The original plant was to produce 3,000,000 mpty of 64% Fe concentrates. There was no
pelletizing plant involved, but one was planned for the near future. The contrates were a
mixture of coarse Humphreys spiral and fine magnetic seperator concentrates. They were to be
used as sinter feed (Germany had a large surplus sintering capacity required to treat the low
grade domestic ores). The mine, started on the tops of the mountains, produced mostly
weathered ore and concentrates were high in hematite. In the very beginning, it seemed that
because there was little magnetite in the feed that the magnetic separation facilities could
have been deffered for a while.
The Mine in 1965
Mining was by open pit methods and continued to be for the life of the operation. There was
little true stripping, only grubbing and removal of a shallow topsoil in places and the
removal of some included low iron waste where wide enogh. Mining started on the hilltops and
all haulage in the beginning was downhill. Trucks had to be equipped with hydraulic retarders.
The mine was equipped withh the best machinery available in the mid-60s, almost all of US
manufacture. Europe and the japanese had not yet entered the large open pit mining machinery
business. As mining progressed and layback stripping increased, the machinery suite would
increase. In the beginning, the mining rate was 7,000,000 mtpy to produce the required
3,000,000 mtpy of 64% Fe concentrates. The size and types of mining machinery sound like the
60s: Bucyrus-Erie 40R rotary drills, P&H 1600 electric shovels 6.0 cuyd (in ore) smaller
diesel shovels in waste, Euclid R45 end dump trucks. (Reliable front-end loaders were a few
years in the future and front shovels were a long way into the future). Table 1 tabulates mine
data after the first year of full operation.
Little stripping was required in the beginning but as the deposit was mined below the base
level of erosion, considerable layback was required as can be seen in the idealized cross
section. Mine planning was carried out in considerable detail with frequent assays on blast
hole drill cuttings. Assays were made for total Fe and divalent Fe. Magnetite content was
determined with a Davis Tube. Ongoing core drilling and plan and section mapping was done by
the engineering department.
The initial concentrator was based on true autogenous grinding operating of primary crushed
ore of 8 to 10 in. with no other grinding media. In later years, as the feed became almost
all magnetite hard ore, some mills were converted to semi autogenous grinding with the
addition of some large grinding balls.
The primary crusher was 60-in. gyratory of German manufacture. This machine was a near
disaster because of serious breakdowns and maintenance problems. Only the skill of the on-site
maintenance people prevented total and extended shutdown. The problems were with top
suspension as well as the eccentric. The crusher was redesigned, and as a second one built
according to this design by a Norwegian consultant, A.A. Sandakat. The crusher was rated at
2500 mtph with dump points on both sides. The primary crushed ore passed via apron feeder,
impact belt to the shuttle belt distributing feed over the six original silos. Each silo served
an autogenous mill grinding line. There was an emergency bypass system for storing a small
quantity of primary crushed ore in a surge pile.
The Autogenous Mills
There were six identical autogenous mill grinding lines each fed from a flat bottom silo of
2800 mt capacity. The silos had four draw points each equipped with an apron feeder delivering
feed to a common collecting conveyor. Instrumentation of the feed system and mills was state of
the art at the time. The four feeders operated on a sequencing time clock that helped maintain
a feed of uniform size distribution. The mills were close circuited on 20 mesh vibrating
screens, the undersize of which was ideal Humphreys spiral feed.
The autogenous mills were of Harding (USA) design but built under license in Germany. They
were 22 ft. in diameter. They were on the growth path for autogenous mills. The Quebec Cartier
mills in 1960 were 18 ft. in diameter and today SAG mills have reached 36 ft. in diameter. The
Bong mills had two 800 hp motors, one on either side of the main drive ring gear. On soft,
highly weathered ore, the capacity was 200 mtph but on a mix of 60% hard, (unweathered) ore and
40% soft (weathered) ore, the capacity dropped to 150 mtph. The vibrating screens were 6.5 ft.
by 14 ft., and on weathered ore the oversize returned to the mill was about 10% of the screen
feed. The screen undersize was pumped to the Humphreys spirals concentrator section.
