Puththalam Hocim Cement Open Pit Mine and Manufacturing Plant
Cement Production - Open pit lime mine
Location
Holcim Group operates a cement manufacturing plant in the Puttalam District, Sri Lanka. Limestone and other raw materials for the cement operations are transported by rail from a quarry which is located in Aruwakkalu. The distance between quarry and plant is 41 km.
History
The limestone at Aruwakalu which is exploited for the Puttalam cement works has excess silica due to the presence of marl intermixed with the limestone Serious problems were experienced at the Puttalam plant due to the impure nature of this limestone and the Geological Survey Department was called upon to carry out additional drilling investigations in order to locate pure limestone in 1975 - 1976
These surveys revealed that pure Miocene limestone is present in the south - western part of the Aruwakalu and opened a new quarry in this area .In addition to these surveys the Geological Survey Department carried out drilling investigations in the Dutch Bay adjacent to this area and proved about 35 million tons of Miocene limestone of acceptable quality on the bed of the Dutch Bay to a depth of about 50 meters. Environmental problems have dissuaded the Cement Corporation from exploiting these reserves.
Geological settling and mineralization of limestone deposit
The limestone deposit was occurred in Miocene period. Before millions years ago Sri Lanka and India was together and then it was started to divide. Then big lagoon was created between Sri Lanka and southern part of India. In that lagoon there were large amount of coral reefs which contain fossils. Then the lagoon dried and corals were also dried. It is believed that Kala Oya flowed through this location. Then clay layers and sand layers were deposited on this died limestone. Sand layer contains pure sil ica. Then red earth included to the cavities which present in limestone. It is believed that red earth is an Aeolian deposit which has come with wind from South India. Red earth has very fine particles.
Geology and mineralogy
The limestone deposit was occurred in Miocene period. Before millions years ago Sri Lanka and India was together and then it was started to divide. Then big lagoon was created between Sri Lanka and southern part of India. In that lagoon there were large amount of coral reefs which contain fossils. Then the lagoon dried and corals were also dried. It is believed that Kala Oya flowed through this location. Then clay layers and sand layers were deposited on this died limestone. Sand layer contains pure silica . Then red earth included to the cavities which present in limestone. It is believed that red earth is an Aeolian deposit which has come with wind from South India. Red earth has very fine particles. The deposit consists with 6 layers.
- 1st layer – Red earth
- 2nd layer – Low grade limestone
- 3rd layer –Clay layer
- 4th layer – Low grade limestone
- 5th layer – High grade limestone
- 6th layer – The base
The red earth layer consist with Al2O3, Fe2O3, and Ilmenite etc. The red earth has mixed with 2nd limestone layer through its pockets. Due to that it is considered as low grade limestone. The last layer is the base. It consist with higher grade limestone. But the problem is the moisture content of that is high. Also it is difficult to mine for that level due to increasing of water level.
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3). Like most other sedimentary rocks, limestone is composed of grains; however, most gra ins in limestone are skeletal fragments of marine organisms such as coral or foraminifera. Other carbonate grains comprising limestones are ooids, peloids, intraclasts, and extraclasts. These organisms secrete shells made of aragonite or calcite, and leave these shells behind after the organisms die.
Limestone often contains variable amounts of silica in the form of chert or siliceous skeletal fragment and varying amounts of clay, silt and sand carried in by rivers.
Some limestone do not consist of grains at all, and are formed completely by the chemical precipitation of calcite or aragonite. Secondary calcite may be deposited by supersaturated meteoric waters. This produces speleothems, such as stalagmites and stalactites. Another form taken by calcite is oolitic limestone, which can be recognized by its granular appearance.
The primary source of the calcite in limestone is most commonly marine organisms. Some of these organisms can construct mounds of rock known as reefs, building upon past generations. Bel ow about 3,000 meters, water pressure and temperature conditions cause the dissolution of calcite to increase nonlinearly, so limestone typically does not form in deeper waters. Limestone may also form in both lacustrine and evaporite depositional environments. Calcite can be either dissolved or precipitated by groundwater, depending on several factors, including the water temperature, pH, and dissolved ion concentrations. Calcite exhibits an unusual characteristic called retrograde solubility, in which it becomes less soluble in water as the temperature increases.
