Chromite Sand Research Articles

OPTIMISATION OF A RESIN-COATED CHROMITE SAND FOR RAPID SAND CASTING APPLICATIONS

Applications of three-dimensional printing (3DP) to sand casting have been well-established in the last two decades. The preferred raw material is silica (quartz) sand, as it is the most readily available and cost effective sand. However, silica sand as a refractory material has some technical limitations, including high thermal linear expansion, low refractoriness, and thermal conductivity. Therefore, it is not suitable for all castings. Other refractory sand types are available, including chromite sand, which is abundantly available in South Africa. Analysis of the physical and chemical properties of in-laboratory coating of a locally available chromite sand was conducted through known metal foundry tests that provide an understanding of the quality and suitability of the use of chromite sand as a potential substitute for silica material for rapid sand-casting applications. The results of this study will inform the industry about the optimisation of parameters for the manufacturing of a resin-coated chromite sand and its use in additive manufacturing using a Voxeljet VX 1000 printer.

ASSESSMENT OF LOCAL CHROMITE SAND AS ‘GREEN’ REFRACTORY RAW MATERIALS FOR SAND CASTING APPLICATIONS IN A POST-PANDEMIC WORLD

In a post-pandemic world, health and environmental safety will become a more important issue than it was before the COVID-19 pandemic. Several fields of engineering, including the foundry industry, have already adopted health and safety measures in the workplace. The risk of pulmonary disease caused by the inhalation of fine particles of silica sand, the main moulding material used in the foundry industry, is rampant in that industry. In terms of environmental protection, the new regulatory restrictions on the disposal of used sand create the need for moulding aggregates to be more recyclable than silica. Chromite sand is an alternative refractory sand for metal casting applications, and is abundant in South Africa. The present study investigated whether this aggregate has better overall reusability than silica. The focus of this study is on the use of chromite sand in foundries as a contribution to improving health in the workplace and to reducing the environmental impact of foundry sand.

Ophiolite Chromite Deposits as a New Source for the Production of Refractory Chromite Sands

Chromite foundry sands, mixed with binding resins, are employed in the industry to form molds for high demanding casting of metals and steel. As there is no substitute, these sands highly contribute to placing chromium at the top value of the economic importance parameter in the EU classification of critical raw materials. Finding new sources to produce these sands can contribute to lowering its criticality. Chromite foundry sands must meet strict quality parameters, referred to as Cr2O3 content, Fineness Index, SiO2 content, and Acid Demand. The foundry chromite market is dominated by South Africa production deriving from layered intrusion chromite deposits. Chromite sands from ophiolite chromite deposits, normally used for the metallurgical-grade chromite market, were tested as an alternative starting raw material to produce chromite foundry sands. The study of the silicate impurities assemblage showed that its mineralogy strongly affects the result of the most crucial parameter, the Acid Demand. Ophiolite chromite with serpentine impurities should be depurated to a hardly affordable 0.31% SiO2 content to meet Acid Demand quality threshold, due to high reactivity of this silicate with the acid environment of the test. Those with olivine impurities require to be depurated to a much more easily affordable 2.11% SiO2 content. As a result, ophiolite chromite with an olivine dominated silicate assemblage can be used as an alternative source of chromite foundry sands.

Investigation of complex permittivity spectra of foundry sands

Abstract This paper discusses impedance spectroscopy as a method to characterize different types of quartz sand, chromite sand, and mixtures of sand. Based on two types of measurement cells, the impedance spectra for various dry sands were measured. The spectra differ enough to allow the sands to be distinguished. The results were validated by extracting the relative permittivity from the impedance and comparing it to literature data. After that, the method could be applied with confidence to other material systems. The influence of moisture was investigated with two types of quartz sand, and typical mixtures of form sand, chromite sand, and regenerated (quartz) sand were studied. In each case, the sand composition had a distinct influence on the Nyquist plot of the impedance spectrum. Compared to results from a laboratory foundry system, the new method exhibits a much more systematic dependence on the sand composition. If one succeeds in describing the impedance spectra with an equivalent circuit model parameterized by only a few parameters, these parameters could be used to identify and classify sand mixtures in the field. This would allow one, for example, to implement feedback control strategies in foundry regeneration processes, which would stabilize the processes and improve the quality of the casting products.

