Lost foam casting is a near-net-shape, precise molding green casting process that is increasingly being widely promoted and applied. The sand processing production line in the lost foam casting can realize the automatic processes of casting shakeout, impurity screening, magnetic separation and transportation of iron pellets, as well as the transportation, cooling, recycling, and reuse of used sand, with a recycling rate of more than 95%. The magnetic separator is one of the key equipments in the sand processing production line, and its efficiency in separating iron from sand is directly related to the automatic operation of the sand processing system, the cooling effect and recycling rate of the used sand, and the production cost of the enterprise.
1. Process Flow of Lost Foam Casting
Our company established a 10,000 t/a lost foam casting production line in 2017, with a sand processing production line capacity of 40 t/h and a production rhythm of 3 – 4 minutes per box. The production process.
After pouring, the castings in the sand box are automatically turned over under the control of the system PLC program. The (casting) parts and sand are separated through the vibration shakeout machine, and the large particles and impurities are screened by the vibration conveying screen. The used sand is sent to the magnetic separator through the 1st stage elevator for magnetic separation of iron pellets to achieve the separation of iron and sand. The separated iron enters the storage hopper and is returned to the furnace, and the used sand is cooled and cooled by the fluidized bed cooler. Then, it is vertically transported to the intermediate sand storage and sand temperature regulation storage through the 2nd and 3rd stage elevators. The sand temperature regulator under the sand temperature regulation storage further cools the used sand, and then it is vertically transported to the molding sand storage for reuse in molding, thereby realizing the automatic recycling and reuse of used sand. The dust is collected and centrally treated in the bag filter through the dust removal pipeline and the induced draft fan. During the process, the sand loss is small, and the reuse rate of used sand can be more than 95%.
The separation of iron and sand in the production process is of great significance to the automatic operation of the system: when the iron can be sorted out in time, the production line can achieve automatic continuous operation. That is, the operators only need to perform molding during their shift, and the production line will automatically carry out horizontal transportation, box turning, screening, cooling, vertical transportation (lifting), storage, and remolding according to the control of the system PLC program, repeating the cycle; when the iron cannot be sorted out in time and enters the fluidized bed cooler with the sand, on the one hand, it hinders the fluidized cooling of the used sand, and on the other hand, it accumulates at the bottom of the bed, restricting the normal transportation of the used sand, causing blockage, and forcing the production to stop for cleaning, screening, recycling, and resuming production. When the ventilation holes are blocked, it will also restrict the fluidized cooling of the used sand. That is to say, the working condition of the magnetic separator is directly related to the automatic operation of the production line, the cooling and reuse of the used sand, and the normal operation of the workshop production, and it is an important process equipment.
2. Structure and Principle of Magnetic Separator
The magnetic separator consists of a transmission device (including a motor + reducer + transmission chain, etc.), a magnetic drum (with magnetic blocks inside), an adjustment handle, a sorting chamber, a feeding plate (multi-stage baffle), a feeding chute, an iron discharge chute, a sand discharge chute, and dust removal parts, as shown in Figure 2. The material of the magnetic block is ordinary (Y20), the working temperature is < 80°C (instantaneous 100°C), and the magnetic field strength is 1,750 Gs.
During production, the materials after the initial screening by the vibration shakeout machine and the vibration conveying screen after the box is turned over are transported to the magnetic separator by the 1st stage elevator. The materials enter the sorting chamber through the feeding chute and are scattered and sprinkled multiple times by the feeding plate, and naturally fall onto the magnetic drum. The iron pellets and iron filings are adsorbed on the drum under the action of the magnetic blocks in the magnetic drum, and the sand falls under the action of gravity and enters the sand discharge chute and enters the next process equipment (fluidized bed cooler). With the continuous rotation of the drum, the iron pellets adsorbed on the drum are brought into the non-magnetic field area (above the iron discharge chute), and fall off the drum under the action of gravity and fall into the iron discharge chute, and then are discharged to the iron storage hopper through the chute and are discharged regularly. The fine sand powder (dust) enters the dust removal pipeline under the action of the induced draft fan in the system.
