The Kenya Flourspar
The fluorite (CaF2) deposits were discovered by prof. Al Amin in 1967 in the areas of Kimwarer, Choff and Kamnaon. He was searching for semi-precious stones, and initially mistook the purple fluorite for gemstone.
He therefore collected the sample and took them for analysis in Mombasa. It was later found to be fluorite.
A hand-mining operation was started and the fluorspar was supplied to the Bamburi Portland Cement Company in Mombasa. At its peak, the operation, which relied on donkeys to transport the fluorspar, produced around 400 tons per month of high grade fluorspar. In 1971 the Fluorspar Company of Kenya (FCK) was established, under the auspices of the Kenya government, to exploit the deposits on a larger scale. Crushing and jigging plants were later put up at mining site for large scale production of the metallurgical grade. Kenya fluorspar was a state parastatal upto 1996 when it was privatized.
It is currently owned by local and foreign investors. The company has seven active mining sites available from which ore can be mined depending on the grade and quality required. In 2003 the company obtained made a major investment in earth-moving equipment and plant upgrading. The deposit has been commercially mined since 1971. Initially, metallurgical grade fluorspar was produced. In 1975 a 100,000 ton per annum acid grade fluorspar concentrator was commissioned and by 1979 metallurgical grade fluorspar was no longer produced as primary product, but can still be produced as secondary product for smaller consignments.
The Kimwarer ore deposit has now been mined for over 35 years It is an EPZ (Export processing zone). LOCATION Kenya Fluorspar is a concern based in the Kerio Valley in the district of Eldoret and Iten. FORMATION OF FLOURITE (CaF2) The fluorite is an industrial mineral formed 150 to 200 million years ago. It is formed by replacement reaction known as mesomatism. The magma or hot water containing fluorine and other minerals was forced up from deep within the Earth. When this brine reached the calcium rich, limestone bedrock (CaCO3), fluorite crystals formed along the walls of fractures and voids in the rock.
Flat layers of fluorite also formed parallel to the limestone beds, replacing the host rock. The fluorine replaced the CO32- from the CaCO3 to form CaF2. The CO32- was dissipated as CO2 and water. This process is further elaborated in equation below: Ca2- + F22- CaF2 (fluorite) GEOLOGY AND OCURRENCE Fluorite (also called fluorspar) is a halide mineral composed of calcium fluoride, CaF2. It is an isometric mineral with a cubic habit, though octahedral and more complex isometric forms are not uncommon. Crystal twinning is common and adds complexity to the observed crystal.
The fluorite deposits are believed to be of hydrothermal origin in the Post-Miocene era when lavas formed a protective cap over Precambrian rocks in the area. The main deposit of fluorite is located at Kimwarer and is exposed on a series of five hills, four of which have been mined. Two other minor deposits knows as Choff and Kamnaon are approximately 2 to 5 km north of the present mine site and have been partially mined in the past for the production of metallurgical-grade spar. The Kimwarer ore bodies contain by far the largest ore reserves. The ore body contacts are distinct due to the buff colour of the ore in contrast to he grey gneiss, white marbles or pegmatites. Typically the fluorite is finely crystalline and disseminated through a siliceous matrix. Zones of high grade ore are often purplish or creamy yellow in colour with cavities filled with distinct cubic crystals. The Choffs have two ore bodies running parallel to each other for more than 3 km. The Choffs are separated by ridges and dips. The ore body pinches and swells and its width varies between 4 metres and 26 metres. This ore is soft and friable hence easy liberation in the plant and provides a good yield. The phosphate in this vein is medium to high.
The Kamnaon ore body has more than five partly parallel veins with varying dips of between 35-45 degrees. There are a number of other sites which are yet to be explored. The Fluorite at Kerio Valley occur as a vein deposit, especially associated with galena, sphalerite, barite, quartz, calcite and metallic minerals, where they often form part of the gangue (the surrounding “host-rock” in which valuable minerals occur). It is a common mineral in deposits of hydrothermal origin and has been noted as a primary mineral in granites and other igneous rocks and as a common minor constituent of dolostone and limestone.
The deposit at Kerio Valley is steeply diping at 40o, with strike varying with mine: Mine| Strike| Percentage CaF2| No. of mines| mines operating| Cheberen| North-South| 37%| 1-6| 1and3| Kamnaum| North west-South east| 40%| 1-5| 1| Choff| East-West| 50%| 1-12| 6and9| The deposit is stratified from Kamnaum to Cheberen1. The fluorite is polychromous i. e occurs in different colours depending on the trace elements. The different colours include: * Colourless * Orange * Green * Purple * Base yellow. GRADES OF FLOURSPAR The fluorite can be processed into grades depending on the use: 1.
