Supercharge your crushing process

For many years the Barmac VSI, with its cascade system, has provided progressive processing operations with the ability to maximise production of the products they require while at the same time minimising total operating cost.

Barmac operation
The autogenous Barmac VSI features a rock-lined rotor at the heart of its operation. The rotor launches a non-stop rock stream into the Barmac VSI's rock-lined crushing chamber. Material fed into the top of the machine is accelerated in the rotor and is continuously discharged into the crushing chamber at velocities of up to 85 metres (275 feet) per second.

This process replenishes the rock lining while at the same time maintaining a rock-on-rock chain reaction of crushing and grinding. A second stream of material can be cascaded in a controlled quantity into the crushing chamber, causing an increase in the particle population within the chamber, thus improving energy transfer.

Cascade
Material is fed straight into the Barmac VSI rotor or can be set to bypass the rotor and directly enter the crushing chamber by way of the cascade feed system. In combination with other variables such as rotor diameter, speed and crushing chamber profile, this can enhance power efficiency, reduce wear, and provide an efficient means of controlling the grinding and crushing action to either maximise or minimise fines.

Using the cascade feed system is like supercharging the Barmac VSI. Crushing efficiency is improved by increasing the particle cloud density in the crushing chamber, thereby increasing the frequency and probability of collisions. This gives the operator more value for money by providing more inter-particle action where it is needed most - in the crushing chamber.

The cascade feed system provides:

• Higher crusher throughputs for the same power consumption.
• Maximum use of available power without fear of motor overload.
• Gradation control to optimise product fractions.
• ‘Free' crushing at no extra expense to the operator.
• Shape control.
  
  

Testing cascade
Tests run for a customer in the USA showed that using cascade can increase the amount of any given fraction, even with the coarsening of the Barmac VSI product curve, as shown in figure 1.

This particular customer was interested in the production of ½" product. They experienced an increase in the TPH of this fraction, for any given power requirement, and an overall increase at 25-30% cascade. The amount of ultra fine material produced is reduced, which was beneficial for this customer's requirement of meeting the local standards and reducing the production of waste material.

The product gets finer as the machine is loaded up. The green, blue and red lines on the graph in figure 1 depict this. All have 0% cascade and operate at the same tip speed, with the only difference being the higher tonnage drawing more amps.

It is important to note the finer product curves at the higher throughputs when appreciating the reasons why cascade works. This can only be explained by the increase in material density in the chamber, which increases the chance of a good, rock breaking, impact. As machine throughput is further increased it is handled by cascade (shown in figure 1 by the yellow and brown curves). The product curves are coarsening and the benefits of increased chamber density are offset by the static energy input (constant motor amps).

It must be noted the feed curve represents the feed to the circuit not the Barmac VSI. The product curves represent the Barmac VSI product, not the circuit product. It is important to consider this when interpreting the results.

Figure 1


On the other side of the Atlantic
Tests for a customer in France have also proven the benefits of using cascade. Figure 2 shows a closed circuit test – the grading curves depict this – and this time the feed curve shown is the feed to the Barmac VSI, not the circuit.

This customer was interested in the 4.75mm material. The product curve coarsens as the cascade percentage is increased. The motor was kept at full power of 200kW so the same amount of material goes through the rotor, with the same power and wear part consumption.

The effect on the feed to the Barmac VSI is an increase in the coarser fractions (12.5mm) and a reduction in the middle fraction just above the cut size. This suggests the Barmac VSI is converting more of the middle fraction into product and there tends to be a build up in the coarser fractions when the Barmac VSI recirculation material is combined with the new feed. This demonstrates why the effect of the long-term build up of coarse material on the circuit must always be considered.

There is a similar increase in crusher, and hence circuit, loading. There is an increase in the ¾” and 1” material that is building up to the new equilibrium level and a reduction in the fraction just above the cut size. This indicates that with this material the Barmac VSI operating with higher cascade is more efficiently turning the middle fraction into product, but it will struggle with the coarser fraction until the new equilibrium is found.

Again more material at the desired cut and less of the waste ultra fines is produced. Unfortunately there is an increase in the quantity of 100mesh. However, as a percentage of total production, it is less. We see a 7.3% increase in the production of 4.75mm material and a 7.8% increase in 100mesh.

Customers need to decide if they can live with this. The key is to find the most appropriate level of cascade to run at to meet an operation's requirements.

Figure 2


Precise control of the crushing process is the key to profitability
Experience has proven that the key to profits in the aggregate production and minerals processing industries lie in the ability to produce consistent products of high quality.
Cascade gives accurate control of product specification and grading by allowing external hydraulic adjustment of the cascade ratio while the crusher is operating. Adjustment can be hand operated at the crusher or be fully integrated into automated plant systems.

Automating the Barmac VSI crushing process allows the operator to keep crushing at maximum operational limits, but within safety margins, and contributes significantly to the efficiency and availability of the crusher. It allows the operator to take full advantage of the features offered by the Barmac VSI by gaining precise control of the machine's ability to cascade and by gaining complete control over product grading and cost per tonne.

Automation makes it possible to perform analysis and corrective actions based on process variations. It allows the operator to program in the right crusher settings quickly and easily for a desired product, and maintain those settings to maximise production.

The Barmac VSI comes with an automation package that includes the VSI Operational Control System (VOCS) and the Automatic Crusher Regulation (ACR) system. VOCS monitors crusher bearing and motor winding temperatures, along with machine vibration levels, while the ACR monitors and controls crusher load and cascade levels.

The cascade process


1. The spreader plate and cascade port shields assist in centralising the feed and controlling the flow to the control gate.
2. The position of the control gate controls the flow of material into rotor. Excess material unable to flow through to the rotor, overflows through the cascade ports.
3. The rotor accelerates material and continuously discharges it into the crushing chamber. Particle exit velocity ranges from 50-85 m/s (165-275 ft/s).
4. Material from the rotor and the cascading material recombine. A constant cloud of airborne particles moves around the crushing chamber. Particles are retained for 5-20 seconds before losing energy and falling from the crushing chamber.