and (e) price for transporting 25 tonnes of loose material per km.
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Figure 4. Flowchart of the research program.
Figure 4. Flowchart of the research program.
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Figure 5. Process costs: (a) mechanical regeneration, and (b) thermal regeneration, (c) transport.
Figure 5. Process costs: (a) mechanical regeneration, and (b) thermal regeneration, (c) transport.
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Figure 6. Mechanical and thermal regeneration price relationships and fresh sand consumption for successive regeneration cycles of 1 tonne of spent moulding sand: (a) in 2019, (b) in 2020, (c) in 2021, (d) in 2022, and (e) in 2023 (the green dotted line indicates fresh sand supply for mechanical regeneration).
Figure 6. Mechanical and thermal regeneration price relationships and fresh sand consumption for successive regeneration cycles of 1 tonne of spent moulding sand: (a) in 2019, (b) in 2020, (c) in 2021, (d) in 2022, and (e) in 2023 (the green dotted line indicates fresh sand supply for mechanical regeneration).
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Figure 7. Summary for costs of 320 mechanical and thermal regeneration cycles for each year.
Figure 7. Summary for costs of 320 mechanical and thermal regeneration cycles for each year.
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Figure 8. Summary of the cost of purchasing fresh sand for 320 mechanical and thermal reclamation cycles for each year.
Figure 8. Summary of the cost of purchasing fresh sand for 320 mechanical and thermal reclamation cycles for each year.
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Figure 9. Summary of the cost of transporting fresh sand and spent moulding sand for 320 mechanical and thermal reclamation cycles for each year, for an assumed distance of 50 km from the sand mine and landfill site.
Figure 9. Summary of the cost of transporting fresh sand and spent moulding sand for 320 mechanical and thermal reclamation cycles for each year, for an assumed distance of 50 km from the sand mine and landfill site.
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Figure 10. Summary of landfill costs for 320 mechanical and thermal reclamation cycles for each year.
Figure 10. Summary of landfill costs for 320 mechanical and thermal reclamation cycles for each year.
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Figure 11. Scanning images of matrix grains at magnification ×100: (a) bound furan mass after crushing, (b) spent moulding sand, (c) spent moulding sand after mechanical regeneration, and (d) spent moulding sand after thermal regeneration.
Figure 11. Scanning images of matrix grains at magnification ×100: (a) bound furan mass after crushing, (b) spent moulding sand, (c) spent moulding sand after mechanical regeneration, and (d) spent moulding sand after thermal regeneration.
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Table 1. Price of 1 MWh of electricity.
Table 1. Price of 1 MWh of electricity.
Year20192020202120222023
Price per MWh (Eur)48.8353.49166.28161.7997.67
Table 2. Conversion price from 1 m3 of natural gas to 1 MWh.
Table 2. Conversion price from 1 m3 of natural gas to 1 MWh.
Year20192020202120222023
Price per MWh (Eur)18.6018.6069.7793.0234.88
Table 3. Landfill fees for 1 tonne of waste.
Table 3. Landfill fees for 1 tonne of waste.
Year20192020202120222023
Price per tonne (Eur)67.5368.6270.2072.5876.28
Table 4. Price of 1 tonne of foundry sand.
Table 4. Price of 1 tonne of foundry sand.
Year20192020202120222023
Price per tonne (Eur)25.1725.1725.7429.8931.47
Table 5. Price for transporting 25 tonnes of loose material per km.
Table 5. Price for transporting 25 tonnes of loose material per km.
Year20192020202120222023
Price per 1 km (Eur)0.850.941.011.081.12
Table 6. Indicators adopted to calculate the costs of mechanical and thermal regeneration.
Table 6. Indicators adopted to calculate the costs of mechanical and thermal regeneration.
IndicatorValue 2019
(Eur)		Value 2020
(Eur)		Value 2021
(Eur)		Value 2022
(Eur)		Value 2023
(Eur)
Average cost of mechanical regeneration of 1 tonne of spent sand for an energy consumption of 11 kWh/t0.540.591.281.791.07
Average cost of thermal regeneration of 1 tonne of spent sand for an energy consumption of 242 kWh/t19.3619.3672.6096.8036.30
Transport price per tonne (transport by 25 tonne semi-trailer for an assumed distance of 50 km)1.701.882.022.162.24
Table 7. Regeneration cost for 2019 prices.
Table 7. Regeneration cost for 2019 prices.
Cost of the Component of the Regeneration