Acid grade: This grade is used in the manufacture of Hydrofluoric acid (HF). 2. Metallurgical grade: This grade is used as flux in steel and cement industry. 3. Ceramic grade: This grade is used in the manufacture of ceramics. Only the acid grade is currently produced by Kenya Fluorspar. Metallurgical grade was also produced but stopped. MAJOR IMPURITIES AVAILABLE IN FLUORITE 1. Silica (silicon dioxide), SiO2 2. Calcite (calcium carbonate), CaCO3 3. Phosphorus pentaoxide , P2O5 4. Iron(III)oxide, Fe2O3 STAGES IN MINING AND PROCESSIG OF FLOURITE (CaF2) 1. Exploration 2. Overburden removal 3. Mining . Processing EXPLORATION This is normally done to obtain sufficient information about the deposit in order to enable a safe and economic design and to avoid difficulties during mining of the fluorite. The Kenya Fluorspar utilizes the diamond core drilling machine to obtain the drill cores. This machine can drill upto 140m-180m depth. The drill cores are then logged upto beyond the fluorite zones. The drill cores are then taken to laboratory for testing. The cores undergo through assessment, determination and classification. Diamond core drilling equipment. LAND CLEARING AND OVERBURDEN REMOVAL
LAND CLEARING This involves the removal of all the trees and stumps including the roots from the mining site. In Kenya Fluorspar, currently land clearing and overburden removal is done at choff 9. The main equipment used for land clearing at choff9 is a D355 Komatsu tractor-mounted dozer. The brush is disposed by dumping in gullies. D355 dozer. OVERBURDEN REMOVAL This involves the removal waste soil or rock that overlies the ore. At Kenya Fluorspar the overburden depth depends on the stripping ratio and dipping angle. Dozer is the main equipment used for stripping the overburden at choff 9.
This equipment pushes the overburden over the high wall. After the overburden has been stripped by the dozer, it is loaded onto trucks by a hydraulic hoe excavator for dumping in gullies. The waste is then pushed into gullies by the D355 dozer. Overburden is removed at a ratio of 3:1 to 8:1 (waste to ore) depending on the ore body, with over 700,000 tons of waste being stripped per annum at current production levels. Overburden is normally trucked away using 25 ton trucks provided by contractors. Overburden stripping continues until the fluorite is exposed. ORE EXTRACTION (MINING) Mining is by open cast methods.
The exposed ore is extracted by drilling and blasting. A total of approximately 360,000 tons of ore is mined annually. The geology has indicated that the ore body extends beneath the valley floor level and an open pit is being planned. Drilling and blasting The Kenya Fluorspar utilizes two types of drilling equipment (rig) to drill holes. These include: 1. Down the hole hammer (DTH). This is mainly used in solid hard formations. The hammer is just behind the bit. 2. Top hammer drill rig (TH). This is mainly used in loose formations. The hammer is on top of the drill string (pipes).
Crawler mounted DTHTop hammer drill rig The drill pattern to be applied depends on the: 1. Size of the drill bit to be used. 2. Geological condition of the area. All the bits are 102mm The drill pattern is square, 3m by 3m (burden by spacing) when there is a free face and staggered in absence of a free face. The depth of the drill holes differs, with most holes 10m and others 7m and 4m. Subdrilling is normally 3m. Both the DTH and TH are crawler mounted with TH complete with compressor mounted on it. Kenya Fluorspar has 3 operating crawler mounted drilling machines (2 DTH and 1 TH).
BLASTING This operation involves the following stages: 1. Preparation of ANFO 2. Hole charging 3. Initiation of explosives PREPARATION OF ANFO This involves mixing the ammonium nitrate with fuel (diesel) in the ratio 0. 075ml of diesel for every 1kg of ammonium nitrate. This operation is done by an explosive mixer in Kenya Fluorspar. HOLE CHARGING This process involves placing booster/ primer at the bottom of the drilled hole. The boosters normally used include: cast booster, pentolite booster and gelignite. The Kenya fluorspar utilizes the pentolite booster for the priming purposes.
Before placing the primer it is connected to a detonator. These detonators include: 1. Plain detonators 2. Instantaneous electric detonators. After placing the primer, ANFO is added depending on the hole depth as shown in the table below: Hole depth| Height of ANFO| Stemming| 10m| 7m| 3m| 7m| 5. 5m| 1. 5m| 4m| 3. 5m| 0. 5m| These holes charged are then connected by 25MS (millisecond) nonel shock tubes, short period detonating delay (SPDD) type. The rows are connected by 17MS surface trunk lines (also nonel shock tubes). The nonel shock tube has the following advantages: 1.
