THE ORIGINAL MINE AND PLANT AS COMPLETED IN 1965

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

Opening first bench in 1961 at Zaweah I orebody. 6cy shovel, 45 ton truck

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.

45-ton Euclid truck dumping at crusher 1965 start-up period

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 Concentrator

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.
General View of the crushing plant and concentrator 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

Original installation of six 22-ft autogenous mills 1965

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

Humphrey Spirals in concentrator 1976. The spirals were the primary concentration device

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 minerals.


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 domestic water.


The Bong Power Plant

Original 1966 power plant. Three 12,000hp Sulzer diesel electric generating units 10,000 KVA, 8000kW at .8 P.F. 6900 Volts 60 cycle

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.
Residential area at Bong in 1966. Townsite continued to expand as more production facilities were added 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.


The Railroad

14 year old Loco 03reaches 1,000,000km in April 1977 with Mr. Schintu at controls

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 equipment.


The Port

Shiploader at port 1966

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.


Capital Cost


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

Vocational Training at the Bong Mining Co.

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.


© James V. Thomson and Wolfgang Jacobs