Bentonite and pulverized coal are always used as greensand moulding bonding agent and anti-sticking sand material in greensand moulding, and Zhuyuansu  is a kind of moulding material which can replace bentonite and pulverized coal; it is a patent technology. It has the advantages of small addition amount, stable process performance, good appearance quality of castings under the same process conditions, obvious reduction of sand dropping and sticking defects, yield-improving and cost-saving. In particular, it is worth mentioning that there is no pulverized coal in moulding sand, in the process of casting molten iron, no air pollution brought by combustion of pulverized coal, no MgO brought by combustion of pulverized coal, or no MgS generated by the combination of S and Mg in pulverized coal. In terms of nodular cast iron, magnesium compounds are reduced, which plays a key role in improving the surface decaying graphite of nodular cast iron. For the exhaust manifold used at high and low temperature, the risk of cracks in the exhaust manifold is reduced due to the reduction of graphite layer in the surface layer.
Raising of the question
Since the use of wet sand moulding, Hengtong Casting had been using pulverized coal as anti-sticking material for wet sand moulding.Under the conditions at that time, pulverized coal has played a proper role in anti-sticking sand of castings. Taking Hengtong Casting’s production of 3613 as an example, in 2017, 322,211 pieces of 3,613 castings were produced and 3,033 sand-dropping wastes were produced, which ranked the firs in waste analysis of the sand-dropping wastes of castings. In order to reduce the waste rate of castings and improve product quality, it was imperative to reduce the waste of sand dropping and sticking.
The market has more stringent requirements for the appearance quality of castings, and also has higher requirements for our casting production units. In addition, sand mixing process is the Company’s largest source of pollution, pulverized coal is light and black; bentonite is light and white, in the process of storage, handling, adding and sand mixing, dust is flying fuill of the whole workshop, which is obvious to all in our company. With the increasingly stringent environmental requirements of the country, it is difficult for pulverized coal and bentonite to meet the requirements of casting production.
Practical production shows that, instead of adding new sand, collapsible resin coated sand is used to replace new sand. Because the residual inert resin film on the surface of collapsible resin coated sand has hydrophobicity, it weakens the bonding strength with hydration film and affects the properties of moulded sand. With the increase of the number of resin coated sand in used sand, the hot-wet tensile strength and surface wear resistance of moulding sand decrease, which leads to the increase of waste rate; if new sand is added, it is necessary to increase the amount of bentonite and pulverized coal, a part of used sand should be discharged regularly to keep the sand balance in the sand treatment system, which increases the production cost, and increases the amount of solid waste discharged in the casting process and pollutes the environment. Therefore, after the new moulding material is added, the toughness and moulding property of the moulding sand should be improved, the hot and wet tensile strength of the moulding sand, the sand-entrainment resistance and the surface wear resistance and the fluidity of the moulding sand should be improved; and on the premise of not reducing the wet compressive strength of the moulding sand; and on the premise of not reducing the wet compressive strength of moulding sand, the adverse effects of the addition of resin coated sand core on the properties of clay sand are eliminated, and the anti-sticking properties of moulding sand are enhanced
Influence of the characteristics, technical indexes and application methods of Zhuyuansual powder on the properties of moulding sand containing a large amount of resin coated sand
Enterprises need a new kind of moulding material to improve the toughness and moulding ability of moulding sand, increase the hot-wet tensile strength, sand-entrainment resistance and surface abrasion resistance of moulding sand, and the fluidity of moulding sand after addition; and on the premise of not reducing the wet compressive strength of moulding sand, the adverse effect of adding resin coated sand core on the properties of clay sand is eliminated, and the anti-sticking property of moulding sand is enhanced.
Benefiting from the introduction of experts from casting association, a new kind of moulding material — Zhuyuansu powder, has been put into market application. The Zhuyuansual powder is a Zhuyuansual clay green sand mixture additives which is developed on the basis of high quality sodium bentonite, high carbon sand-sticking-proof material, alpha starch and other organic materials.
Characteristics of Zhuyuansual powder:
1. The addition of bentonite and pulverized coal is reduced by about half as much as that of ordinary bentonite and pulverized coal.
2. Molding sand has good toughness and improves the starting strength.
3. Good liquidity, easy to fill and compact.
4. Good preservation to prevent air-drying loss of strength, especially hot sand.
5. The addition of moisture content is reduced by 15-20%, which can prevent formation of pores.
6. The mud content of old sand is decreased and the addition of new sand is lowered. The addition of new sand can be reduced by 60% and core sand with less than 10% can be totally eliminated, thus maintaining the balance of the amount of sand used.
7. It effectively solves the brittleness of moulding sand caused by the extensive use of resin coated sand and triethylamine sand.
