Component composition of the processed forming mixture and mineralogical recommendations for its re-use

Modern industrial production is aimed at the development of waste-free technological processes, wider use in the production of waste and by-products. The largest amount of waste is generated from the activities of mining and metallurgical enterprises. One of the problems of foundry production is the disposal of multiton solid waste, about 90% of which is spent molding mixture. Part of the spent mixture is reused as recyclable, but the main mass of it is collected at municipal solid waste landfills. This work is devoted to the study of the component composition of foundry production waste – spent molding mixture with the aim of its reuse. According to the results of the research carried out by the authors, mineralogical recommendations regarding its reuse were provided. Involvement in the reuse of spent molding mixture will allow to free up part of the solid household waste landfills, reduce the negative man-made load on the environment, and additionally obtain conditioned restored molding sands.


Introduction
Currently, Ukraine has accumulated about 45 billion tons of industrial waste, located on an area of more than 160,000 hectares.About 1.7 billion tons of new waste is generated every year, while 5-12% of waste is reused, while in developed countries industrial waste is used for 60-80% [1][2][3].
The largest amount of waste is generated from the activities of mining and metallurgical enterprises.One of the enterprises that uses the metallurgical cycle of processing raw materials is the Kryvyi Rih Repair and Mechanical Plant (Metinvest -KRMZ LLC) in the city of Kryvyi Rih, the main activity of which is the production of machines and equipment for the mining and mining processing industry, parts and components for it, as well as production of steel and iron castings, welded metal structures, stampings, non-standardized equipment for objects under construction or reconstruction.This enterprise includes a full production cycle, from metal smelting to the production of finished products in the form of machines and mechanisms.
The main waste of the enterprise, which can be involved in processing, is generated in the steel casting, mechanical assembly, repair and tool shops, and metal structures shop [4,5].The waste of these subdivisions includes: dust from gas cleaning traps (emissions from electric steel melting furnaces in the form of suspended particles undifferentiated by composition), foundry slag, slag, spent molding mixture, welding production waste, abrasive metal dust.Welding production waste and slag are completely disposed of at the enterprise for internal needs, all other generated waste is processed by the enterprise for its own needs in small quantities, and the main volume is transferred to third-party organizations or stored on the factory territory.One of the problems of foundry production is the utilization of multi-tonnage solid waste, about 90% of which is spent molding mixture, which currently belongs to substances of the 4th category of danger [6][7][8].A part of the spent mixture is reused as a turnover, for the Kryvyi Rih Repair and Mechanical Plant, this indicator is 10percent of its total mass.The rest is collected at municipal solid waste landfills.

Methodology
The current state of production waste was analyzed and compared with global trends.Waste samples were selected by the authors.Chemical analyzes were carried out, granulometric and component composition and the content of impurity elements were investigated.The current state of the situation with industrial waste was analyzed and compared with global trends.
Detailed mineralogical studies of spent molding mixture waste (burnt earth) were carried out.According to the results of the obtained data, mineralogical recommendations for the technological scheme of the processing of the used molding mixture and its reuse are provided.