Process		Regeneration Price of 1 Tonne of Spent Moulding Sand with Organic Binder (Eur)
ThermalMechanical
First CycleCycle 2
(and Next)		First CycleCycle 2
(and Next)
In the first cycle, the cost of 1 tonne of fresh sand, in subsequent cycles the cost of replenishment25.171.8925.178.81
In the first cycle cost of transporting 1 tonne of fresh sand, in subsequent cycles cost of the replenished part1.700.131.700.60
Cost of regeneration of 1 tonne of spent moulding sand19.3619.360.540.54
Cost of transporting the waste to the landfill0.130.130.600.60
Storage fee for waste generated after reclamation5.065.0623.6423.64
Total51.4226.5751.6534.19
Table 8. Regeneration cost for 2020 prices.
Table 8. Regeneration cost for 2020 prices.
Cost of the Component of the Regeneration
Process		Regeneration Price of 1 Tonne of Spent Moulding Sand with Organic Binder (Eur)
ThermalMechanical
First CycleCycle 2
(and Next)		First CycleCycle 2
(and Next)
In the first cycle, the cost of 1 tonne of fresh sand, in subsequent cycles the cost of replenishment25.171.8925.178.81
In the first cycle cost of transporting 1 tonne of fresh sand, in subsequent cycles cost of the replenished part1.880.141.700.60
Cost of regeneration of 1 tonne of spent moulding sand19.3619.360.590.59
Cost of transporting the waste to the landfill0.140.140.660.66
Storage fee for waste generated after reclamation5.155.1524.0224.02
Total51.7026.6852.3234.74
Table 9. Regeneration cost for 2021 prices.
Table 9. Regeneration cost for 2021 prices.
Cost of the Component of the Regeneration
Process		Regeneration Price of 1 Tonne of Spent Moulding Sand with Organic Binder (Eur)
ThermalMechanical
First CycleCycle 2
(and Next)		First CycleCycle 2
(and Next)
In the first cycle, the cost of 1 tonne of fresh sand, in subsequent cycles the cost of replenishment25.741.9325.749.01
In the first cycle cost of transporting 1 tonne of fresh sand, in subsequent cycles cost of the replenished part2.020.152.020.71
Cost of regeneration of 1 tonne of spent moulding sand72.6072.601.281.28
Cost of transporting the waste to the landfill0.150.150.710.71
Storage fee for waste generated after reclamation5.275.2724.5724.57
Total105.7880.1054.3236.28
Table 10. Regeneration cost for 2022 prices.
Table 10. Regeneration cost for 2022 prices.
Cost of the Component of the Regeneration
Process		Regeneration Price of 1 Tonne of Spent Moulding Sand with Organic Binder (Eur)
ThermalMechanical
First CycleCycle 2
(and Next)		First CycleCycle 2
(and Next)
In the first cycle, the cost of 1 tonne of fresh sand, in subsequent cycles the cost of replenishment29.892.2429.8910.46
In the first cycle cost of transporting 1 tonne of fresh sand, in subsequent cycles cost of the replenished part2.160.162.160.76
Cost of regeneration of 1 tonne of spent moulding sand96.8096.801.791.79
Cost of transporting the waste to the landfill0.160.160.760.76
Storage fee for waste generated after reclamation5.445.4425.4025.40
Total134.45104.8060.0039.17
Table 11. Regeneration cost for 2023 prices.
Table 11. Regeneration cost for 2023 prices.
Cost of the Component of the Regeneration
Process		Regeneration Price of 1 Tonne of Spent Moulding Sand with Organic Binder (Eur)
ThermalMechanical
First CycleCycle 2
(and Next)		First CycleCycle 2
(and Next)
In the first cycle, the cost of 1 tonne of fresh sand, in subsequent cycles the cost of replenishment31.472.3631.4711.01
In the first cycle cost of transporting 1 tonne of fresh sand, in subsequent cycles cost of the replenished part2.240.172.240.78
Cost of regeneration of 1 tonne of spent moulding sand36.3036.301.071.07
Cost of transporting the waste to the landfill0.170.170.780.78
Storage fee for waste generated after reclamation5.725.7226.7026.70
Total75.9044.7262.2640.34
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Łucarz, M. Economic and Environmental Aspects of Applying the Regeneration of Spent Moulding Sand. Sustainability 2024, 16, 8462. https://doi.org/10.3390/su16198462

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Łucarz M. Economic and Environmental Aspects of Applying the Regeneration of Spent Moulding Sand. Sustainability. 2024; 16(19):8462. https://doi.org/10.3390/su16198462

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Łucarz, Mariusz. 2024. "Economic and Environmental Aspects of Applying the Regeneration of Spent Moulding Sand" Sustainability 16, no. 19: 8462. https://doi.org/10.3390/su16198462

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Łucarz, M. (2024). Economic and Environmental Aspects of Applying the Regeneration of Spent Moulding Sand. Sustainability, 16(19), 8462. https://doi.org/10.3390/su16198462

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