Summary: Basalt is the best material in the stone used in the repair of roads, railways, and airport runways. It features wear resistance, low water absorption, low electrical conductivity...
Basalt is the best material in the stone used in the repair of roads, railways, and airport runways. It features wear resistance, low water absorption, low electrical conductivity, high compression resistance, low crushing value, excellent corrosion resistance, and strong asphalt adhesion. It is internationally recognized as an ideal aggregate for railway and road construction.
When it comes to the crushing and sand making of hard rocks such as basalt and granite, many people worry that spare parts wear severely, replacement frequency is high, output fails to meet design requirements, efficiency is low, and the finished sand has a poor grain shape. Indeed, making sand from hard rocks such as basalt is really difficult!
Difficulties of basalt crushing and processing
1. Basalt has high compressive strength, good rock toughness, high hardness, strong abrasiveness, and great difficulty in crushing, which makes it difficult for the actual processing capacity of the crushing equipment to reach the theoretical output capacity.
2. After the crushing process of basalt, the final products have poor grain shape, and it is difficult to control the needle‑flake content of the finished coarse aggregate to meet specification requirements.
3. After the sand making process of basalt by vertical shaft impact crusher, the content of stone chips and coarse particles in sub-5mm aggregate is relatively high, while fine particles are insufficient. Meanwhile, the sand has a higher fineness modulus and a low stone powder content. If customers adopt a rod mill to process basalt for sand making, the output of a single unit is low, while water consumption, steel consumption and power consumption are all high.
Technological countermeasures of basalt crushing processing
During the preparation period of a hydropower station, the sand and gravel processing system encountered the above problems. The raw materials are dense massive basalt and amygdaloidal basalt, with dry compressive strength ranging from 139.3–185.7 MPa and 163.3–172.9 MPa respectively. The total amount of concrete to be processed by the system is about 1.2 million m³, and the production capacity of the system is 154,000 T/month. Among them, the processing capacity of raw material is 560t/h, the production capacity of finished aggregate is 396t/h, and the production capacity of finished sand is 140t/h.
1. Equipment selection
In view of the characteristics of basalt, it was decided to adopt the process of "four-stage crushing, vertical shaft impact crusher and rod mill combined sand making (commonly used sand making process)". The main workshop layouts include: coarse crushing workshop, medium crushing workshop, screening workshop, sand making workshop, inspection and re‑screening workshop, coarse and fine aggregate storage yard, etc. In addition, the equipment load rate should be kept low during selection, with sufficient production capacity reserved.
2. Grain shape control of finished aggregate
In view of the difficulties of poor grain quality and large content of small and medium needle-like particles in finished aggregate after basalt processing, the quality of finished coarse aggregate can be controlled mainly through the following measures:
On the one hand: control the crushing ratio of medium and fine crushing, continuous feeding, realize fully feeding, lamination crushing and other measures to control the grain quality.
On the other hand: In view of the characteristics of high content of needle-like particles after basalt crushing, shaping machine is adopted. The first screening workshop after medium fine crushing does not produce small size finished particles, but only large size and medium size finished particles. Aggregates are fed into the ultra-fine crushing workshop (equipped with 3 sets of vertical shaft impact crushers with shaping functions). After shaping in the secondary screening workshop, small-sized particles and particles below 5 mm are produced.
3. Control of sand formation rate, fineness modulus and stone powder content
In view of the characteristics of low sand formation rate, high fineness modulus of finished sand and low stone powder content of basalt sand, the following corresponding measures are mainly adopted:
First of all, improve the rotor speed of the vertical shaft impact crusher, improve the linear velocity of aggregate in the crushing cavity, improve the sand formation rate, and the stone powder content of the produced sand, and reduce the fineness modulus of the sand at the same time;
Secondly, adjust the feeding grade of vertical shaft impact crusher, which can improve the sand making effect very well;
Thirdly, the content of stone chips in < 5mm aggregate produced by the system after coarse crushing and medium &fine crushing is high. The process makes this part of aggregate not to be finished products, so that all < 5mm aggregate after medium and fine crushing are fed into the vertical shaft impact crusher for shaping, in order to control the quality of finished sand;
Fourthly, the vertical shaft impact crusher processes the aggregates from the secondary screening workshop. The sub-5 mm particles enter the rod mill for re-crushing to adjust the fineness modulus and stone powder content of the finished product.
Fifthly, when a vertical shaft impact crusher is used to produce manufactured sand, the lower the moisture content of the processed aggregate, the better the sand making effect. According to this characteristic, the whole dry method is adopted in the system to improve the sand making effect of the sand making machine.
4, Stone powder content of finished sand
The system produces sand for normal concrete and sand for RCC. The biggest difference between the two types of sand is the different content of stone powder, which is 6-18% in the former and 12-18% in the latter. In the process, the following measures are mainly adopted to control the quality of the two finished sand:
Firstly, no finished products are discharged after medium and fine crushing. All crushed materials undergo secondary screening and classification before being fed into vertical shaft impact crushers. The 3–5 mm aggregate is separated and conveyed to a rod mill for further crushing. The process adopts the all-dry production method. To properly regulate the stone powder content and fineness modulus of qualified concrete sand, vertical shaft impact crushers and dry processing technology are applied.
Secondly, vertical shaft impact crusher and rod mill are used to make sand for RCC. Meanwhile, stone powder is recovered from the fine sand lost during sand washing. All the stone powder recovered by the recovery device is mixed into the sand for RCC to improve its stone powder content.
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