Mine 14 — Technical Report

This article outlines the recovery methods for the Mine 14 Project, focusing on the design and operation of a Coal Preparation Plant (CPP) intended for export metallurgical coal production.

The Mine 14 Project is designed to process run-of-mine (ROM) coal through a Coal Preparation Plant (CPP) to meet export metallurgical coal specifications. Because the ROM coal's ash content exceeds 15% on an air-dried basis (adb), processing is required to reduce ash below 8% on a dry basis (db) to meet most market specifications. Dr. Peter J. Bethell of Bethell Processing Solutions LLC was engaged to design the CPP flowsheet, which serves as the basis for permitting and construction specifications.

The plant design philosophy centers on maximizing coking coal yield at a given ash level with minimum moisture content. A two-phase construction plan is being investigated. Phase 1 involves the preparation plant without the ultra-fine coal circuit, while Phase 2 adds the ultra-fine circuit and froth flotation. The facility will be located adjacent to the Mine Gasification System (MGS), with coal trucked from Mine 14 to the ROM stockpile. Clean coal will be stockpiled and loaded onto rail for transport to an export terminal.

The flowsheet utilizes Dense Medium Cyclones (DMC) for coarse/intermediate cleaning and a Reflux Classifier for fines. Phase 2 incorporates froth flotation to treat ultra-fines. The plant is designed for a throughput of 300 tph. Regulatory requirements for permitting are being quantified, and permit assembly is in process. The design allows for flexibility in producing high levels of processing efficiency at varying product quality levels, with anticipated coking coal ash levels between 7% and 8% db.

Critical Data

Overview

Summit intends to sell Mine 14 coal into export metallurgical coal markets. The ROM coal requires processing due to high ash content exceeding 15% adb, which is too high to meet typical metallurgical coal specifications. All coal washability data collected show that ash in both coal seams can be reduced below 8% db. The CPP flowsheet was designed by Dr. Bethell and is being used as the basis for specifications for the CPP being permitted.

The plant will be located adjacent to the MGS on land controlled by Milner. Coal will be trucked from Mine 14 to the ROM stockpile at the proposed preparation facility. Clean processed coal will be stockpiled and loaded onto rail for transport to an export terminal. As much processing refuse as practical will be provided to the MGS and blended into its fuel supply, resulting in minimal ash disposal requirements for the preparation facility.

Key Process Stages

The plant circuit design centers on maximizing coking coal yield at a given coking coal ash level with minimum moisture content. Consideration was also given to minimizing the moisture content of the MGS power plant feed coal to maximize its calorific value (CV). With minimum dilution and low coking coal ash levels around 7%, power plant feed CV on the order of 3,300 kcal/g (ar) can be expected, corresponding to 50% ash on an as-received basis. If coking coal ash levels are high at 8% or above, and open pit mining dilution is high at greater than 20%, then power plant feed CV on an as-received basis could be reduced to approximately 2,200 kcal/kg or less, with ash levels approaching 63% on an as-received basis. These figures are based on the Phase 2 plant design, which incorporates froth flotation treatment of the ultra-fines.

The sizing envelope used in plant design was based on data for the two coal seams to be mined at Mine 14 as well as data from coals of similar Hardgrove Grindability Index (HGI). An attrition test was also performed on the 2011 bulk samples. Historic data and recent sampling were reviewed for the No. 4 and No. 10 seams. All coal samples reviewed exhibited minus 0.15 mm percentages coarser than the maximum fines case, except for the 2006 bulk samples which showed marginally higher ultra-fines in one case and significantly higher fines in the other. These samples were believed to be weathered and oxidized, and therefore excessively fine, so these data points could be considered non-representative outliers. In the event excessively fine oxidized coal is encountered while mining, the plant layout allows space for an extra press to handle the additional ultrafine tonnes.

The yield envelope was developed from samples taken in 2011 of the No. 10 seams as well as historic Summit data, after assuming a maximum open pit dilution for the proposed mine. Plant yields between 55% and 75% were considered in the circuit design, although yields moderately above and below this range will be processed with marginal efficiency loss and magnetite consumption increases. The circuit design provides flexibility in producing high levels of processing efficiency at varying product quality levels. This was accomplished by using dense medium separation for the plus 1 mm material and a Reflux Classifier for the 1 mm x 0.15 mm material. Both processes have the ability to adjust cut-point density and therefore product ash while maintaining high levels of processing efficiency.

The Phase 1 flowsheet does not incorporate ultrafine coal cleaning, with the minus 150 micron material passing directly to the power plant after dewatering. Froth flotation was added in Phase 2 to generate additional coking coal. Plant feed enters at 300 tph up the raw coal plant feed conveyor. The Deslime Banana Screen is 2.4-m wide by 6.1-m long, a multi-slope screen with a single deck of screen panels with 1-mm opening. The sump has 19,000-liters capacity to provide adequate volume to retain slurry circulating in the system when the plant is shut down.

