Question 2 of 11

Question 1.2 (6 marks)

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.

1.2.1 — BCE vs. BWE. A bucket-chain excavator (BCE) carries a continuous chain of buckets around a boom-mounted digging ladder, similar in principle to a dredge ladder; the chain digs continuously as the whole machine (or ladder) slews or advances, and is best suited to soft, unconsolidated, wet or swampy ground (e.g. tin placers, oil-sand overburden) where a wheel cutting head would clog. A bucket-wheel excavator (BWE) instead mounts a large-diameter wheel of buckets on the end of a boom; the wheel rotates about its own axis while the boom slews horizontally and luffs vertically, cutting a bench face in a sequence of horizontal or vertical slices. BWEs achieve far higher continuous output (2,000–15,000+ bank m³/hr for large units) and are the standard continuous-mining machine for large lignite, oil-sand and phosphate operations in moderately consolidated material; BCEs are now comparatively rare and confined to soft alluvial/placer duty.

1.2.2 — capital and erection cost. A large modern BWE (bucket-wheel diameter 5–18 m, output up to ~10,000 bank m³/hr) typically carries an on-site delivered capital cost in the order of USD 20–150 million depending on size class, with erection (site assembly of a machine shipped in major sub-modules — superstructure, boom, wheel, crawler undercarriage) adding a further 10–20% of the ex-works price and 6–18 months of site labour, since these machines are far too large to ship assembled. Auxiliary conveying (mobile boom stackers, bridge conveyors, spreaders) typically adds a comparable capital sum again, so the BWE is normally costed and budgeted as one complete continuous-mining SYSTEM, not as a single machine line item.

1.2.3 — materials handling, face to disposal. At the face, each bucket is fitted with replaceable digging teeth (for hard, abrasive, blocky material — individual hardened-steel teeth absorb impact and are replaced piecemeal) or a continuous cutting lip (for softer, more homogeneous, less abrasive material — a lip gives a cleaner cut profile and lower unit digging resistance, but a single damaged section requires replacing the whole lip segment). Once cut, material falls by gravity through the open bucket back as the bucket rotates over the top of the wheel, discharging onto a short wheel-boom conveyor; this feeds, via a slewing/luffing transfer point, onto the machine’s own main discharge boom conveyor, which in turn discharges either directly onto a mobile bridge conveyor (for immediate spoil placement) or onto a semi-mobile/shiftable conveyor system feeding the plant or an external spreader.