Getting your float circuit layout right
Sunday, 03 July, 2005
After confirming that the instruments and valves are working correctly, you conclude that the float circuit can only handle 80% of the design flow. Clearly a potential disaster. However, all is not lost.
The following article examines why this situation occurs and offers some ways to alleviate the problem if confronted with it.
How did this happen?
The most basic objective of the flotation circuit designer is to ensure that the plant's hydraulic capacity allows the design flow to pass through the plant in a controlled manner. This requires the correct sizing of slurry control values, which in turn requires determining the available driving head. There are two general situations in the flotation circuit where the driving head needs to be carefully considered: between cell banks (1) and between flotation cells and conditioner tanks (2).
1. Between cell banks
The driving head between cell banks is used to determine the size of the slurry control value. It is determined by three factors:
- The physical height between the two cell banks known as the 'step' height;
- The relative slurry levels or froth depths;
- The relative slurry densities (the slurry density changes along the cell row as the solids and/or air addition rate varies).
This equation [image] is used to calculate the driving head DH:
2. Between flotation cells and conditioner tanks, thickener feed or pump hoppers
This situation is not dissimilar to the one described above. The driving head is still determined by the physical height between two slurry levels and the relative pulp densities. In this case it is important to look at the actual slurry levels in the flotation cell, which has froth, and the downstream vessel, which does not. It is also important to consider whether the conditioner is aerated or not. Let's look at the situation presented in Figure 1 again, but in this case assume an un-aerated vessel (conditioner, thickener etc) instead of the second flotation cell and the calculation becomes:
You can see that DH is significantly less than in the cell-to-cell case. In this situation an additional 200 mm height difference between the two pulp levels is recommended.
What these two calculations illustrate is that consideration of the physical layout of the flotation circuit alone is not sufficient to prevent flow restrictions. An understanding of the process conditions is also required. In both cases the available driving head is significantly less than the physical step height. This difference can easily result in the under-sizing of the slurry control valve. Taking case 2 as an example, the difference in valve size for a nominal flow of 1000 m3/h is 22%. If a DH equal to the step height of 0.8 m is used, a 400 mm diameter valve is selected. If the correct DH of 0.54 m is used, a 450 mm diameter valve is required.
What can I do to fix this?
There are really two ways to fix this problem if you find it in your plant: increase the driving head or install larger control valves. In either case it is critical to correctly assess the existing situation before deciding on any modifications.
Increasing the driving head
Generally speaking, this is not practical as it is very difficult and expensive to change the physical height between cell banks once the plant is constructed. However, there are some situations in which it is possible to better utilise the existing plant layout. One such example is when an inter-stage conditioner is employed. Many such conditioners are installed with the same step height upstream and downstream of them. This can be problematic when trying to push aerated pulp into an un-aerated conditioner. However, an opportunity also occurs because un-aerated pulp in the conditioner creates additional driving head when transferring to the next aerated stage of flotation. In this situation it is possible to lower the outlet level in the conditioner, thus transferring the excess downstream driving head back to the bottlenecked upstream side. The disadvantage of this change is a reduction in conditioning time, as the operating level of the conditioner is lower. Depending on the situation, this may be up to a 10% loss.
Increasing the slurry control valve capacity
Here we have two options: increase the valve size or change the valve profile. Increasing the valve size is usually preferable to changing the valve profile. Changing the valve profile generally involves compromising the range over which the valve controls effectively. If the valve size needs to increase, it is important to consider first if a larger valve will physically fit. Areas to consider for both pinch and dart valves are outlined below.
- Is the distance between vessels sufficient for a longer valve?
- Are the valve inlet and outlet nozzles large enough?
- Will a larger valve fit into the dart box?
- Is the driving cylinder large enough?
- Can the dart seat be changed?
- Is the downstream piping from the valve large enough?
In either case a well-designed flotation cell should make such an upgrade possible. Cells with removable dart valve seat plates or oversized pinch valve connection nozzles that allow the installation of larger valves are invaluable. After all, even if your original float plant design is fine, you might want to increase the flow in the future.
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