|
 Krebs Spirals
Krebs Coal Spirals are used to make density-based separations on nominal 14M x 100M (1.19mm x 0.149mm) size coal. These devices consist of a 0.96-meter Ø helix with between 2 and 3.25 turns, mounted on a 160-mm Ø center column. To provide greater capacity per volume, either two (duplex) or three (triplex) helices can be mounted on one center column. Each helix mounted on a center column is typically referred to as a "Start". Each helix contains:
- feed boxes are used to properly distribute the feed slurry to the helix
- product-discharge boots are used to collect the products separated by the spiral. Each product discharge boot has two (2) adjustable-position product splitters which direct the "clean-coal" (outer stream), "middlings" (middle-stream), and "refuse" (inner stream) to their respective discharge ports in the boot.
Spirals are further grouped into rectangular-shaped "Packs", consisting of between two and ten duplex or triplex spirals, which are fed from a central radial distributor. For example, a ten-pack of duplex spirals contains 20 starts, a ten-pack of triplex spirals contains 30 starts.
Beneath the spiral pack is a discharge piping system which collects the clean-coal, middlings, and refuse from all the product-discharge boots and directs them to a central discharge location.
Equipment
- 3.25-turn Coal Urethane General Purpose Spiral (CUGP)
- 2-turn Coal Middlings Retreat Spiral (CMR)
- 2.25-turn primary and 2-turn secondary Coal Compound Spiral (CMP/2
3.25-turn Coal Urethane General Purpose Spiral (CUGP) The general purpose, single-stage coal spiral is the main product employed to separate 14M x 100M (1.19mm x 0.149mm) size coal. These units can typically treat between 2.2 and 3.85 st/h of dry solids per start, depending on yield and difficulty of separation. When fed a slurry containing between 30 and 35 percent solids (w/w) this equates to between 28 and 35 gpm slurry per start and a maximum of 44 gpm of feed slurry. The capacity limitation on any spiral is refuse capacity, which is typically around 1.65 st/h per start.
Since coal spirals produce three discharge streams (clean-coal, middlings, and refuse), each spiral start actually achieves two density separations:
- a clean-coal / middlings separation, termed the "clean-coal split"
- a middlings / refuse separation, termed the "refuse split".
Coal spirals exhibit general separation characteristics where the 14M x 28M (1.19mm x 0.595mm) size coal is separated at higher densities than the 28M x 60M (0.595mm x 0.250mm) size coal. The separation of the 60M x 100M (0.250mm x 0.149mm) size and 100M x 200M (0.149mm x 0.074mm) separate at densities equal to or higher than the 14M x 28M (1.19mm x 0.595mm) size coal. There is minimal cleaning effected on the minus 200M (0.074mm) size particles. Typical 14M x 28M (1.19mm x 0.595mm) size coal D50’s are between 1.80 and 1.90 SG for the clean-coal split and 2.10 and 2.25 SG for the refuse split. The 28M x 60M (0.595mm x 0.250mm) size are typically separated at between 1.70 to 1.80 SG for the clean-coal split and between 1.90 and 2.10 SG for the refuse split.
2.25-turn primary and 2-turn secondary Coal Compound Spiral (CMP/2)
Research has shown that the majority of the density separation on a coal spiral occurs in the first two-turns of the spiral. In addition, analyses of operating spirals indicates that when the middlings are directed to refuse to effect a lower-density separation, some 28M x 60M (0.595mm x 0.250mm) size coal is typically lost in the middlings stream.
Taking these conditions into account, Krebs/SWMS developed the compound coal spiral (patent pending). This unit is comprised of a 2.25-turn duplex or triplex primary spiral and a 2-turn simplex secondary spiral, all on the same center column. This arrangement provides a two-stage system within the same physical volume as a conventional 5-turn spiral column.The compound spiral is piped so that the middlings produced by the primary-stage, duplex spirals can be repulped and fed to the secondary spiral beneath them. A dilution water header, fitted with individual metering valves, are installed along each side of the spiral pack to permit individual adjustment of dilution water to each middlings-retreat spiral.
This arrangement has several advantages over conventional two-stage spirals:
- The compound spiral provides more compact installations, requiring a single floor to contain both primary and secondary spiral stages, rather than the two-to-three floor requirement of a conventional two-stage system.
- The compound spiral requires only a single radial distributor for the primary stage rather than the conventional’s individual distributors for both the primary and secondary stages.
- All adjustments between the primary and secondary stages are done at one location (the primary-stage splitters, secondary-stage splitters, and secondary-stage dilution water are all adjusted at one location). In contrast, a conventional two-stage system requires setting the primary-stage on one floor, moving to another floor to adjust dilution water, and then moving to another location to adjust the secondary-stage splitters.
- The compound Spiral is extremely well suited for tailings pond recovery projects where feed quality is often changing; the second stage "catches" the extra coal particles misplaced to middlings in the first stage
2-turn Coal Middlings Retreat Spiral (CMR) The 2-turn middlings retreat spiral is used where a low-yield feed requires mininal upgrading. This unit features a proprietary helix profile which promotes movement of refuse toward the center column. These units are typically used in Europe and Africa rather than in North America. Q: What is the particle topsize effectively cleaned in a spiral?
A: Although 2mm is generally accepted as the largest topsize, a target size of 1.19mm should be used.
Q: What is the lowest feed rate that should be fed to a spiral?
A: Roughly 28 gpm is required to establish proper conditions on the spiral trough.
|