Produces low noise and vibration 2. It has less fly rock. It is because of the above advantages that make it useful near factory where less vibration are required. The nonel shock tube is very expensive. INITIATION OF EXPLOSION Once charging is complete the explosion can be initiated depending on the detonator used. Initiation can be done using the following: 1. Safety fuse which burns at a rate of 2 minutes per metre. This is used incase of plain detonators 2. Blasting machine incase of electric detonators. Secondary blasting is also done incase large boulders are produced in the primary blasting.
Large boulders can also be broken using the rock breakers. LOADING AND HAULAGE The blasted ore is loaded into trucks by an hydraulic excavator (Komatsu) hoe. The loaded ore is hauled a short distance of between 2 km and 7 km by contracted trucks to the crushing plant. These trucks range from 10tons to 20tons. The ore stock pilled separately depending on the source. PROCESSING OF FLOURITE (CaF2) The processing of fluorite at the processing plant involves the following stages: 1. Blending 2. Crushing 3. Grinding 4. Conditioning 5. Floatation 6. Thickening 7. filtration 8. Stock pilling
BLENDING Blending in normally done at the primary crusher by a wheel loader before feeding into the crusher in ratio of: 2cheberen:1choff:1kamnaum This blending ratio ensures that the feed is 40% CaF2 CRUSHING Types of crushers 1. Primary crusher: Jaw crusher 2. Secondary crusher: cone crusher 3. Tertiary crusher: cone crusher PRIMARY CRUSHING The ore feed to the hopper through a 12in grizzly screen. It is then feed to the primary crusher through an apron feeder and belt conveyor. The ore is first crushed in a primary stage crushing plant from a feed of 350 mm to less than 30mm in size.
The ore that is less than 60mm but more than 30mm is passed through the cone crusher where it is further reduced to less than 30mm. Any material that is more than 30mm is passed through the tertiary crusher. GRINDINGTypes mills used in Kenya Fluorspar 1. Rod mill 2. Ball mill 3. Re-grind mill. It is then conveyed to the grinding circuit where the material is added to a rod mill along with water to form slurry. The rod mill instantly grinds the ore after which it is sized using a hydrocyclone. Any oversized material is passed to the ball mill for a further grinding to the desired size of 80%.
The ore is now ready for conditioning and flotation. CONDITIONINGThis process takes place in the conditioner. This where the depressants and collectors are added. 1. Depressants a) Sodium fluorite which depresses the P2O5 b) Sodium bifluorite which also depresses the P2O5 2. Collectors a) Betacol b) TapiocaThis process renders the fluorite hydrophobic by the addition of a surfactant or collector chemical. FLOATATIONFlotation is the process that concentrates the ore. This is done by agitating the ore slurry in cells with air bubbles.
By adding a combination of fatty acid reagents, the fluorspar in the ore attaches itself to the air bubble to float to the top of the cell. This product is skimmed off leaving the waste in the bottom of the cell. The process is conducted in a series of rougher, scavenger, and cleaner cells that successively concentrates the ore from 40% CaF2 in the feed material to a minimum of 97. 0% CaF2 in the final concentrate. The water in the final product is then removed in a thickener and a rotary drum filter. This produces a filter cake concentrate containing approximately 11. % moisture. The samples are analyzed in the company’s assay and research laboratory. The waste product is pumped to the tailings dam and settled water is recycled to the plant for reuse. A FLOATATION CELLPERCENTAGE CaF2 CONCENTRATED PER SERIES OF FLOATATION CELL FLOATATION CELL| PERCENTAGE CaF2 CONCENTRATED| ROUGHER CELLS| 80%| SCAVANGER CELLS | 85%-87%| CLEANER CELLS| 97%| FLOATATION CIRCUITDEWATERINGThe water in the final product is then removed in a thickener and a rotary drum filter. This produces a filter cake concentrate containing approximately 11. % moistureTHICKENER ROTARY DRUM FILTEROperational Sequence * Cake Formation With the overflow weir set to a maximum the “apparent submergence” is normally 33-35% so the slurry levels between 0400 and 0800 hrs. Once a sector enters submergence vacuum is applied and a cake starts to form up to a point where the sector emerges from the slurry. The portion of the cycle available for formation is the “effective submergence” and its duration depends on the number of sectors, the slurry level in the tank and the bridge setting which controls the form to dry ratio. Cake Washing and Drying After emerging from submergence the drying portion of the cycle commences and for non-wash applications continues to about 0130 hrs where the vacuum is cut-off. If cake washing is required the wash manifolds will be located from about 1030 to 1130 hrs and the remaining time to vacuum cut-off at 0130 is the portion allocated to final cake drying. * Cake Discharge After vacuum for the entire sector is cut-off air blow commences at about 0200 hrs in order to facilitate cake discharge.