Study of the properties of a aqueous sodium silicate film on the surface of a refractory filler

We investigated the effect of the method of solidification of aqueous sodium silicate on the formation of its film on the surface of a refractory filler. The structure of liquid-glass films on the surface of a refractory filler cured by convective drying, blowing with carbon dioxide, and with the addition of organic ether (butyl acetate) was studied. Quartz and chromite sands were used as a refractory filler.

Hadfield manganese austenitic steel: a review of manufacturing processes and properties

In this paper, manufacturing processes and properties of Hadfield manganese steel was studied. Due to good flexibility and excellent resistance to wear, the high strength steel is widely used in various industries such as cement, mining, road construction and railroads. Induction furnace is suitable for melting of Hadfield steel. Typically, silica, olivine and chromite sand are used to make mold in the casting process of Hadfield steel. This steel is in standard state, an alloy of Fe, C and Mn. But in certain applications, other elements such as Cr, Ti, Al, Mo, B, V, etc are added. Mn is used as the most important austenite stabilizer in Hadfield steel and its role is to delay the austenite to martensite transformation. Mn is a carbide forming element and forms Mn3C and (Fe, Mn)3C in Hadfield steel. The optimal structure for the Hadfield steels is the fully austenite structure and the single-phase. In non-heat-treated castings, the structure of the unit consists of austenitic and grain boundary carbide phase. To provide optimum toughness, the structure of the Hadfield steel should be austenitic single-phase. Twin strain has a great contribution to plastic deformation of this steel. Two phenomena of dislocation accumulation and the formation of twins during plastic deformation of these steels are the main reason of the strain hardness of this steel. The high rate of work hardening in Hadfield steel is due to strain induced transformation of γ to α or ε martensite, mechanical twinning, dynamic strain aging, and the confrontation between dislocations with stacking faults. In the early stages of wear and slow wear conditions, the wear resistance of Hadfield steel is low. In conditions of slow wear, the surface is not sufficiently work hardening and thus the wear resistance of this steel is low. But with the work hardening of the surface, wear resistance rises sharply. The welding and weldability of this steel are strongly influenced by the heat input. In any case, the formation of carbide phases in the weld metal is acceptable, and often the weld metal has higher strength and lower toughness than the base metal. Carbides deposited in the heat affected zone (HAZ) of welded Hadfield steel, usually are Mn7C3 and Mn23C6.

Enrichment Chromite Sand Grade Using Magnetic Separator

To fulfil requirements of mettalurgical grade 48% Cr2O3 with Cr:Fe ratio 3:1 need the right beneficiation, one of them is using magnetic separator. Chromite sand sample obtain from Konowe-Southeast Sulawesi have low grade, so it needs process to enrichment Cr:Fe Ratio. Cr:Fe ratio feed was 0.55, after the process for size variation, roll speed, feeder speed and electric current decline Cr:Fe Ratio around 50.5%. When size -65+100 mesh Cr:Fe ratio was 0.282. At roll speed 20 RPM, feeder speed 50 Kg/Hr and electric current 1.5 A have Cr:Fe ratio 0.3525. Therefore, it needs more beneficiation before using magnetic separator for chromite sand from Konowe-Southeast Sulawesi.

Study on Removal of Copper Rice Paint Film and Copper Recovery Efficiency

The effects of chromite sand particle size, mixing ratio of sand and copper wire, motor speed and collision friction time on the removal of copper wire paint film and copper recovery efficiency were studied. The results show that the conditions for efficient copper recovery are: chromite sand particle size is 0.40mm, chromite sand and copper wire mixing ratio is 1:1, centrifugal motor speed is 2700r/min, collision friction time is 2min. Under these conditions, the polyester enameled copper wire film was completely removed and the copper recovery rate reached 98.75%.

