Caisson Thickeners

Caisson Thickeners

To discuss “Large Diameter Thickeners in the Phosphate Industry” is to discuss large diameter slimes thickeners in general. Since 1948, Dorr-Oliver has supplied these machines to the phosphate industry, originally as a means to reduce pumping costs, but more recently for environmental reasons as well. With the exception of the Caisson thickener, all the types of thickeners to be discussed have been used by Southeastern phosphate producers. Perhaps in the future its greater reliability and the economics of the time will dictate the use of the Caisson concept for phosphate slimes.

The Caisson Thickener was developed to take advantage of the dry well concept of withdrawing underflow into the center column. This concept required a center mechanism where it was possible to service or remove the underflow pumps.

The large diameter center column or caisson is sized to house the underflow pumps. Mechanisms have been built to service caissons between four meters and eight meters inside diameter. Several pumps are used to draw off the underflow. Two pumps, 180 degrees apart, are used. This arrangement is due to the length of the discharge trench necessary around the large diameter caisson, and also due to the greater amount of solids to be handled. Multiple pumps, each with lines sized for their capacity, and stand-by pumps for handling maintenance requirements, are essential for proper operating flexibility.

Multiple pumps, important during peak operating conditions, are necessary to remove excess material and reduce high loads created by surges. They may be equally important during start-up, when it may be necessary to pump less material until the layer of solids with lighter slimes has settled near the center, or for recycling until the unit reaches equilibrium. The smaller lines, each sized for the capability of its respective pump, allow reduced quantities of flow to be pumped and keep line velocities high enough to prevent heavier solids from settling in the vertical lines exiting the caisson. They also minimize the problem encountered in evacuating a large quantity of, solids and prevent short-circuiting of the underflow pump draw-off to the more dilute settled slurry or clear liquid. If geological conditions permit, the caisson diameter can be expanded below the level of the discharge trench to create a larger area to house the pumps. This configuration provides added head on the suction side of the pump. The area is obtained without added structural cage costs that would be necessary if the column itself were of the larger diameter, yet the opening in the center mechanism is adequate for servicing of the pumps.

The hydraulic system for the center mechanism’s hydrostatic bearing is also housed in the caisson. It is usually located on the first deck below the main gear. It is also quite practical to build a house on top of the caisson to house the control panels for the drive mechanism and the underflow pumping system.

Since the start-up of the first caisson thickener with a hydrostatic bearing in 1964, Dorr-Oliver has sold eighteen additional units. All but one are caissons. Of the nineteen, one is being erected, the others are in operation. The hydrostatic bearing thickener that is not a Caisson model is 152 m in diameter and is in a phosphate plant in Florida. It shares the siphon feed and lack of access to the center mechanism with older thickeners in this area. In over six years of operation, it has proven to be a rugged and reliable piece of process equipment. We feel that the success of these units strongly demonstrates the benefits of the caisson hydrostatic bearing concept.

  1. When one large thickener can be used in place of two smaller ones, there is a substantial reduction in the cost of the total installation.
  2. The center mechanism and hydrostatic bearing system are more reliable than the conventional ball bearing system.
  3. The center mechanism bearing has high dead load capacity and high radial stability.
  4. Friction in the support bearing is significantly reduced and results in longer operating life and lower maintenance.
  5. Unbalanced moment on the rake arms is resisted within the bearing itself.
  6. Underflow tunnels can be eliminated.
  7. The caisson serves as a pump house and allows easy access to underflow lines.
  8. Positive head on the underflow pump intake lines provide optimum pumping conditions.
  9. Pumps can be serviced at the bottom of the caisson. If major repairs are required, the pumps can be removed through the center mechanism and across the truss with the crane.
  10. The feed line under the bridge and the underflow lines on tope of the bridge are accessible for routine maintenance.
  11. The drive mechanism is easily accessible for routine maintenance.

The caisson hydrostatic bearing concept has proven to be a reliable unit in applications with iron ore tailings, copper tailings and a counter-current decantation leaching system. We feel that the concept has a great deal to offer to the phosphate industry.

large diameter thickeners