The Humphreys Spiral Concentrator Section
The Bong concentrator contained one of the largest Humphreys spiral installations of all time
consisting of some 2304 spirals arranged in a 1:1:1 rogher, cleaner, recleaner configuration.
Each of the six sections contained 128 five-turn roughers, 128 five-turn cleaners and 128
five-turn recleaners. The roughers produced a tailing and a middling from the two bottom turns.
The middling was recirculated to the feed. The rougher tailing was actually a middling
containing magnetite that passed the magnetic section, which is described later. The cleaner
and recleaner spiral tailings are dewatered in cyclones with the overflow returning to the
rougher spirals. The cyclones were installed on their sides and gravity fed. The recleaner
(final) concentrate passed to a dewatering cone, the underflow of which fed horizontal table
filters. In the initial plant, the mixed hematite-magnetite concentrate provided the bulk of
the plant product; however, there was some magnetite concentrate from the magnetic section. The
initial concentrates represented a 40% weight recovery and about 74% iron unit recovery,
assuming a 42% Fe head and mostly weathered ore. The calculated tailing would assay about 22%
Fe, but some of this iron is from ferromagnesian silicates in the ore and these are not ore
The Magnetic Section
In the beginning, it was underloaded due to the low magnetite content of weathered ore. The
section, however, was designed for the higher magnetite that would result in a few years after
start up. The feed to the magnetic section was the rougher spiral tailings and was actually
a magnetite middling. Most free magnetite above 200 mesh would be captured in the spiral
system. The rougher tailing was first dewatered/deslimed in hydroseparators of 40 ft. diameter,
one to each of the six plant sections. The underflow passed to a single drum cobber magnetic
separator, the tailings of which passed to a desanding cone ahead of the tailing thickeners.
The cobber concentrate was reground in 10-ft. 6-in. by 14-ft. ball mills operating open
circuit. The mills discharged to three stage magnetitc separators and the concentrates filtered
and joined the spiral concentrates. The magnetic tailing joined the spiral tailings passing
to the thickeners. The mixed concentrate was distributed to the railroad loadout bins.
Tailings Disposal and Water Supply
Tailings having been treated with floctulating agents in the thickeners passed by gravity to a
large settling area that was initially swamp land; however, retaining dams were constructed.
Return water from the tailing pond was pumped to a steady head tank on the roof of the
concentrator and this pumping was the second largest power use in the plant. Makeup water came
from the water system originating at the Saint Paul River, about six or seven miles from Bong.
At the river, water was pumped to a clarifying basin and then, at a rate of about 20,000 gpm,
was pumped via 28-in. pipeline to th eBong reservoir. The river pumping station had a total of
about 2000 hp. Most water was used untreated in the concentrator after most of it had passed
through the cooling system of the power plant. At Bong there was a small treatment plant for
The Bong Power Plant
The Bong area had no potential for affordable hydroelectric power. As a result, a truly
remarkable diesel power plant was built. There were three, nine cylinder, 12,000hp, 150rpm
Sulzer two-cycle engines operating on "Bunker C" heavy oil. Their thermal efficiency
was 38% and this is about the best of any thermal fossil fuel generating system. They were
directly connected to 10,000 Kva AC generators (60cycle) producing power at 6900 volts. The
engines were started by electric motors energized by Caterpillar diesel electric sets. The
scavenger air for the two-cycle engines was provided by turbo blowers driven by engine exhaust.
The Sulzer engines are of Swiss make and are widely used in large ships.