High-quality manufacturing method of complicated castings based on multi-material hybrid moulding process

A multi-material hybrid patternless moulding process for complicated castings has been proposed. Moulding sands used in the hybrid moulding process include silica sand, ceramic sand, chromite sand, zircon sand, and steel shot sand. Experimental method was used to study the effects of moulding sands on the temperature field, mechanical properties, and dimensional precision of the iron castings. Under the condition that the wall thickness on different sides of the casting is the same, when the wall thickness is greater than 10 mm, the heat storage capacity of the moulding sands from strong to weak is steel shot sand, zircon sand, chromite sand, ceramic foundry sand, and silica sand. Tensile strength of the obtained castings from high to low is zircon sand, chromite sand, steel shot sand, ceramic sand, and silica sand. Contraction rate of the obtained castings from high to low is steel shot sand, zircon sand, chromite sand, silica sand, and ceramic sand. Therefore, steel shot sand and zircon sand can be used as chilled sand, and even can be used instead of cold iron when the casting wall thickness is greater than 10 mm. Zircon sand and chromite sand can be used to obtain high mechanical properties, and silica sand and ceramic sand can be selected to obtain high dimensional precision of the castings. Finally, a typical iron casting piece was tested by experiment using the hybrid moulding process. Excellent performances of iron castings confirm the feasibility of the hybrid moulding process.

Effect of Temperature on Chromite-Based Moulding Sands Bonded with Sodium Silicate

Abstract In the paper, a research on effects of baking temperature on chromite sand base of moulding sands bonded with sodium silicate is presented. Pure chromite sand and its chromite-based moulding sand prepared with use of sodium silicate were subjected to heating within 100 to 1200 °C. After cooling-down, changes of base grains under thermal action were determined. Chromite moulding sand was prepared with use of 0.5 wt% of domestic made, unmodified sodium silicate (water-glass) grade 145. After baking at elevated temperatures, creation of rough layer was observed on grain surfaces, of both pure chromite sand and that used as base of a moulding sand. Changes of sand grains were evaluated by scanning microscopy and EDS analyses. It was found that changes on grain surfaces are of laminar nature. The observed layer is composed of iron oxide (II) that is one of main structural components of chromite sand. In order to identify changes in internal structure of chromite sand grains, polished sections were prepared of moulding sand hardened with microwaves and baked at elevated temperatures. Microscopic observations revealed changes in grains structure in form of characteristically crystallised acicular particles with limited magnesium content, intersecting at various angles. EDS analysis showed that these particles are composed mostly of chromium oxide (III) and iron oxide (II). The temperature above that the a.m. changes are observed in both chromite-based moulding sand and in pure chromite sand. The observed phenomena were linked with hardness values and mass of this sand.

Determination of Chromite Sands Suitability for Use in Moulding Sands

Abstract Using the available analytical methods, including the determination of chemical composition using wavelength-dispersive X-ray fluorescent spectroscopy technique and phase composition determined using X-ray diffraction, microstructural observations in a highresolution scanning microscope equipped with an X-ray microanalysis system as well as determination of characteristic softening and sintering temperatures using high-temperature microscope, the properties of particular chromite sands were defined. For the study has been typed reference sand with chemical properties, physical and thermal, treated as standard, and the sands of the regeneration process and the grinding process. Using these kinds of sand in foundries resulted in the occurrence of the phenomenon of the molding mass sintering. Impurities were identified and causes of sintering of a moulding sand based on chromite sand were characterized. Next, research methods enabling a quick evaluation of chromite sand suitability for use in the preparation of moulding sands were selected.

Characterization of Foundry Waste Sand Streams for Recycling Applications in Construction Industry

The local South African foundry industry discharges large volumes of waste sand each year, with estimates for silica and chromite sands of 250,000 and 25,000 tons, respectively. As a developing country, South Africa is moving toward a greener environment, so the management of industrial waste is an imperative area of study. In this work, we present the laboratory analysis results of the characteristics of three waste sand streams (molding, shot blast, and dust) and discuss the feasibility of reuse options in the construction industry (including concrete mix and cement manufacturing). The results reveal that each stream has its own characteristic properties. In addition, we determined the effect of the molding binder on waste sand, especially on the shakeout sand stream. The results also demonstrate that resin knock sand features much coarser grains and less clay, sulfate, and organic content than greensand. We found the chemistry of sand stream to have the same main constituents as cement kiln feed. With respect to recycling options, we determined that the shakeout stream can be reused in both concrete mix and cement manufacturing. However, special precautions should be taken regarding the clay content of sands originating from greensand foundries. Due to its finer grain size, dust streams can only be recycled in cement manufacturing. Lastly, we found that shot blast sand failed to comply with the requirements of any of the explored recycling options due to it high density, low silica content, and high metallic debris content. The effective beneficiation of sand may involve waste stream segregation and particle size classification since the recycling options we explored (concrete mix) mainly depend upon aggregate size and the impurities present (metallic debris).