Results
The authors investigated the waste of the steel foundry shop -spent molding mixture (burnt earth), which is formed from foundry molds and rods during the production of metal blanks.After knocking out hardened castings from foundry molds on grids and sieving on sieves, metal blanks and waste are obtained in the form of burnt molding mixture.According to the technological regulations of the Kryvyi Rih Repair and Mechanical Plant, about 10% of the mixture is reused, the rest is transported by road transport to the temporary storage site (figure 1).Based on the results of granulometric, chemical, and detailed mineralogical studies of the composition, structure, and texture of the used molding mixture by the authors, it was established: • The spent molding mixture was represented by a loose, lumpy material of burnt sands of gray, dark gray to black color with a brownish, yellowish tint.The lumpiness of the mixture (figure 3) is due to significant heating of the molding mixture during the pouring of the molds.The presence of a metal component (hooks, nails, forming pins) in the spent mixture is associated with their use as an auxiliary material in the assembly of casting molds.After knocking out on the grates, part of the hooks, nails, and forming pins remains in the mixture and falls together with the burnt sand to the temporary storage area.During the selection of initial samples, in addition to the above-mentioned metal objects in the composition of the used molding mixture, individual allocations of residual metal in the form of "beads", dendrites, and irregular shapes were occasionally observed.The size of the discharges ranged from 1 to 10-20 cm.Large fragments are most often manually selected by the employees of the enterprise from the composition of burnt earth, were identified by the authors during the selection of initial samples at the enterprise and did not fall into the composition of the examined samples of burnt earth (figure 4).The authors established that the average content of coarse-grained material (particles larger than 0.5 mm) in the composition of burnt earth is 5.1%, the main part of the material (85.4%) is represented by particles with a size of -0.5+0.1 mm.The amount of the most fine-grained (siltstone-pelitic) component is 9.5% (figure 5).
Mineralogical studies were performed by the authors using macroscopic and microscopic methods.According to the results of detailed mineralogical studies, the average component composition of the used molding mixture was determined (table 1).The results of a complete silicate analysis obtained by the authors for the material of the spent molding mixture revealed that the product-forming chemical component is silica, aluminum, iron and calcium oxides are of secondary importance (table 2).The significant content of silicon oxide is due to the presence of quartz as the main mineral component in the molding mixture.The increased content of aluminum oxide is associated with the presence of silicates -products of thermal effects on the clay minerals of the primary molding mixture.A noticeable admixture of calcium and magnesium oxides is due to the presence of carbonates represented by calcite and dolomite.The presence of iron oxides is caused by several factors: the presence of iron-containing minerals (magnetite, ilmenite, hematite, goethite, chromite) in the sand of the primary molding mixture; migration of iron from the molten metal to the side of the mold; the presence of iron in the composition of substances used to ensure the strength of forms, their non-stick properties, etc.The relatively high content of alkalis (sodium, to a small extent potassium) causes the use of liquid glass in the composition of the original molding mixture.The content of rare, scattered chemical components in the used molding mixture was determined by the authors using the method of semi-quantitative spectral analysis (table 3).
The obtained data were compared with the data on the concentrations of the corresponding chemical components, which reflect the average content of each of them in the composition of the earth's crust.The authors assumed that the Clarke number corresponds to the safe concentration of the corresponding chemical element in both natural and man-made formations.In order to compare data on the average content of chemical components in the composition of burnt earth and their clarke, the Clarke indices of concentrations were determined -by dividing the values of the average content of an element by its Clarke number (table 3).According to the obtained data, all the investigated chemical elements can be divided into three groups: (i) impurity elements, the content of which is less than clarke (clarke concentration lower than 1.0): silver, cobalt, copper, niobium, nickel, porcelain, lead, zinc; (ii) elements, the content of which is close to the Clarke number (clarke concentration from 1.0 to 2.0): germanium, tin, titanium, vanadium, zirconium; (iii) elements, the content of which in the composition of burned earth significantly exceeds the Clarke number (clarke concentration from 5 to 250): bismuth, chromium, molybdenum.
According to the authors, the reasons for significant fluctuations in the content of impurity elements in the material of the spent molding mixture compared to the amount of these elements can be as follows: -the presence of the investigated impurity elements in the primary material of the molding mixture; -the presence of these chemical elements in the composition of substances used to ensure the strength of forms, their non-stick properties, etc.; -different behavior of chemical elements in the technological process (redistribution between the mold and the product, transition to the gaseous phase and evaporation, etc.).
The raw material used in the manufacture of casting molds mainly contains quartz sand and clay minerals (kaolinite, beidelite, hydromuscovite, etc.).They are characterized by a low isomorphous capacity, which is why the content of impurity elements in their composition is very low: for the studied chemical elements, it usually does not exceed 1•10-3 -1•10-6 wt.%.The sand may contain accessory minerals, which include the studied chemical elements (ilmenite, chromite, zircon, monazite, rutile, pyrolusite, etc.).
In the process of preparing foundries molds, in accordance with the features of the technologies used, the following chemical components are added to the molding mixture: liquid sodium glass; soluble sodium silicate; lignosulfonate grade A; PLH-1 brand refractory clay; chromite concentrate; furan resin SQG-120; phenolic resin SQJ610; Permabind CO phenolic resin; phenolformaldehyde resin NOVANOL 160, 165N; zirconium and magnesite non-stick mixtures.
Thus, in the composition of the components that are added to the clay-quartz molding mass, it is possible to distinguish: -inorganic substances that practically do not contain the studied impurity elements (sodium liquid glass, refractory clay); -organic substances that do not contain the investigated impurity elements (lignosulfonate, furan resin, phenolic resin, phenol-formaldehyde resin); -mineral components, which include investigated impurities (chromite concentrate, zirconium and magnesite non-stick mixtures).
That is, in the composition of substances added to the molding mixture, there are two of the 36 investigated chemical components -chromium and zirconium.The technological process is accompanied by: -partial combustion of organic substances (lignosulfonate, furan resin, phenol resin, phenol formaldehyde resin) with the formation of amorphous carbon present in the form of "shirts" on quartz particles and their cementing substance; -sublimation of parts of impurity elements characterized by low melting and boiling (evaporation) temperatures: bismuth (melting point 271°C, boiling point, evaporation 1564°C), phosphorus (respectively, 44°C and 277°C), zinc (420°C and 907°C), ytterbium (819°C and 1194°C), strontium (769°C and 1382°C), thallium (304°C and 1473°C), lithium (18°C and 1342°C), stybium (630°C and 1587°C), mercury (-39°C and 357°C); -redistribution of a part of refractory elements-admixtures (chromium, manganese, titanium, cadmium, molybdenum, cobalt, nickel, zirconium, tungsten) between the outer surface of the casting and the inner surface of the mold.