Material is pumped from the sump to one 900 mm diameter DMC. The feed pressure is maintained at 9D where D represents cyclone diameter times circulating media density. A pressure transducer and gauge positioned at the DMC feed pass signals through the plant PLC to display operating pressure. Overflow from the DMC reports to a static sieve for media recovery and then a 3 m x 6 m single deck drain and rinse banana screen. Underflow from the DMC passes to the refuse Drain and Rinse sieve and screen. Additional media drainage occurs on the drain section of the screen, with material returning with the static sieve through to the DMC sump.

Coal from the clean coal drain and rinse screen reports to the dryer to reduce moisture content to an acceptable level. Effluent from the dryer reports to the dilute medium circuit for additional magnetite recovery. The rinsed material from both clean coal and refuse screens as well as the dryer effluent pass to the dilute medium sump. From there, material is pumped to the magnetic separator, with magnetite concentrate reporting to the Dense Medium Sump and tailings reporting to the deslime screen to supplement clarified water.

Material passing through the deslime screen at nominally minus 1 mm passes into the classifying cyclone feed sump, from where it is pumped to the raw coal cyclones. Six units of 380-mm diameter cyclones separate the minus 1 mm feed into nominally plus 150 micron and minus 150 micron fractions. Nominal 1 mm x 150 micron material after sieving to remove oversize passes into the Reflux Classifier for separation into clean coking coal and power plant feed. The overflow from the Reflux Classifier, comprising clean coal plus some slimes, removes ultrafine material which has not been cleaned in the device and can therefore be high in ash.

The sieve effluent passes to the effluent cyclone sump and then to effluent cyclones, with the sieve product being processed by a Screen Bowl Centrifuge, a high gravity force machine designed to produce a low moisture product. Feed entering the unit is processed and split into an effluent stream which is discarded, a low moisture product, and a screen drain which is recirculated through the effluent cyclones back to product. Effluent from the two sieves and the screen drain from the Screen Bowl Centrifuge report to the effluent sump, from where they are pumped to two 380-mm effluent cyclones. Reject material exiting the bottom of the Reflux Classifier is dewatered on the high frequency refuse screen.

In the Froth Flotation Circuit for Phase 2, raw and effluent cyclone overflow report to a bank of five 14 m³ cells. In this flotation process, hydrophobic coal particles are separated from hydrophilic shale, clay, sandstone, and other particles. Subjecting the fine particle slurry to a stream of stable air bubbles causes the coal particles to stick to the bubbles while other minerals remain in suspension. Froth coal concentrate flows over the cell lips and passes into a pressure filter feed sump. The clarified water returns to the plant for re-use. Solids are thickened to approximately 30% solids by weight and pumped to the pressure filtration system. In Phase 2, two of these units serve to dewater the clean coal concentrate from the froth flotation circuit. Slurry is introduced into the units under pressure via positive displacement pumps. A reversible belt configuration allows coal discharging onto the power plant feed belt during Phase 1 to report to the coking belt in Phase 2.

Additional Interesting Data and Summary

The minus 150 micron material passing after dewatering reports directly to the power plant. A system of heat exchangers or steam injection will be incorporated into the process. The plant will be run from a PLC with full automation of sump level and density control achieved through a system of level detectors, control valves, and online density measurement.

The coal processing facility will be located adjacent to the MGS. Although the requirements for water and electrical power have not been finalized, both are readily available at the site. Plant materials include magnetite, frothers, flocculants, and biodiesel. Because of the somewhat remote location of the MGS, stockpiling of these materials may be necessary to ensure availability between delivery shipments.

The DMC feed pressure must be maintained at 9D for optimal separation performance. The signal from the pressure transducer passes through the plant PLC to display operating pressure. A pressure transducer and gauge positioned at the DMC feed enable continuous monitoring of the operating conditions. Additional media drainage occurs on the drain section of the refuse screen, with material returning to the DMC sump. The effluent from the two sieves and the screen drain from the Screen Bowl Centrifuge report to the effluent sump, maintaining the closed-loop media recovery system.

Key Processes

• Dense Medium Cyclone (DMC) for coarse and intermediate coal cleaning
• Deslime Banana Screen for size classification at 1 mm
• Raw Coal Cyclones for classifying minus 1 mm feed into plus and minus 150 micron fractions
• Reflux Classifier for cleaning 1 mm x 150 micron material
• Screen Bowl Centrifuge for dewatering clean coal concentrate
• Froth Flotation (Phase 2) for ultra-fine coal recovery
• Pressure Filtration for dewatering flotation concentrate
• High Frequency Refuse Screen for reject dewatering
• Magnetic Separator for magnetite recovery in the dilute media circuit
• Effluent Cyclone Circuit for fines recovery and water clarification

Source: Mine 14 , Technical Report, Region: N/A. Project website: Maxim Power Corp.

Project website: Mine 14, Technical Report