Influence of sand base preparation on properties of chromite moulding sands with sodium silicate hardened with selected methods

Abstract The paper presents a research on the relation between thermal preparation of chromite sand base of moulding sands containing sodium silicate, hardened with selected physical and chemical methods, and structure of the created bonding bridges. Test specimens were prepared of chromite sand - fresh or baked at 950°C for 10 or 24 hours - mixed with 0.5 wt.% of the selected non-modified inorganic binder and, after forming, were hardened with CO2 or liquid esters, dried traditionally or heated with microwaves at 2.45 GHz. It was shown on the grounds of SEM observations that the time of baking the base sand and the hardening method significantly affect structure of the bonding bridges and are correlated with mechanical properties of the moulding sands. It was found that hardening chromite-based moulding mixtures with physical methods is much more favourable than hardening with chemical methods, guaranteeing also more than ten times higher mechanical properties.

Effect of Temperature on Chromite-Based Moulding Sands Bonded with Sodium Silicate

Abstract In the paper, a research on effects of baking temperature on chromite sand base of moulding sands bonded with sodium silicate is presented. Pure chromite sand and its chromite-based moulding sand prepared with use of sodium silicate were subjected to heating within 100 to 1200 °C. After cooling-down, changes of base grains under thermal action were determined. Chromite moulding sand was prepared with use of 0.5 wt% of domestic made, unmodified sodium silicate (water-glass) grade 145. After baking at elevated temperatures, creation of rough layer was observed on grain surfaces, of both pure chromite sand and that used as base of a moulding sand. Changes of sand grains were evaluated by scanning microscopy and EDS analyses. It was found that changes on grain surfaces are of laminar nature. The observed layer is composed of iron oxide (II) that is one of main structural components of chromite sand. In order to identify changes in internal structure of chromite sand grains, polished sections were prepared of moulding sand hardened with microwaves and baked at elevated temperatures. Microscopic observations revealed changes in grains structure in form of characteristically crystallised acicular particles with limited magnesium content, intersecting at various angles. EDS analysis showed that these particles are composed mostly of chromium oxide (III) and iron oxide (II). The temperature above that the a.m. changes are observed in both chromite-based moulding sand and in pure chromite sand. The observed phenomena were linked with hardness values and mass of this sand.

Effective Laboratory Method of Chromite Content Estimation in Reclaimed Sands

Abstract The paper presents an original method of measuring the actual chromite content in the circulating moulding sand of foundry. This type of material is applied for production of moulds. This is the case of foundry which most frequently perform heavy casting in which for the construction of chemical hardening mould is used, both the quartz sand and chromite sand. After the dry reclamation of used moulding sand, both types of sands are mixed in various ratios resulting that in reclaimed sand silos, the layers of varying content of chromite in mixture are observed. For chromite recuperation from the circulating moulding sand there are applied the appropriate installations equipped with separate elements generating locally strong magnetic field. The knowledge of the current ratio of chromite and quartz sand allows to optimize the settings of installation and control of the separation efficiency. The arduous and time-consuming method of determining the content of chromite using bromoform liquid requires operational powers and precautions during using this toxic liquid. It was developed and tested the new, uncomplicated gravimetric laboratory method using powerful permanent magnets (neodymium). The method is used in the production conditions of casting for current inspection of chromite quantity in used sand in reclamation plant.

Effects of Casting Geometry on the Mechanical Properties of A201 Aluminum Alloy Castings in Chromite Sand Molds

Systematic change of riser size together with variation of thickness and length of A201 aluminum alloy plate castings were studied to investigate the correlation between mechanical properties and porosity content. The molds with end chill for plate castings were made of 100% chromite sand by CO2 process. If soundness throughout the whole plate casting has to be guaranteed, the riser diameter should be proper design to the dimension of the plate. The tensile strength and elongation of A201 aluminum alloy plate casting is governed by riser diameter, thickness, and length of the plate.