Conclusions
One of the problems of foundry production is the disposal of multiton solid waste, about 90% of which is spent molding mixture [10,11].The existing methods of using the spent mixture can be divided into three groups: (i) use of spent molding mixture without additional processing; (ii) methods that ensure processing and recycling of production waste at the same enterprise; (iii) methods of processing and utilization of spent molding mixture in various branches of the national economy.
According to the results of the research carried out by the authors, the following mineralogical recommendations were proposed regarding the technology of processing the spent mixture for the purpose of its reuse: -due to the presence in its composition of spent mixture of metal inclusions (forming pins, nails, particles of excess metal in the form of beads, dendrites, formations of irregular shape) it is necessary to remove them from the composition of the mixture; at the same time, it is necessary to take into account that individual metal particles can be weakly or non-magnetic; -the spent molding mixture is a loose, lumpy material of the sand fraction with an admixture of psephyto-siltstone-argillite and pelite components.The lumpiness is due to the presence of impurities used in the preparation of the molding mixture and its significant heating and sintering during the pouring of the molds; -when drawing up a technological scheme for the processing of the mixture, it is necessary to ensure the destruction of lumpy aggregations of the mixture, the rubbing of quartz particles from silicate-hydroxide "shirts", the reduction of the number of binding organic and inorganic components on the surface of quartz particles to a level that will allow the separation of aggregations into separate particles; -restoration (regeneration) of the spent molding mixture can be carried out using mechanical, hydraulic or pneumatic methods.
The spent mixture without preliminary processing can be used as an additive in the production of construction and road construction materials, it must first be cleaned of metal inclusions.In the production of silicate bricks, the spent mixture can completely replace quartz sand.The use of burnt earth will increase the degree of sintering of the brick mass due to liquid glass and alkalis in its composition.Using the spent molding mixture, you can get building solutions, wall stone, concrete products, cinder blocks, foam concrete.
The spent mixture without preliminary treatment can be used for reclamation of spent quarries, backfilling of road surfaces, in the technological processes of solid household waste landfills for overlapping layers as an insulating material in the middle and upper parts of landfills.In road construction, the spent mixture, classified as a special soil and classified as man-made soil (industrial waste), can be used as a component of the upper part of the road surface, for the formation of embankments.
Modern industrial production is aimed at the development of waste-free technological processes, wider use of waste in production and by-products.The reuse of spent molding mixture will allow to free up part of the solid household waste landfills, reduce the negative man-made load on the environment, obtain additional commercial products for third-party consumers, and additionally obtain conditioned reclaimed molding sands.

Figure 1 .
Figure 1.Site of temporary storage of industrial waste.

Figure 2 .
Figure 2. The material of the spent molding mixture.

Figure 3 .
Figure 3. Lumpy particles (a) of the molding mixture with individual metal inclusions and metal pins (b) in its composition.

Figure 4 .
Figure 4. Separation of residual metal in the spent molding mixture.

Table 1 .
The average component composition of the spent molding mixture.

Table 2 .
The average content of chemical components in the spent molding mixture.

Table 3 .
The average content of impurity elements in the material of the spent molding mixture.