3. Existing Problems
Our company’s lost foam casting production line was installed and put into production in April 2018. Within less than 3 months of operation, the problem of poor separation effect of iron and sand appeared. The amount of iron-sand mixture selected per shift was about 240 kg, with an iron content of 5% – 8%, accounting for about 50% – 60% of the total iron, and there was still a part (about 40% – 50%) that could not be sorted out in time. The effect of the magnetic separator was poor. There was more sand and less iron in the iron storage hopper sorted by it, and the iron in the sorted used sand could not be removed in time and entered the fluidized bed cooler with the sand, accumulating on the surface of the fluidized bed cooler, restricting the fluidized condition and cooling effect of the sand, resulting in the molding sand temperature being as high as 70°C or even above 80°C, seriously exceeding the process requirement of ≤ 60°C), causing the pattern clusters that have undergone multiple procedures such as molding, mold repair, dipping in three coats of coating, and drying to deform in the sand box, directly affecting the quality of the casting products. Moreover, it was necessary to stop production specifically to arrange the cleaning of the iron-sand mixture in the fluidized bed cooler, which took at least half a shift each time. When the sand temperature was too high to perform molding production, it was necessary to stop production and specifically circulate the sand for cooling (air cooling method), resulting in a passive production situation of circulating and cooling the sand for half a shift and producing for the other half a shift, shortening the normal production time. The old sand and iron mixture cleaned from the fluidized bed cooler needed to be manually screened for a second time, and the available sand was recycled and reused after screening, increasing the labor intensity of the employees. At the same time, the iron material deposited on the fluidized bed cooler accumulated in the fluidized bed cooler, and as the deposition continued, it caused blockage, restricting the normal transportation of the sand; due to the blockage, the wear of the facilities and equipment in the previous process of the magnetic separator was aggravated, the seal was damaged, resulting in material leakage, sand ingress into the bearings, shaft wear, magnetic drum leakage, and sand ingress into the drum, as shown in Figures 4 – 7, and in severe cases, production had to be stopped for maintenance, increasing equipment failures and cost consumption, seriously restricting the continuous production of the workshop and hindering the development and operation of the company.
4. Problem Analysis
- After measurement, the temperature of the sand entering the magnetic separator was above 152°C, far exceeding 80°C. The increase in the temperature of the magnetic block led to the attenuation of the magnetic properties, and the iron pellets could not be sorted out in time and fell into the fluidized bed cooler with the sand, accumulating on the bed surface and even blocking the air nozzles, restricting the fluidized cooling effect and making it impossible to fully exchange heat with the cooling water in the bed, restricting the cooling of the sand temperature, and also restricting the normal transportation of the sand.
- At the inlet, the falling point of the material was directly above the drum, and the gap between the drum and the last-stage feeding plate was 40 mm , and some of the sand would flow back and fall into the iron discharge chute, increasing the sand discharge.
- At the outlet, the gap between the discharge plate and the drum was 50 mm , and some of the sand would fall into the iron discharge chute along with the airflow and inertia of the drum rotation, increasing the sand discharge.
- The material of the discharge chute pipe was Q235, which was easily magnetized, and the local part of the blanking pipe had an upward warping phenomenon, as shown in Figure 11. The collected iron pellets were adsorbed on the inner wall of the chute, and accumulated at the warping point along with the fine powder and sand particles, which was not conducive to the smooth falling of the materials and caused blockage.
- The particle size of the molding sand was small (850 μm – 425 μm), and the weight was light. Especially after repeated use, after wear and crushing, the particle size became even smaller, and the valve opening was too small, resulting in some sand entering the iron discharge chute.
- The mesh size of the screen was large. A 6-mesh (4 mm × 4 mm) screen was selected, and some of the sorted iron pellets “escaped through the net”.
5. Solution
Based on the structure and characteristics of the equipment, it was decided to transform the magnetic separator. The structure after the transformation.
- A new magnetic drum was made, using a jumping roller system (by using different strength grades of magnetic blocks to achieve multiple separations of sand and iron, thereby improving the sorting efficiency). The thickness of the stainless steel roller skin was changed from 3 mm to 4 mm, and the magnetic block was replaced with neodymium iron boron instead of the original ordinary magnetic block Y20. The working temperature was increased from 80°C – 100°C to 200°C; the magnetic field strength was increased from the original design of 1,750 Gs to ≥ 3,000 Gs, and the magnetic system was adjusted reasonably, as shown in Figure 15. During installation, the transmission direction was adjusted according to the magnetic pole strength, and the left installation was changed to the right installation.
- The last-stage feeding plate was extended by 40 mm to exceed the top of the drum, so that the materials could be sorted smoothly without backflow after entering the sorting chamber and avoid mixing into the iron discharge chute side.
- A baffle was added at the outlet to form a curtain-like blanking to improve the sorting effect.
- The discharge chute was remade and installed, and the material was changed to stainless steel, with a blanking angle of ≥ 50°, to ensure a straight pipe and smooth blanking.
- An inspection door was added to timely inspect and adjust the sorting of materials.
- An adjustment plate was added to the feeding plate to adjust the amount of material feeding and ensure a uniform material layer on the drum.
- The opening of the dust removal valve was appropriately adjusted to ensure a slight negative pressure in the sorting chamber.
- The screening mesh was changed from 6-mesh (aperture 4 mm × 4 mm) to 12-mesh (aperture 1.7 mm × 1.7 mm) to refine the screening aperture and timely screen out the sorted iron materials.
