Spiralling in to control
Although different trough designs have led to many models, spirals can be categorised into two broad varieties: washwater and washwaterless.
For both washwater and washwaterless spirals, the mechanism of separation is the same and involves primary and secondary flow patterns. The primary flow is essentially the slurry flowing down the spiral trough under the force of gravity.
The secondary flow pattern is radial across the trough. Here, the upper, more fluid layers of lower density particles move away from the centre while the lower, more concentrated layers of higher density particles move towards the centre.
The innermost zone is generally comprised of higher density particles transported downward, that is primary flow. The rising component of the flow has a certain capacity to lift lower density particles and transport them outward to the intermediate zone, that is, secondary flow.
The intermediate or transition zone is a region of free motion above the bed, which is relatively less concentrated and more fluid than the inner zone. Particles in this region move with the secondary flow and are transported according to their relative position within the bed.
In the outer zone, particles may settle into the lower layers and be transported towards the centre of the spiral. Particles of higher density will have a tendency to migrate into the lower, inward-moving stream.
Particle size is also a significant factor in the mechanics of separation. The finest highest SG particles will be distributed in the main (inner-most) concentrate band whereas the coarser particles of the same SG minerals are often the most difficult to recover to the concentrate.
In applications where the particles are relatively fine or higher density particles are the predominant component of the slurry, the addition of washwater provides greater separation efficiency. In these situations the innermost zone may lose its fluid nature since water can be removed along with the higher density material or crowded-out by the particles themselves. Replenishing the slurry with washwater enhances the upward and outward movement of the lower density particles.
The Outokumpu H9000W spiral, and especially the washwater system, was developed to meet the needs of modern ore producers. The open washwater cup minimises the possibility of plugging while providing variable flow and point control at the spiral trough.
The amount of washwater and its distribution down the spiral trough can be adjusted to meet operating requirements. Point control minimises the total water requirement by efficiently directing the water into the flowing pulp at the most effective angle.
Testing in the early stages is critical to determining the most efficient spiral model and circuit.
During testing, feed distribution, pulp density and feed rate are adjusted to establish the optimum separation parameters for a specific mineral suite. Outokumpu’s spiral test rig at its Perth laboratory, for example, can test and simulate all three stages at once. This means that even the often difficult to model scenarios involving recirculating streams can be handled.
It is important to note the following general rules:
By maintaining a consistent distribution to each spiral, consistent products are achieved
Generally, low pulp density will produce high heavy mineral concentrate grades while high feed pulp densities will result in lower concentrate grades with the higher recovery of heavy minerals
A spiral will normally achieve a minimum 3:1 upgrading ratio (ratio between head feed grade of heavy minerals and concentrate grade). Therefore, as with most gravity concentrators, a multi-pass flowsheet is generally needed to achieve a desired grade and recovery of minerals.
Another element key to achieving the desired processing goal is accurately determining a spiral’s performance. Sometimes, the high recovery of heavy minerals (HM), which includes alumino-silicates, can cause processing problems in subsequent separation circuits.
In these instances, it is necessary to differentiate HM recovery from valuable heavy mineral (VHM) recovery and recognise that, even though the overall HM recovery may be lower the VHM (for example, TiO2 and ZrO2) recovery might be higher and thus the flowsheet is more efficient.
Ideally, a testwork facility with the expertise in both mineralogy and processing should be chosen.
The spiral concentrator is a proven, effective, low-cost device for the gravity beneficiation of industrial minerals and other ores. Made from lightweight, corrosion and abrasion resistant materials, spirals need a minimum of maintenance and upkeep, while at the same time providing an environmentally desirable process.
Steve Benson is manager – physical separation technology for Outokumpu.
22-Feb-2006