Ch. 5— Mining and At-Sea Processing Tectinologies • 203 



The jig concentrates are further refined on shak- 

 ing tables before transport to shore for final gold 

 separation and smelting into bullion. It is expected 

 that about 22 pounds of gold concentrate will be 

 produced by mining and processing approximately 

 50,000 tons of ore per day. The actual amounts of 

 concentrate produced will depend on the quantity 

 of heavy minerals associated with the gold at each 

 location. 



Environmental Effects. — The ore processing 

 plant on the Bima returns 99.9 percent of the proc- 

 essed material to the seabed as tailings. Since the 

 tailings do not undergo chemical treatment, local 

 turbidity caused by particles that may remain in 

 suspension is likely to be the most significant envi- 

 ronmental impact. During pilot plant tests in 1985, 

 Inspiration Resources found that turbidity could 

 be minimized by discharging fine tailings through 

 a flexible pipe near the seabed. Other potential 

 environmental impacts could occur if diesel fuel is 

 spilled, either as it is being transferred to the Bima 

 or as a result of accidental piercing of the hull. 



Operating Conditions. — The Bima operates 

 only between June and October (five months per 

 year) because ice on Norton Sound prohibits oper- 

 ations during the winter months. Thus, without 

 breakdowns or downtime due to weather and other 

 causes, a theoretical yearly production of about 7.5 

 million cubic yards of ore is possible. During tests 

 in 1986, Bima operated only a small fraction of the 

 time available. This was due more to the nature 

 of the trials than to downtime related to winds and 



waves. Assuming a mining efficiency (bucket fill- 

 ing) of 75 percent and an operating efficiency of 

 80 percent (allowing for time to move and down- 

 time due to weather), yearly production is limited 

 to 4.5 million cubic yards. If gold grades of 0.012 

 to 0.016 ounces per cubic yard of ore are assumed, 

 the yearly gold production would be between 1 .75 

 and 2.20 short tons (before any losses due to proc- 

 essing and refining). 



The Bima will have a crew averaging 12 persons 

 per watch, 3 watches per day. Personnel are trans- 

 ported to and from Nome daily by helicopter. The 

 operation also requires extensive maintenance, sup- 

 ply, and administrative facilities onshore. These fa- 

 cilities will be manned by an additional 46 persons 

 during the operating season. During the winter 

 months, the Bima will be laid up in Nome harbor, 

 and most of the operating personnel will be on 

 leave. 



Capital and Operating Costs. — Capital and 

 operating cost estimates (table 5-6) are based on 

 a number of assumptions and, like the other 

 scenarios in this report, must be considered first 

 order approximations. The estimates rely in part 

 on published information that the Bima gold min- 

 ing project will have a life of 16 years and will re- 

 cover about 48,000 troy ounces of gold per year 

 at operating costs of less than $200 per ounce. 



The Bima was constructed at a cost of $33 mil- 

 lion in 1979. It is assumed that its purchase in 1986 

 as used equipment (sold because of the fall in the 



I 



Table 5-6.— Offshore Placer Gold Mining Scenario: 

 Capital and Operating Cost Estimates 



Millions of dollars 



Capital costs: 



Exploration and pilot plant mining tests 



Used dredge (BIMA) 



Dredge transport and insurance from Indonesia to Nome 



Shipyard modifications 



Onshore facilities and infrastructure 



Auxiliary vessels 



Total capital costs 



Annual operating costs: 



Fuel and lubricants 



Personnel and overhead 



Maintenance and spares 



Services 



Annual operating costs 



SOURCE: Office of Technology Assessment, 1987. 



$ 3 

 3 

 2 

 5 

 2 



2 



$17 



$1.5 

 3.0 

 1.5 

 1.0 



$7.0 



