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38 www.coalage.com June 2016 fine coal recovery Installation of Sub325 Centrifuge for Dewatering of Ultra-fine Coal by t. anthony toney, nick massini and cliff raleigh Since the late 1980s, screen bowl centri- fuge technology has been widely applied in coal preparation for recovery and de- watering of fine and ultra-fine coal. Screen bowls are efficient and cost effective and, to a large extent, have replaced vacuum filters for recovery of these size fractions. However, compared to vacuum filters, screen bowls generally do not recover all of the ultra-fine coal. Very recently, high G solid bowl centrifuge technology has been harnessed and adapted to recover and de- water fine and ultra-fine coal historically not captured in fine coal centrifuges. This article follows the initial on-site testing, installation, and then long-term testing of the Sub325 solid bowl dewater- ing technology to recover and dewater ul- tra-fine coal. Losses from the original fine- coal circuits are quantified and found to be much larger than shown on a typical plant flow sheet. Furthermore, supplemental benefits generated from the recovery of additional fine high-quality coal are real- ized and quantified. Screen bowls lose a significant amount of fine clean coal through the screen open- ings (which is typically recirculated, but only partially recovered) and lose ultra fines to the bowl effluent. New technology was needed to improve recovery of this former- ly lost coal and bridge the gap between the screen bowl centrifuge's improved moisture and the filter's improved recovery. Somerset Coal International (SCI) now offers a way to recover the lost revenue from the ultra-fine coal stream with the Sub325 solid bowl centrifuge. The high-speed cen- trifuge multiplies the centrifugal forces to more effectively recover and dewater fine coal. This is accomplished while still reject- ing the super-fines and colloidal clays that increase the moisture in the filter product. The result: high recovery with selective rejection of unwanted material. Sub325 Fine Coal Recovery System SCI has partnered with Centrysis Corp. to "crack the code" for recovery of ultra-fine coal at acceptable moistures. Centrysis has adapted existing proprietary technol- ogy used worldwide for cost-effective fine particle recovery. The Sub325 has been customized for the recovery and the dewatering of fine coal through a combination of: • Unique internal geometry; • Ultra-high G forces (more than five times typical screen bowl G forces); • Variable rotational speeds (for site spe- cific needs); • Adjustable differential speeds (vari- able while operating); and • A dedicated PLC to monitor and control the machine operating parameters. Like other bowl centrifuges, the SCI unit is a robust unit designed for many hours of uninterrupted operation. It has low connected horsepower, a relatively small footprint, requires no operator and requires little maintenance other than predictable rotating unit replacement. The three process connections are feed, product and effluent. For process capacity application, generally one SCI Sub325 unit is used to process the screen drain and a portion of the bowl effluent from one to two large screen bowls. On-site testing uses a full-scale pro- duction unit mounted on a 55-foot, low- boy trailer as shown in Figure 1. The unit is delivered to site, process connected, electrically connected and running coal generally within a shift of arrival. Test- ing is conducted over a one- to two-week period. The test setup includes an interme- diate sump/pump to collect various pro- cess streams for delivery to the SCI unit. Process connections are made to collect screen bowl screen drain and screen bowl affluent streams. Process flows are mea- sured, densities are checked and reliable/ repeatable gallon per minute and ton- per-hour calculations are made for each stream. Combinations of the streams are also processed for comparison. The test unit is now a well-traveled unit. It has been to more than 27 loca- tions and 39 one- to two-week tests have been conducted. Test locations include sites in Pennsylvania, Virginia, West Vir- ginia, Ohio, Indiana, Illinois, and east and west Kentucky. An extended four- month test unit was installed in western Pennsylvania and an extended test unit is installed in Australia. Test Observations One major trend noted in on-site testing is this: The plant flow sheet frequently un- derstates the tonnage of coal in the screen drain by a factor of two or more. In al- most every site test, we find the measured amount of screen drain tonnage is double, or more, the tonnage shown on the flo sheet. This is significant. Plant P&IDs; and mass balances are often seen as the only sure thing in an operating plant. However, these are the best guesses prior to construction and rarely get updat- ed with actual operating production data. Figure 1: On-site testing with the Sub325 centrifuge.