The blow, depending on the position of the tip of the scraper blade, will cut-off at approximately 0300 hrs. Drum filters are normally operated with a low pressure blow but on certain applications a snap blow is applied and to avoid the snapping out of the caulking bars or ropes wire winding of the cloth is recommended . Blow is used on scraper and roll discharge mechanisms but on belt discharge filters vacuum cuts-off when the filter media leaves the drum. The final product has composition shown in the table below: PRODUCT| PERCENTAGE COMPOSITION| Fluorite, CaF2| More than 97%|
Silica, SiO2| Less than 1%| Carbonate, CaCO3| Less than 1%| Iron III oxide, Fe2O3| Neglible| Phosphorus penta oxide, P2O5| Less than 0. 5%| TAILINGS DAMTailings, also called slimes, tails, leach residue, or slickens, are the materials left over after the process of separating the valuable fluorite from the uneconomic fraction (gangue) of the ore. These are the wastes from the floatation cells. Tailings are distinct from overburden or waste rock, which are the materials overlying an ore or mineral body that are displaced during mining without being processed.
The tailing are pumped from the processing plant to the to tailings dam for disposal. The height is about 15m and dimensions are 150m by 30m. it is trapezoidal in shape. The dam also has drain pipes to remove the seepage water. Hydraulic cyclones are used to build the dam. The cyclones separate the tailings into fine and coarse material. The coarse materiel from the cyclone underflow is used to build the wall of the dam will the material and water is disposed into the dam. The water is recycled and pumped back for reuse into the factory. The will built until the dam touches the adjacent hill.
LABORATORY QUALITY CONTROLL SECTIONThe company conducts two types of tests – chemical and metallurgical. The chemical section performs wet chemistry analysis (mainly titration) and operates 24 hrs a day throughout the year and each shift is headed by a qualified chemist. The metallurgical section mainly concentrates on research and development work. The sample (pellet) is subjected to an XRF machine which scans the sample imparting energy to it. The sample then radiates heat of various wavelengths. The wavelengths are analyzed by a computer program which displays the information instantly on the screen.
Strict quality control of run-of-mine ore, exploration and final product analysis is maintained by sampling. KFC prides itself in the quality of its assays, which are always crosschecked with those of European laboratories, namely SGS (Netherlands), HOFF and BSI in the United States. Weight sampling and analysis: All analyses are being performed by SGS Kenya Ltd (Swiss company) at the cost of the company. Apparatus in the laboratory: a) Two Jaw crushers (14in. and12in. ) b) Pulverizer c) Oven for drying d) Splitter e) XRF machine f) Analytical balance g)
A pilot plantSample preparation i. Drying ii. Splitting iii. Crushing iv. Pulverizing. CARGO CONDITIONThe fluorite is Chemically inert, non-reactive, non-flammable, non-hazardous mineral sand. No hazard labelling required in accordance with IMO regulations. No special precautions for storage however, in order to maintain the purity of the material, it must not be allowed to be contaminated by any other material as it is used as the base product in sensitive chemical processes to produce other products and substances.
TRANSPORT Once the acid spar is produced, the fluorspar is loaded into 10 ton trucks and hauled to the railway siding at Kaptagat, a distance of 37km. From here it is loaded into 36 ton rail wagons for the 860 km journey to the Mbaraki wharf at Mombasa. The wagons are offloaded at the wharf where the fluorspar is stockpiled and covered while awaiting shipment. The storage capacity at Mbaraki is approximately 25,000 tons. The process of off-loading of wagons and trucks is labour intensive by means of manual labour.
Loading of the vessels is conducted using an automatic Demag Ship Loader, owned and operated by Bamburi Portland Cement Company. An average rate of 2,500 tons per day can be loaded on a continuous basis even on public holidays. The quantity loaded and moisture analyses are conducted by an independent surveyor on a continuous basis to ensure a representative result of the product loaded. Since Mombasa is one of the main ports in East Africa, chartering of vessels to carry bulk cargoes to any destination in the world can be easily arranged. |