24-MMP-B5 Mineral Processing Design and Operations · Undated paper
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 09-MMP-B5, Mill Design and Operations — May 2019, 3 hours. Candidates were instructed to answer any 6 of the 8 questions (each of equal value, 16.7%); all 8 are solved below as a complete study resource.
Reference texts: Wills' Mineral Processing Technology (B.A. Wills & J. Finch, 8th ed., Butterworth-Heinemann) — Ch. 4 Comminution, Ch. 8 Screening, Ch. 9 Classification, Ch. 12 Froth Flotation, Ch. 13 Leaching, Ch. 14 Solid-Liquid Separation; Mular, Halbe & Barratt (eds.), Mineral Processing Plant Design, Practice and Control (SME, 2002); Mular & Poulin, CIM Special Volume 47 (1998) preliminary capital cost estimation; Doll & Barratt (2010) SAG mill design correlations; SME Mining Engineering Handbook (3rd ed.).
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
(a) A conventional thickener is a large circular tank with a central feed well, slow-rotating rake arms on a central drive shaft, a peripheral overflow launder, and a sloped or conical floor with a central underflow discharge. The feed is a dilute slurry (typically a few percent to 20-30% solids) drawn from an upstream unit operation — tailings, final concentrate, or leached residue — dosed with a high-molecular-weight flocculant just ahead of (or inside) the feed well to aggregate fine particles into fast-settling flocs. Two products leave the unit: a clarified overflow (low-solids water, recycled to the process or discharged) and a thickened underflow (a much higher-%-solids slurry sent to further dewatering, tailings disposal, or downstream processing). The dominant reagent is a synthetic polyacrylamide-based flocculant (anionic, cationic or non-ionic depending on the slurry chemistry), occasionally supplemented by a coagulant (e.g., lime or alum) to neutralize particle surface charge ahead of flocculation.
(b) A high-capacity (or "high-rate") thickener achieves far greater throughput per unit floor area than a conventional unit by (i) diluting and dispersing the feed evenly across a large-diameter feed well so flocculant contacts every particle uniformly, (ii) using a deep, steeply-sloped compression zone (sometimes a deep-cone or "paste thickener" geometry) that gives settled solids a long residence time under their own weight to consolidate to a high final underflow density, and (iii) fitting a mechanically robust, torque-monitored rake with picket fences or rakes designed to channel water upward through the settling bed (assisting dewatering) without disturbing the compacting solids. These design features let a high-capacity unit process several times the tonnage of a conventional thickener of the same diameter, at a correspondingly higher underflow density (useful for paste backfill or dry-stack tailings).
(c) Key operating and process control features include: continuous bed (mud-line) level or interface detection, typically by ultrasonic or nuclear density gauges, to keep the compression-zone interface at its optimum depth; rake torque monitoring, which shuts down or raises the rake mechanism automatically if underflow density rises too far and threatens to stall or damage the drive; flocculant dosage control, combining feedforward dosing (proportional to measured feed flow and solids content) with feedback trimming from overflow turbidity or clarity sensors; and underflow density control via a variable-speed underflow pump, which is throttled to hold the target discharge density without starving or flooding the compression zone.