Other Bong Ancillary Facilities
There was a large central maintenance shop that worked on units that could be brought to the
shop. Minor repair in the mine and concentrator was done at these locations by maintenance
people assigned to the operating unit. Railroad maintenance and major repairs were done in the
central shops on work order back-charged to the operating units. There was a very large
warehouse that always seemed to have an ever-expanding inventory. Liberia was a long way from
Europe and North America and, while a few emergency items could be brought in by air freight,
most supplies and parts arrived by ship on a minimum six week delivery time. Suppliers and
sources were almost nonexistent in Liberia in the early 1960s.
The townsite was first class in every respect. In 1966, the company had about 2000 Liberian
and 340 expatriate, mostly German, employees. There was no segregated housing as a matter of
policy, and the only condition governing housing assignment was wage status. Over the years,
the number of expatriates was steadily reduced, even though production was more than doubled.
There were 130 comfortable air conditioned staff houses. There were a few staff houses for
single men and a few single women, mostly secretaries from Germany. There was an excellent
club and visitors´ quarters and several recreational facilities. All Germans working at Bong
were required to have a fair to good understanding of English because it was the only way to
communicate with Liberian workers. About 38% of the Liberian labor force lived in company
houses and the remainder lived in the surrounding villages.
A 50-mile standard gauge railroad was constructed from Bong to the free port area of Monrovia.
It was a single track with passing track about 32 miles from Monrovia. Trains were radio dispatched with contact with Bong and the port. All vehicles and stations involved with the
railroad and port were radio equipped. There was a slight downrange with the load. All rail
was 90 pound. There were three major bridges and some swamp land problems near the port that
required much rockfill before a stable roadbed could be established. Trains consisted of 35
swivel coupling, rotary dump cars of 65 tons capacity. Port bound trips required two hours and
return of empty trains required one and a half hours. At the port, locomotives disconnected for
servicing and the cars were advanced through the rotary dumper without uncoupling, pulled by a
mechanism that is indigenous to the rotary car dumper. There were four 100-ton, 2000 hp,
German-built locomotives and only one was required to pull a 35-car loaded train. There was an
initial fleet of 95 of the 65-ton rotary car dump cars. There was adequate maintenance of way
The port was a state-of-the-art facility employing rotary car dumping, stockpiling and
reclaiming machinery and systems provided by German manufacturers who always pioneered and
excelled in this field. The facilities could unload trains at about 2000 mtph. The stockpiles
were a blending system that assured quality control on shipments. Ships of almost any length,
beam and hatch arrangement could be loaded at a rate of about 4000 mtph and it was possible to
reclaim with the bucketwheel at 2000 mtph and at the same time unload a train at the rate of
2000 mtph direct to ships via a bypassing conveyor arrangement, thus achieving a net loading
rate of 4000 mtph.
The capital cost of the initial facilities is shown on the capital cost table. At $78,050,000,
the cost per annual ton was $25.33 for sinter feed product. This is very much in line with the
Quebec and Labrador projects in Canada in the early/mid-1960s where only sinter feed was being
produced. The Canadian projects also involved mines, concentrators, railroad, ports, power
facilities, townsites and infrastructure. Capital injection in projects like Bong is an ongoing
thing. The initial cost as reported here probably existed for a month or two until some new
capital item was acquired, but the next major capital investment was to come in 1967.
Labor Training in the Early Years
The training of the Liberian labor force was an ongoing effort. In the beginning, expatriate
staff was about 350. By 1986 it had been reduced to 198, even though production had more than
doubled and the facilities were more complex after the addition of a pellet plant. Liberians
are as capable as any other people, providing they are literate, meaning the ability to read
and write and understand simple mathematics. Sophisticated, better educated Liberians live in
Monrovia and are not much interested in living in isolated mining camps. For the most part, the
people Bong had to train in the early days had little education and were not from a mechanical
culture. Machines of any kind were strange to them and almost any mechanical system, be it ever
so simple, had to have an expatriate operator. However, from the beginning, Bong was dedicated
to educating the Liberians through the high school level. By 1986, there were 336 Liberians of
staff grade. Also in 1986, there were 55 apprentices/trainees destined for staff grade
positions or highly skilled operator positions.