8. The surface finish of the castings is improved, and the defects of sand washing and scarring are eliminated.
9. It has good reusability and can keep low addition for a long time.
10. Dust and flue gas pollution during casting is reduced.
11. The discharging device of sand mixer can be reduced by one.
12. The open-box sand after casting is easy to disperse, which can simplify the crushing equipment.
13. No spontaneous combustion of pulverized coal, which eliminates potential safety hazards.
14. The overall cost is decreased by 10-15%.
Model and technical indexes
1. The existing sand treatment bentonite silo is used to store the Zhuyuansual powder; the original pulverized coal silo is used to store the bentonite, and the bentonite is used as a standby to solve the fluctuation caused by the strength of the moulding sand of different products and to adjust the strength fluctuation.
2. The original sand mixing process remains unchanged.
The role of Zhuyuansual powder in the casting process of moulding sand is as follows:
1) To the high temperature effect of molten iron, the organic fibre of Zhuyuansual powder produces a large amount of reducing gas, which prevents the molten iron from being oxidized and can reduce the iron oxide on the surface of molten iron, thus reducing the possibility of chemical reaction between metal oxides and moulding sand. The reducing gas in the mould cavity mainly comes from the volatile matter produced by the pyrolysis of the Zhuyuansual powder.
2) After heating, the minerals of Zhuyuansual powder begin to soften and have plasticity. If the temperature range from softening to solidification is wide and the time is long, the expansion stress of quartz particles caused by heating in the changing temperature range can be buffered, thus the sand inclusion defect of castings caused by thermal expansion of sand moulds can be reduced.
3) The Zhuyuansu powder produces colloid after heating, and the volume expansion part of the colloid plugs the pore between the sand grains on the surface of the sand mould, which makes the molten iron difficult to penetrate.
Performance comparison of moulding sand before and after test
In order to prevent the fluctuation of moulding sand quality and the increase of casting waste, the original sand mixing process remains unchanged, as shown in Table 2 and Table 3.
Sand mixing process remains unchanged as shown in Table 2.
The properties of moulding sand before and after using Zhuyuansus are shown in Table 4.
From Table 1, Table 2 and Table 3, it can be seen that bentonite and pulverized coal in moulding sand can be replaced by Zhuyuansus, under the situation without adding new sand (our Company’s original sand mixing process did not add new sand), comparing the moulding sand mixed by clay used sand containing a large amount of resin coated sand core with the moulding sand mixed with used sand, bentonite and pulverized coal under the same technological conditions, the wet compressive strength, air permeability and compactness are similar, and the hot-wet tensile strength is obviously improved, and it can be seen that the adoption of Zhuyuansus and clay moulding sand mixed by clay sand and used sand with a large amount of resin coated sand core can eliminate the adverse effects of a large number of resin coated sand core on the properties of the sand.
Based on the above test results, Zhuyuansus is gradually implemented to replace bentonite and pulverized coal in moulding sand system. In order to ensure the normal production and avoid the large fluctuation of moulding sand performance to lead to the generaton of batches of waste, we adopt the method that adding Zhuyuansus on the basis of the original bentonite and pulverized coal green moulding clay used sand, in the working process, gradually replacing bentonite and pulverized coal in used sand by Zhuyuansus, and after a period of circulation, bentonite and pulverized coal in used sand are completely replaced by Zhuyuansus. The blending ratio and properties of moulding sand during the transition period are shown in Tables 4 and 5.
During the use of Zhuyuansual powder, the precondition of adjusting the addition of Zhuyuansual powder is that there is no big fluctuation in the test results of sand. According to the size of casting and the difference of drawing, the addition of Zhuyuansual powder and bentonite can be adjusted in time, and the produced castings shall be inspected in time, the appearance of casting and the waste products shall be inspected, so as to prevent the emergence of batch problems, resulting in waste of cost.
The properties of moulding sand in transition period is shown in Table 6.
Effect of Zhuyuansual clay sand on surface graphite of ductile iron
Zhuyuansual powder is composed of high-quality sodium-based bentonite, high-carbon sand-sticking-proof material, alpha-starch and other organic substances, Pulverized coal is added to traditional tidal sand, and the sulfur content of pulverized coal is about 0.5%, the Zhuyuansual powder is used to replace pulverized coal and bentonite, as the sulfur content of Zhuyuansu powder is about 0.2%, the sulfur content of moulding sand in the whole sand treatment is further reduced after a certain period of circulation, which provides a basis for solving the problem of excessive graphite layer thickness of surface layer. Sodium bentonite and pulverized coal, and Zhuyuansual graphite layer are used to test the different parts for the same casting. Figure 7 sampling site.