Because of impurities, such as clay, sand, organic remains, iron oxide and other materials, many limestone exhibit different colors, especially on weathered surfaces. Limestone may be crystalline, clastic, granular, or massive, depending on the method of formation. Crystals of calcite, quartz, dolomite or barite may line small cavities in the rock. When conditions are right for precipitation, calcite forms mineral coatings that cement the existing rock grains together, or it can fill fractures. Travertine is a banded, compact variety of limestone formed along streams, particularly where there are waterfalls, and around hot or cold springs. Calcium carbonate is deposited where evaporation of the water leaves a solution supersaturated with the chemical constituents of calcite. Tufa, a porous or cellular variety of travertine, is found near waterfalls. Coquina is a poorly consolidated limestone composed of pieces of coral or shells.
The process of the quarry
Check the samples of limestone
The Carbonate content, Chloride content, free Silica content and Clay content in limestone samples are tested at Aruwakkalu quarry lab. There are three methods to collect samples.
- Test hole sampling
- Drill hole sampling
- Ripping sampling
Test hole sampling
To get information about the quality of undiscovered material of the quarry a test hole campaign is carried out once a year. This is done on a 20m grid where the area overburden has been removed and in the direction of the quarry advancing. GPS coordinates of easting and nothing are taken on drill holes where samples are collected. These samples are analyzed using the XRF (x - ray fluorescence analysis of elements) at the Palavi main laboratory and the chemistry of it is known.
Drill hole sampling
Beds are prepared after blasting the reject material and clay layer by ripping using the heavy duty dozer for drilling. The drill machine is having a dust collector which dust is sucked with air. This equipment is helpful to gather samples from the drill holes. A separate worker works with drilling machine to collect dust samples.
Ripping sampling
Carbonate content, Chloride content and free Silica content is tested in ripping samples. But normally, the ripping materials are from base so it has high Chloride content than the high- grade materials. Therefore, most of ripping materials are taken as finish grinding materials.
Over Burden removal
Over Burden is upper soil layer on the deposit. It content ripple marks and classified layers. 1m from the surface is a humus layer. Below that it’s red soil. This soil is very rich in Titanium Oxide metals. Metals such as Ilmenite and Rutile, up to 7% of the soil. Other than that, the red earth is made of magnetite, spinel, zircon, garnet, monazite and hematite (iron oxide). The thickness of the over burden varies from west to east. It changes from 25m to 5m. First select the area which is to be removed the soil. Study the fauna and flora of the area for one year. Then select the overburden dump area. Then start removing. 375 A dozer is used to remove the overburden. Main parts of this dozer are ripping unit, blade and the track. This D375A dozer mainly does ripping operations at the quarry site. The blade is used for push and removes for material. Rate of Diesel consumption is very high in this machine. Total weight of this machine is 85MT approximately. Diesel per hour is 90L.
Selecting blasting bed
The limestone deposit contains low grade, high grade limestone and clay. From drill hall samples the correct depth to certain material can be identified.
- Low grade limestone
- High grade limestone
- Finish Grinding Limestone
Finish grinding lime stone materials are high grade limestone which is rich in lime content though low in clay content than that of high grade limestone. The tolerance limit of chloride for finish grinding is not specific and relatively high contents are permissible.
By a test hole campaign conducted once a year in a particular area, the lab clarifies the height of high grade one and low grade one. By the samples from the bed the lab decides whether this is a high grade one and low grade one. According to the requirement quarry engineer is informed to blast the particular bed. The requirement can be
- High grade – to optimize or for clinker manufacture
- Low grade – to optimize or for clinker manufacture
- Finish grinding – rarely to optimize or to add as additives to clinker
Drilling blasting bed
Drilling is done by a drilling machine. It’s atlas copco CM470. Drilling process done by hydraulics. Has both rotating and hammering capability. Contains an auto drill bit changer and a drifter. Can use to drill vertical, horizontal and angle. Here we drill only vertical holes but when necessary angle drills also being drilled. Compressed air is used to take material out of the hole. It contains a dust collector to collect dust comes out from the drill hole. That is helpful to take samples of the bed to find out the chemistry of the bed. The machine is used to drill 10ft,'20ft, 30ft. Sometimes 40ft is also drilled. It depends on the bench height.
Blasting
Dynamites are used as primary explosives and ANFO is used as Secondary explosives. Charged holes are connected as a loop. Then all the loops are connected in parallel to a main line to reduce the overall resistance. If there were any hole which has not blasted previously also fixed to the main line or to a loop. Check whether the detonator has blasted by using the meter. If it shows a high resistance means the blast has gone it not it is still good. Then the main line is connected to the generator. There are 3 sirens before the blast. The detonator blasts first. Then dynamite blasts. Because of that cracks are generated. Then ANFO burns and release high pressure gas. That goes through those cracks and breaks the material to small parts.
