Minerals Report - International Seabed Authority

Based on data collected during the first six years of Fe-Mn crust
studies, Hein et al. (122), developed criteria for the exploration for and
exploitation of Fe-Mn crusts, which was later expanded (123) to include 12
criteria. Regional Criteria: (1) large volcanic edifices shallower than 1000-1500
m; (2) volcanic edifices older than 20 Ma; (3) volcanic structures not capped by
large atolls or reefs; (4) areas of strong and persistent bottom currents; (5) a
shallow and well-developed OMZ; and (6) areas isolated from input of
abundant fluvial and eolian debris. Site-Specific Criteria: (7) subdued smallscale
topography; (8) summit terraces, saddles, and passes; (9) slope stability;
(10) absence of local volcanism; (11) average cobalt contents ≥0.8%; and (12)
average crust thicknesses ≥40 mm. Depending on the mining systems used,
crust thickness may turn out to be more important than grade; if true, then the
crust thickness criterion would increase and grade criterion would decrease.
That relationship is inevitable because cobalt grade decreases with increasing
crust thickness.
6.1. Mining Systems
Crust mining is technologically much more difficult than Fe-Mn
nodule mining. Nodule mining concepts developed by mining consortia in
the last decades consist of a hydraulic dredge and a slurry lift system (124).
Recovery of nodules is relatively easy because they sit on a soft-sediment
substrate. In contrast, Fe-Mn crusts are weakly to strongly attached to
substrate rock. For successful crust mining, it is essential to recover Fe-Mn
crusts without collecting substrate rock, which would significantly dilute the
ore grade. Five possible Fe-Mn crust-mining operations include
fragmentation, crushing, lifting, pick-up, and separation (125). The proposed
method of Fe-Mn crust recovery (126) consists of a bottom-crawling vehicle
attached to a surface mining vessel by means of a hydraulic pipe lift system
and an electrical umbilical. The mining machine provides its own propulsion
and travels at a speed of about 20 cm/s. The miner has articulated cutters that
would allow Fe-Mn crusts to be fragmented while minimizing the amount of
substrate rock collected (Fig. 9).
INTERNATIONAL SEABED AUTHORITY 229