Study to improve the breaking strength of the no-bake chromite sand-furan resin cores

This study was carried out to determine the effects of number of washings, curing time and heat treatment on the breaking strength of no-bake chromite sand-furan resin cores. The Hevi chromite sand produced by a single stage washing of chromite sand was found to exhibit lower breaking strength in service. Forty five samples of no-bake chromite sand mixes were prepared with furan resin at room temperature and 39% humidity from chromite sand washed in single and two stages and cured at 2, 4 and 24 hours. Eighteen of the samples washed in single and two stages were tested for breaking strength without heat treatment, while the 27 obtained from single stage washing sand mix were heated to 900 o C and held at 12, 14 and 16 minutes before cooling. Re-washing gave an appreciable increase in breaking strength only for samples cured for 24 hours. The results obtained showed that highest breaking strength occurred in heat treated and cured samples with the highest occurring at curing period of 24 hours and 900 o C with holding time of 16 minutes. For re-washed samples held in room atmosphere, there was a decrease in breaking strength compared to the single stage washed samples. The implication of this work is that Hevi chromite sand has better breaking strength when heat treated at 900oC and held in the temperature for 16 minutes. Evidently, the air cured samples may have been affected by the air humidity.

Effects of grain refining and modification on mechanical properties and microstructures of Al–7.5Si–4Cu cast alloy

Al–7.5Si–4Cu cast alloy melt modified by Al–5Ti–B, RE and Al–10Sr master alloys were poured in the chromite sand moulds, to investigate comparatively the effects of individual or combined additions of grain refiners and modifiers on the mechanical properties, microstructures, grain refining and modification, and intermetallic compounds of the alloy. The results show that the mechanical properties and the microstructures of Al–7.5Si–4Cu cast alloys are improved immensely by combining addition of 0.8%Al–5Ti–B, 0.1%RE and 0.1%Al–10Sr grain refiners and modifiers compared with the individual addition and cast conditions. For individual addition condition, addition of 0.8% Al–5Ti–B master alloy can obtain superior tensile strength, Brinell hardness and finer equiaxed α(Al) dendrites. The alloy with 0.1% RE master alloy shows the highest improvement in ductility because the rare earth can purify the molten metal and change the shape of intermetallic compounds. While the alloy with 0.1% Al–10Sr modifier shows only good improvement in yield strength, and the improvement of other performance is unsatisfactory. The Al–10Sr modifier has a significant metamorphism for the eutectic silicon, but will make the gas content in the aluminum alloy melt increase to form serious columnar grain structures. The effects of grain refining and modification on mean area and aspect ratio have the same conclusions obtained in the mechanical properties and the microstructures analyses.

Effect of mould sand type and casting wall thickness on properties and microstructure of multivariate Al–7·5Si–4Cu alloy

The effects of different mould sands and casting wall thicknesses on the properties and microstructures of multivariate Al–7·5Si–4Cu alloy, which was modified by Al–10Sr, RE and Al–5Ti–B master alloys, were investigated. The melt was poured into multistep moulds made from three mould sands (quartz, alumina and chromite) to produce specimens for mechanical property testing and metallography. The results demonstrated that the sample with chromite sand exhibited the best mechanical properties owing to its higher cooling rate. Tensile strength and elongation decreased monotonically with increasing wall thickness. No obvious trend of wall thickness on hardness of the alloy was found. When the cooling speed decreased, the grain refinement and transformation effect became weaker, the dendrite arm spacing (DAS) increased obviously and the properties and microstructures became poorer. A series of fitting models was established and used to predict properties of Al–7·5Si–4Cu alloy from the DAS.

Effects of moulding sands and wall thickness on microstructure and mechanical properties of Sr-modified A356 aluminum casting alloy

The effects of different cooling conditions on the mechanical properties and microstructures of a Sr-modified A356 (Al-7Si-0.3Mg) aluminum casting alloy were comparatively investigated using three moulding sands including quartz, alumina and chromite into multi-step blocks. The results show that the mechanical properties and microstructures using chromite sand are the best. As the cooling speed increases, the dendrite arm spacing (DAS) decreases significantly and the mechanical properties are improved, and the elongation is more sensitive to the cooling speed as compared with the tensile strength. The increase of the properties is primarily attributed to the decrease of the DAS and the increase of the free strontium atoms in the matrix. In particular, the regression models for predicting both the tensile strength and the elongation for Sr-modified A356 aluminum casting alloy were established based on the experimental data.