Transport the material from quarry to the mix bed
WA 600 loader is used to lord the material. Bucket capacity of the loader is 6.1m³ approximately and normal load of the loader is 10 MT approximately. Operating weight of the machine is 45MT. Engine horsepower of the machine is 327kW. Procedures are done according to the operation and maintenance manual of the loader. Diesel per hour is 50L. Dumpers are used for hauling the material from quarry to the mix bed. Overall weight of one dumper is (with material) 60MT approximately .its dump body capacity is18m³. There are three main dumper trucks are operation at presently. Normally HD 325 dumpers are available. In further expansion of cement project it is planned to import large capacity dumpers.
Mix heaping
Whatever the material from the quarry do not satisfy the requirement. To make almost all up to the requirement, a mixing procedure is used. That’s called mix heaping. To build the mix heap which gives the requirement, have to haul material from the quarry. Chemistry of the material to be hauled is entered to special software called QSO Expert, short term. That’s special software used in cement industry. When we enter the chemistry of two different quality material, it gives the ration which those material to be optimized to get the requirement. Optimizing work can do for maximum 3 different quality materials. Normally they build 2 types of heaps. One is high grade and the other one is low grade. For high grade the optimizing ratio is about 30% to 70%. For low grade it’s almost 50%- 50%. Those heaps are situated in two ramps. One is called old ramp and the other one called new ramp.
Wet Season Plan
The rain season is between Octobers to February. During this period the quarry operations are not done well as dry season. Therefore the end of the dry season, the materials should be stocked ck in the Palavi plant and at the loading ramp in the quarry. But both hauling and dispatching of materials is difficult due to the rain. If the material is transporting during the rainy season following plan is implemented.
- Material is dispatched from covered mixed heaps.
- Free fall faces are built closer to cover easily.
- If the covered material is insufficient, arrangement being made to transport as it is from the quarry pit.
- Revised moisture targets are used in this season.
- If necessary loaded limestone wagons during rai ns shall be covered with proof cloth.
Quality controlling
The quality of dispatch materials is controlled and assured in the quarry lab. The Quality control and assurance is done from the quarry to the point of that customer use the cement. According to the SLS and ISO standards, the quality control is done by step wise to achieve their quality targets.
The quarry lab has the quality targets separately for low- grade limestone, high - grade limestone and Finish grinding limestone. In the quarry lab, the quality of materials is getting by doing wet analysis. The Carbonate content, Chloride content, free Silica content and Clay content for the finish grinding limestone are analyzed in the quarry lab. According the results of these tests, the rough idea of quality of materials is get after the materials are dispatched.If the high Chloride content materials are dispatched, it is affected to block the preheater system and interrupt the cement production. Therefore the Chloride content of materials has targets. But after the bypass system was installed to the preheater system, there can be prevented blocking the preheater system due to Chloride. Hence the Chloride content of material is not much more considered these days. But specially, free Silica content of materials are to be attention because free Silica content is affected the kiln productivity. If free Silica content is high, the kiln productivity is low due to the its low burn ability and the wall coats of kiln are damaged. Therefore the quarry l ab works hard on keeping the dispatch materials within their quality targets
Rehabilitation
Holcim is having ISO 14001 certificate on environment. So “Optimum restoration of biodiversity and landscape values after limestone mining of the quarry site” is the main target of rehabilitating the mined areas.
To achieve that, Holcim has joint hands with IUCN (the International Union for Conservation of Nature) who willingly to assists Holcim in effective biodiversity conservation in the quarry area by adhering to IUCN’s general goal of biodiversity conservation. On the other hand, Holcim is also to conserve biodiversity and to enforce their policy of responsible quarry rehabilitation. Rehabilitation of the quarry commonly consists of three main steps. Those are
- Refill the quarry pit to a lower than original profile
- Landscape
- Reforest
Refilling
Refilling is started with reject material. Then put red soil. On top of it the humus layer is put. If the overburden is to be dumped in to the quarry at a level below the original topography, it is possible to refill the complete area. This scenario will result in lowest waste haulage costs and will provide good opportunities for rehabilitation of the old quarry area. This is the method that Holcim use to refill.
Reforesting
Reforesting in Refilled areas was replanted almost 7000 with natural flora in order to fully rehabilitate the refill. Before 2003 topsoil was not replaced, and replanting consisted of one tree species. As studies showed, these areas were later poorly vegetated and not much biodiversity seems to have achieved. But afterwards, the topsoil has been placed, fertilized and a wide variety of plants species are being planted to achieve balanced bio diversity. Some planted species grow well, and the topsoil seed bank resulted in a luxurious growth of pioneer species.