Mechanisms of producing graphite layer: because of the contact between Mg and O in the air during the casting process of spheroidized molten iron, part of Mg will be oxidized to MgO. When molten iron is in contact with the mould wall, pulverized coal will burn. The combustion process of pulverized coal is also the oxidation process of Mg. Furthermore, S in pulverized coal will combine with Mg to form MgS, when the residual magnesium in the surface layer of molten iron is low enough not to ensure spheroidization, decaying graphite and graphite are produced, which is collectively called graphite layer. In theory, graphite layer is unavoidable, but too thick graphite layer has a certain impact on product quality, especially in the exhaust manifold used under the alternating state of high and low temperature, cold and hot, graphite layer is the source of hot cracking. Figure14 is agraphite layer with a wall thickness of 4mm, and Figure 15 is a graphite layer allowed by Honeywell’s high nickel material. Therefore, each engine enterprise has certain regulations on the surface graphite of the exhaust manifold and turbine box. Generally, it is allowed to be between 0.2mm and 0.3mm, or even less than 0.15mm. It is very difficult to meet this requirement in technology. The oxidation resistance of heat-resistant materials for automotive exhausting system is studied in detail in the paper of An Investigation on the Anti-oxidation Property of Heat-resisting Materials Used for Automotive Exhausting System , and then the surface oxide layer (i.e. graphite layer) produced during casting process is the hot crack source of exhaust manifold and turbine box used at high and low temperature. After use of Zhuyuansus, the thickness of the graphite layer is effectively reduced, which is a remarkable achievement. The presence of graphite has been clearly observed in Figure14, while the surface in Figure 15 is completely oxidized and graphite is barely visible. In the metallographic pictures of Zhuyuansus (Figsure 8, 10 and 12), there is no graphite on the surface of the castings, and even no decaying graphite can be seen. The test results are shown in Table 7.
Cost saving, environmental improvement, reduction of solid waste discharge and rational utilization of storage space
1. The quantity reducing of added supplementary materials brought cost savings.
Taking Hengtong Casting sand mixing line as an example, 480 tons of moulding sand are mixed every day, the amount of pulverized coal that needs to be added is 480*0.6%=2.88 tons, and the amount of bentonite that needs to be added is 480*0.7%=3.36 tons. After Zhuyuansual powder is used, the addition is 480*0.7%=3.36 tons.
The cost-effectiveness of using Zhuyuansual powder to replace pulverized coal and bentonite is shown in table 8.
From the above table, it can be concluded that the replacement of pulverized coal and bentonite with Zhuyuansual powder has significant cost-effectiveness and reduces the cost of moulding sand.
2. Reduction of waste brought the improvement of benefit.
Before using Zhuyuansu powder, the sand dropping and fleshy Waste accounted for 1.01%; and after using Zhuyuansu powder the sand dropping and fleshy Waste accounted for 0.57%, improved by 0.44%, which brought the improvement of benefit. According to the annual production program of 100,000 units, it was equivalent to reducing 4,400 pieces of waste products, each deducting residual value, saving 20 yuan, and 4400 units can save 88,000 yuan. It is shown in Table 9.
3. Reduce the labor intensity of workers
Before using Zhuyuansual powder, pulverized coal and bentonite as supplementary materials for sand treatment needed to be added about 6.2 tons per day, after using Zhuyuansual powder, it only needed to add about 3.36 tons of Zhuyuansual powder, and the addition amount can be reduced by 2.88*25 days = 72 tons per month or so.
4. Reduce the discharge of solid waste and save energy
Before using Zhuyuansual powder, four tons of solid waste are discharged every day , after using Zhuyuansual powder, the overall dust in sand treatment decreased, and the solid waste discharged per day decreased to 2.4 tons; and for the whole month, it can be reduced by 1.6*25=40 tons, which reduces the difficulty of solid waste treatment.
The castings appearance have less sticking sands, and the black sand produced in the cleaning process is reduced correspondingly, which can appropriately reduce the shot blasting time of the product and save part of the power consumption.
5. Improvement of the environment
Appearance comparison between Zhuyuansual powder and pulverized coal (Figure 16):
The appearance of Zhuyuansual powder is grey, and the pollution to environment is relatively small;
The appearance of pulverized coal is black, which pollutes the environment greatly. The gas produced by the combination of pulverized coal and bentonite at high temperature is harmful to human body.
6. Reasonable utilization of storage space
The original use of bentonite and pulverized coal requires a lot of space to store bentonite and pulverized coal respectively. After the use of cast elemental powder, it is only need half of the original storage space.
In summary, our Company, as a professional enterprise producing cast iron, takes green moulding as the main moulding method. The daily moulding sand volume in Xunyang is up to 480 tons, and the annual production of castings is more than 1 million pieces, With the use of castal element powder to replace pulverized coal and bentonite, significant results have been achieved in improving the properties of moulding sand, improving the quality of castings, reducing the production cost, reducing the waste rate of castings and purifying the working environment.
Especially when a large amount of collapsible resin coated sand is used and no new sand is added, the performance of moulding sand can be effectively improved by using Zhuyuansual powder to replace pulverized coal and bentonite. The elimination of graphite layer is of great significance to castings used at high and low temperatures. Of course, there are many factors affecting the graphite layer, which need effective control in all aspects to produce high quality castings.