During operation, the materials transported by the elevator entered the sorting chamber through the feeding chute and were scattered and sprinkled multiple times by the feeding plate. The fine sand powder (dust) was brought into the dust removal pipeline under the action of the induced draft fan in the system, achieving the first separation of iron and sand; other materials fell along the feeding plate and the blanking plate. Since it exceeded the top of the magnetic drum and the gap with the magnetic drum was small (15 mm – 20 mm), the materials would not flow back and be returned to the iron discharge chute side; under the action of the magnetic force, the iron pellets in the mixture and the sand formed sand clusters and moved towards the drum, and the iron was adsorbed on the surface of the drum, and the sand fell under the action of gravity, achieving the second separation of iron and sand; with the continuous rotation of the drum, the materials entered the blank area between the magnetic poles in the magnetic system, the magnetic force decreased, the iron pellets slipped along the surface of the drum, and the sand fell under the action of gravity, achieving the third separation; with the continuous rotation of the drum, when reaching the next group of magnetic poles, the iron pellets were adsorbed again, and the sand fell under the action of gravity… After multiple rollovers and multiple separations. At the discharge plate, the added baffle blocked, disrupted, and dispersed the mixture and airflow. Combined with the small gap of 10 mm – 15 mm with the drum, the passing area was greatly reduced, and under the combined action of the rotation of the drum, the negative pressure of the dust removal, and the inertia of the materials, the materials were scattered and separated again, and the iron pellets were adsorbed on the surface of the drum and brought into the iron discharge chute area and discharged through the chute. The sand fell into the sand discharge chute under the action of gravity and entered the next process equipment, the fluidized bed cooler.
During the adjustment process, the added inspection door was used to timely check the blanking and discharge situation, and appropriate fresh air was added to enhance the airflow in the sorting chamber; the height of the adjustment plate was adjusted to make the material layer on the drum uniform, reducing the phenomenon of no-load idling and uneven accumulation of thick material layers on the magnetic drum. The magnetic force of the roller system was fully utilized to promote the sorting effect; the opening of the dust removal valve was appropriately adjusted to reduce the negative pressure in the sorting chamber, and the sorted fine powder could be timely removed.
6. Production Effect
After actual operation, good results were achieved: the sorting efficiency was greatly improved, the cleaning cycle of the fluidized bed cooler was extended to more than 5 months, reducing the forced downtime and laying a solid foundation for the successful completion of the production task; the increase in the mesh size of the screen timely screened out the iron pellets for reuse, reducing the production cost; it also reduced the probability of iron pellets mixing into the molten iron and the impact on the quality of the castings, reducing the frequency and workload of cleaning and grinding; it also reduced the load on each equipment in the automatic production line, reduced equipment failures, and extended the service life of the equipment; it promoted the cooling of the sand temperature and optimized the production process conditions. At the same time, it improved the working environment, reduced dust emissions, and promoted environmentally friendly, clean, and civilized production, creating considerable social benefits.
6.1 Direct Benefits
An annual benefit of 611,300 yuan can be achieved.
- The annual forced downtime can be reduced by 22.5 days, and the production output can be increased by 681.75 tons, creating a benefit of 524,900 yuan.
Before the transformation, the cleaning cycle of the fluidized bed cooler was at least once a week, and it was required at least 47 times a year; after the transformation, it was extended to more than 5 months, and it was required 2 times a year, reducing by 47 – 2 = 45 times. It took 0.5 days for one cleaning, totaling 0.5 × 45 = 22.5 days.
The increased production output (calculated based on 2 shifts per day, 19 furnaces per shift, 1 ton per furnace, and a yield rate of 0.85):
2 × 19 × 1 × 22.5 × 0.85 = 681.75 tons.
The benefit (calculated based on 7,700 yuan per ton of castings and a profit of 10% of the output value):
681.75 × 0.77 × 0.1 = 524,900 yuan. - An annual of 24.64 tons of iron can be timely selected and recycled for reuse.
According to the average weight ratio of iron and sand before and after the transformation, the average value is 2.5%.
681.75 × 2.5% = 17.04 tons;
With the increase in the mesh number of the screen, 1.12% more iron can be recycled and reused.
1.12% × 681.75 = 7.6 tons
17.04 + 7.6 = 24.64 tons
The price is calculated at 3,500 yuan per ton.
24.64 × 0.35 = 86,200 yuan. - The timely separation of iron reduces the load on each equipment and facility in the automatic production line, reduces the loss, and can reduce equipment failures and maintenance costs by 50,200 yuan annually.
- Investment cost: 50,000 yuan.
Among them, 1 set of magnetic drum costs 48,000 yuan; other materials (steel plates, etc.) cost 2,000 yuan.
In summary, the benefit can be achieved: 524,900 + 86,200 + 50,200 – 50,000 = 611,300 yuan.
6.2 Social Benefits
It reduces the labor intensity, improves the working environment, promotes green, environmentally friendly, clean, and civilized production, and creates considerable social benefits.
7. Conclusion
With the development of the national casting industry and the promotion of intelligent manufacturing, the continuous improvement of the level of automated production technology, the lost foam casting automatic production line will be more widely used. Among them, the magnetic separator in the sand processing automatic production line can timely separate the iron pellets in the production process, reduce the equipment load, improve the cooling efficiency, increase the recycling of used sand, improve.