Transporting
Limestone and other raw materials for the cement operations are transported by rail from a quarry which is located in Aruwakkalu. The distance between quarry and plant is 41 km. Vegans are used to load the material. In one time 40 vegans are carried by train engine. The rail way track belongs to the holcim.
Description of the plant
Cement is a material with adhesive and cohesive properties and that binds or unites essentially like glue. Cement binds the sand and coarse aggregate together to fill voids in between sand and coarse aggregate particle and form a compact mass. Hydraulic Cement sets and hardens by action of water such as Portland cement.
Chemical composition of Portland cement:
- Tricalcium Silicate (50%)
- Dicalcium Silicate (25%)
- Tricalcium Aluminate (10%)
- Tetracalcium Aluminoferrite (10%)
- Gypsum (5%)
Cement manufacturing process in brief
Cement is manufactured from a carefully controlled mixture of calcium, silica, alumina and iron. Calcium is generally obtained from limestone. Silica, alumina and iron are generally obtained from clay or shale and iron ore. The limestone which transported through the train is crushed with hammer crusher to reduce sizes 75 mm. Then materials are transferred trough belt conveyor to milling process. Depending on the mineral content of the limestone, additional silica and/or iron ore are added to the milling process to get the chemistry right. The various components are milled and dried in a roller mill. Heavy rollers are held ove r a rotating and the coarse material is milled until it is fine enough.
The milled blended raw material is transported to the top of the kiln preheater tower. The kiln is fired by finely ground coal and is designed to maxim ize the heat transfer from the ignited coal to the clinker. In the preheater, the raw materials are heated to a temperature of about 800°C and carbon dioxide is removed from the limestone. Then the material drops into the rotating kiln where its temperatu re is raised to 1450°C. The rotation and shape of kiln allow the blend to flow down the kiln, submitting it to gradually increasing temperature.
At this stage the various minerals start to fuse together to form predominantly calcium silicate crystals (cli nker). This semi molten material is then cooled as rapidly as possible in a cooler and stored in a clinker silo ready for final milling.
Cement mills are rotating tubes filled with steel balls. A roller press is sometimes used to the size reduction process before the clinker is fed into the mill to produce a very fine grey powder. Controlling the cement fineness assists in determining the strength growth rate of the cement in mortar and concrete. During milling, gypsum is added to control the cement’s setting time. The base cement is stored in homogenizing silos to ensure the most consistent product possible.
Then, it is grounded to a fine powder and then the cement is stored in storage bins or cement silos or bagged. Cement bags should be stored on pallets in a dry place.
Lab operations
In each stage of production lab testing of every materials is done in factory main lab. When hazed materials are taken as secondary energy resources the content of radioactive elements are checked. Also in each production samples are checked using XRF. After cement is produced the strength of cement block is checked in main lab.
Main control unit
All the production steps are controlled by main controlled unit. The production process is controlled in computer system. All the indicators and controllers present in main controlling uni t. When there is an error it is indicated in computer screen.
About Cement
Cement is a glue that holds the rocks and bricks.Also a bonding agent obtained by burning a mixture of limestone and clay together to form clinker,then pulverizing the clinker with gypsum.It is consist with Calcium oxide with silica, alumina and iron oxide.Also minor consist of MgO,SO3, Na2O,K2O and chloride. Joseph Asdin, an English mason invented cement in 1824.Cement called as Portland cement because it produced a concrete that resembled the colour of the natural limestone quarried in Portland(England).
There are various cement in the world. They are,
Portland cement Types
- Ordinary Portland cement
- Portland white cement
- Portland oil well cement
- Portland high alumina cement
Blended cement
produced by intimately and uniformly intergrinding of blending Portland cement and two or more types of fine materials,such as
- Ground granulated blast furnace slag- it is a by product in steel industry
- Fly ash - by product in thermal power plants
- Silica fume- by product in Ferrosilicon Alloys
- Calcined clay- burnt clay
- Volcanic ash- Volcanic eruption
- Limestone- naturally available
Types
- Portland blast furnace slag
- Portland fly ash cement
- Portland masonry cement
Clinker minerals
- Alite - C3S
- Belite- Beta - C2 S – β
- Belite- Gamma - C2S- γ
- Aluminate- Cubic - C3 A – C
- Aluminate Orthorhombic - C3A- O
- Ferrite- Alpha - C4AF- α
- Ferrite- Beta - C4 AF – β
- Free Lime - F.CaO
- Arcanite - CaSO4
- Periclase - MgO
- Quartz - SiO2