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I. Overview
The sand used for modeling has always been made of silica sand. Because the SiO 2 content is as high as 98% or more, a large amount of high concentration of free SiO 2 dust is generated during the casting process, which is extremely harmful to the worker's body. Since 1970, the study has adopted the “seven sands†of limestone processing, which greatly reduced the risk of silicosis. After the success of “Seven O Sandsâ€, the Luda City Labor Health Research Institute and Liaoning’s asbestos mine processing and casting test a new type of Modeling materials. Since 1974, Dalian has four test units were cast, six units were cast iron test. Tests have shown that it has achieved good results in protecting workers' health and simplifying the cast iron molding process and improving the quality of castings .
The use of dolomitic asbestos tailings as molding sand greatly simplifies the modeling process. 5% of bentonite and 3% of water are added to the dolomitic asbestos tailings. After mixing for 5 minutes by the sand mixer, the shape can be directly molded, which can save the circle. More than 95% nails. Mandrel to be dried is not, as long as the burner surface is dry to about 10 mm thick, lead powder was then fused brushing twice, casting boxes can be combined (i.e., tidal die casting), high quality, smooth surface, to facilitate sand . Some units have a rejection rate of 70% in the same type of workpiece silica sand molding, and the scrap rate has dropped to less than 5% when using the dolomitic asbestos tailings. The dolomitic asbestos tailings and silica sand are modeled by the same operation method. The dolomitic asbestos tailings have good gas permeability, and the dolomitic asbestos tailings can be used multiple times. The reusability is better than “seven sandsâ€, 80%. The above can be reused, the amount of sand eaten accounts for about 5%, and the dolomitic asbestos tailings can be preserved for a long time, not afraid of weathering, and not afraid of flooding. And the cost is lower than the "seven o sand".
Second, the technical performance of dolomitic asbestos tailings
(1) Chemical composition of dolomitic asbestos tailings
See Table 1.
Table 1 Chemical composition of dolomitic asbestos tailings
name
Loss on ignition
SiO 2
CaO
MgO
Al 2 O 3 +Fe 2 O 3
content(%)
34.7
12
31
20
2.3
(2) Physical properties of dolomitic asbestos tailings
Hardness: Mohs 4 to 5 degrees;
Shape: Polygon
Refractoriness test: no sintering at 1000 °C,
1050 ° C discoloration,
1100 ° C vitrification begins,
1160 ° C vitrification
Gasification temperature: 750 ° C;
Gas production: 34.7%;
Forming compression test: wet compressive strength 0.2 kg / cm 2 ;
Dry pressing strength: 3 ~ 5 kg / cm 2 .
The performance and cost of dolomitic asbestos tailings and “seven o sand†and silica sand are shown in Table 2.
Table 2 Comparison of performance cost of dolomitic asbestos tailings with “seven o sand†and silica sand
Name
Combined SiO 2 content (%)
Free SiO 2
content(%)
Shape
hardness
(Mohs)
Gas temperature (°C)
Gas production (%)
Refractoriness (°C)
cost
(Yuan / ton)
Silica sand
Seven o sand
Dolomitic asbestos tailings
96~98
5~10
11~12
90
2
2
Circular
Polygon
Polygon
7
3
4~5
530
750
4.3
44
34.7
1730
920
1160
100
50
20
Third, the granularity requirements
According to the requirements of the test, they have produced seven varieties of 10/20, 16/30, 20/30, 30/70, 40/70, 50/100 and -70 mesh, respectively. The combination of varieties can still meet the modeling requirements, but the product of less than 70 mesh should not be too much, otherwise it will affect the quality of castings. Its particle size analysis is shown in Table 3.
Table 3
Granular grade
-10+20
+30
+70
-70
%
0.6
18.8
75
5.6
According to the results of two years of production test, the dolomitic asbestos tailings is a promising new material for cast iron molding, but due to the lower melting point, it is still necessary to carry out some research work directly for casting steel.