One of our copper mine customer, who has 2 sets of φ5.5 m×1.8 m MZS series semi-autogenous mills and 2 sets of MQS series autogenous mills. The annual treatment capacity is 1 million tons. With the continuous increase in production tasks, the annual treatment capacity increased to 1.7 million tons. The heavy workload leads to serious wear of manganese mill liners of the semi-autogenous mill, and the service life is only 5-6 months. Each semi-autogenous mill needs to replace 2.5 sets of liners every year, and the continuous shutdown and maintenance seriously affect the grinding production.
Wear status of mill liners
The design thickness of the lifting liner is 70 mm and the height of the lifting strip is 150 mm, as shown in Figure 1 (a). The appearance of the worn lifting liner is shown in Fig. 1 (b). The thickness of the middle part of the lining plate is only 8-10 mm. The middle part of some lining plates has been worn through, while the height of the lifting strip is only 50 mm.
The semi-autogenous mill is mainly used for rough grinding. Due to the large ore entering the cylinder, the grinding body is a ball with a diameter of 200 mm (single weight of 50 kg). When the mill is working, the local impact force of the grinding body on the cylinder liner is large. The grinding medium and ore body are lifted by the liner plate to a certain height and then dropped or dropped along the parabola. In the process of throwing, the lining plate is impacted and worn, and part of it is damaged When the ball is lifted up and hit the ore body, the sharp angle of the hard block ore body is chiseled into the surface of the lining plate, and along with the rotation of the mill barrel, it cuts along the surface to form a chisel pit and plow ditch; while the finer ore body is in a state of falling in the middle of the cylinder rotary section, sliding relative to the lining plate, resulting in low-stress abrasion.
The results of the metallographic analysis of the worn mill liner material are as follows:
- The chemical constituents are C 0.89%, s 0.01%, Si 0.46%, Mn 11.18%, P 0.077%. The carbon content is lower than 0.9% ～ 1.4% of the national standard GB / T 5680-1998, so the wear resistance of the liner is low.
- Mechanical properties: tensile strength RM = 654.5 MPa, elongation after fracture a = 11%, hardness 195 HBW, impact toughness AK = 148 J / cm2, which are lower than RM ≥ 685 MPa and a ≥ 25% specified in GB / T 5680-2010.
- The metallographic structure can be seen from the metallographic diagram (see Fig. 2) that the austenite grain size is finer, the inclusions are less (grade 2b), and there is obvious carbide precipitation structure (grade x5) at the austenite grain boundary, which is lower than the national standard GB / T 13925-2010 that “the precipitated carbide level is not greater than X3 is qualified”, which leads to the reduction of the toughness of the mill liner.
Change semi-autogenous mill liners material
The mill liners material ZGMn13 was changed to zgmn13cr2 (adding 0.003% – 0.005% boron). The as-cast grain shape of the high manganese steel is regular, and the grain size is uniform and fine, which can prevent the micro crack propagation and force the crack to change direction. Tensile strength RM = 765 MPa, impact toughness AK = 165 J / cm2, hardness 243 HBW, yield strength, fracture toughness, wear-resistance and impact resistance are good.
Change semi-autogenous mill liners heat treatment
The results show that the black carbides precipitate obviously at the austenite grain boundary, and the carbon content is low, which indicates that the temperature of the liner is not heated to 1050 during water toughening treatment, In addition, the cooling rate of water toughening medium is slow, and the temperature of water toughening medium is too high, resulting in carbide precipitation at the austenite grain boundary, which affects the wear resistance and toughness of lining plate.
When zgmn13cr2 material is used to make lining plate, the temperature before water toughening is heated to 1050 ～ 1100 ℃, which makes the liner austenitizing fully, dissolving carbides into austenite fully, controlling the temperature of water toughening medium below 30 ℃, accelerating the cooling rate of water toughening, and making the carbide precipitate in austenite too late.
Add semi-autogenous mill liners thickness
If the thickness of all liners is increased by 20 mm, the total weight of a set of liners is 48 T, but it is only increased by about 6 t, which is not enough to affect the load capacity of the semi-autogenous mill, and the cost is only increased by about 50000 yuan.
The distribution of shell liner and lifting liner is shown in Fig. 3.
After the transformation, the lining plate was put into use in the second half of 2009. Compared with the lining plate before the transformation, it has the following advantages:
(1) It reduces the maintenance, replacement times, and downtime of the liner, improves the work efficiency and increases the output.
(2) The service life of the liner is extended to 8 months, and the service life is increased by nearly 30%.
(3) The wear resistance and impact resistance of the liner is greatly improved by using zgmn13cr2 steel. After 8 months of service, the liner was shut down for inspection. The thickness of the middle part of the liner and the lifting liner was about 10-18 mm, and the thickness of the liner at the inlet and outlet was about 32-40 mm, indicating that the wear resistance of the liner was significantly improved.