Egeralayta Perlite Deposit
Summary of the Djibouti Perlite Project
The completion of early DFS studies on the Egerlayta perlite deposit in the Republic of Djibouti have identified the deposit as an economically viable source of perlite. The intention is to rerun the DFS programme to confirm project economics and design parameters and quickly establish mining and on site first stage processing facilities. Second stage expansion processing facilities will be established close to the principal markets. The Gulf region and India are the targeted markets.
The deposit is exposed at surface permitting direct access to ore without a pre strip. Initial mining stages will be completed by dozer and ripper before traditional open cut mining begins.
The current delineated reserve will facilitate production of: the treated bulk perlite, the expanded perlite and some perlite hydrophobised for a minium mine life of 10 years. The estimated production levels are;
• Perilite bulk treated 50,000 - 75,000 tons per annum
• Perlite expanded 5,000 - 7,500 tons per annum
• Perlite hydrophobised 500 - 1,000 tons per annum
The first feasibility study of the Egeralayta perlite deposit was carried out in 1988-90 by the BRGM. The first phase comprised of:
- The opening up of 2,560 m of track by the bulldozer,
- The execution of 68.5 meters of core drilling in 6 bores.
- Sampling of 13 perlite collected for laboratory tests
- The execution of further geological observations
In the second phase, the laboratory work included:
- 11 standard expansion tests with additional corresponding analyses;
- 2 series of specific expansion tests , with pre-requisite preheating
- The determination of the coefficient of thermal conductivity of the expanded perlite.
Drilling in the downthrown blocks has confirmed the existence of an extensive upper perlitic flow with no overburden, with an average mineable width of 11.6m, varying between 7.3 and 20.5 m. The same lithostratigraphic succession as the dome is observed, that is, from the base up:
- a lower layer (> 2 m) of perlite with barren spherules;
- a middle layer (2 to 19 m) of standard mainly (often) granular perlite,
- An upper layer (< 1 to 7.5 m) of perlite with a more or less pumiceous (fine) texture.
Laboratory tests has proved the good quality of all the material, with the same relationship between the ore lithology of the mineral and scope of industrial usage of the expanded product as well as on the dome:
- the lower layer gives an expanded product suitable for use in the construction ( including the manufacture of lightweight concrete) or in horticulture, when the rate of the non-expandable spherules do not exceed 10 to 20 %,
- The middle layer of classic perlite, which constitutes the majority of the mineable bed, is in all cases, of cryogenic isolation quality and sometimes gives an expanded perlite sufficiently resistant to be used in the manufacture of plasterboards.
- The surface layer of pumiceous perlite seems generally of a better quality than on the dome, with the expanded product of cryogenic isolation quality and even at the norm limits to be used as a filtration additive.
The overall geological reserves of the deposit (dome + downthrown blocks) can thus be estimated at around 23 Mt., of which 80% of the ore is of cryogenic quality and 20% of construction, horticultural and agricultural quality, the reserves indicated by the total mining activities being at 3.27 Mt.Thus, all the results confirm the industrial potential of the Egeralayta.
The Mining License\Permit area is situated 78 kms from Djibouti town on the way to Lac Assal. The Lac Assal is situated 25 kms further on the same high NH-9 leading to Tadjoura. Nearest villages are Karta on south and Aseylayta on the north.
The mining Plan is prepared for approval from the Government of Republic of Djibouti to open the deposit to start commercial production and develop a systematic and scientific mining and provide employment for the local people in the mines for overall social and economic benefits.
The mining License extends over 404.21 Hectares of hilly waste land without any vegetation and dwellings nearby. Elevation of the area ranges from 488-588m above Mean sea level.
The outer boundary (Green) line shows the Prospecting License Area (PL Area) of Egeralayta Massif . The inner boundary line (Red) shows Perlite deposit in the Mining License\Permit. Total extent of the Mining License\Permit Area applied is 404.21 Hectares .
Perlite is a natural volcanic glass containing combined water. This includes obsidian, perlite, Pitch stone and pumicite. The Perlite was first identified in early 1800 AD. The production was started from 1946 in Superior, Arizona and New Mexico with a trade organisation “The Perlite Institute”, in 1949. The crushed and screened perlite expands or “Pops” into anhydrous glass foam when heated quickly to plasticity at about 600° - 900° C while releasing the steam from combined water.
The texture of ranges from classical (Onion skin) to granular (Commercial type) to pumiceous (low density) . The perlite normally contains 3-5 wt. % of combined water (+H20). These type of perlite occur in association with young rhyolite, a high silica volcanic rocks.
The chemical composition shows Si02 70-75%, Ca0 + Mg0 = <2%, The various grades of perlite as observed in the applied area along the slope of hillock ( profile\succession )are:
Standard Perlite (Insitu) in Zone -4 Spherulitic Perlite (Low grade)
Friable Perlite (Coarse Grained-Filter Construction grade) Obsidian grains embedded in perlite
- Intermediate Rhyolite (Altered)
The typical succession is as follows;
The succession thickness shows some variability due to local faulting but in general the thickness of Pumiceous Perlite is more than 2m in Zone 1and 2 resulting in high quality zones for production of light weight classic perlite. Production for the first five years is planned in Zone-1 and 4 to mine pumiceous and standard perlite for foundry and horticultural grades with stable coarse grains of perlite.
Properties of Raw Perlite
The chemical composition and Physical properties of Raw perlite are shown below:
1) Typical Chemical Properties:
Properties of Expanded Perlite
1) Physical and Metallurgical Properties
• Mechanical Resistance: It is considered while using Perlite for Construction and horticulture purposes.
• Coefficient of thermal conductivity: Coefficient of Thermal conductivity is specially considered when Perlite is considered for using for cryogenic purposes.
• Specific Gravity: Lesser the specific gravity of Perlite the more it is preferred.
• Packed Density: Density of Perlite is studied at 3 atmosphere pressure.
• Non Packed Density: Density of Perlite at 1 atmosphere pressure. This property decides the grade of perlite for various applications.
• Expansion tests: These tests will be carried out to check the capacity of individual particles to expand after heating.
• Steriles: The impurities within the Perlite that do not expand on heating are collectively referred to as steriles. Percentage of steriles directly affects the quantity of end product and the yield of the furnace.
• Free Moisture Content: If the moisture content of Perlite is high it is subjected to processing prior to expansion. This increases cost of production and hence Perlite with low moisture content is preferred. This is for processing reasons and product quality.
• The Chemical properties of expanded perlite is more important for filter aid purposes particularly the soluble iron (free iron) should not be more than 1000ppm.
• The pH, it is indicative of the stability of Perlite in acidic or basic environment and is considered for most of the applications for horticultural applications.
Structure of Perlite deposit and its relation to Quality
The perlite bearing mountain is a “Dome” in appearance intersected by several faults and fractures. Being a volcanic glass, the fractures were developed radially in all directions from the slope of the hill.
Concentric fractures are also seen around the hillock. The major fault that occur on either side of Zone -2 is found parallel to the Regional Rift, trending NW-SE directions to a length of 2kms. The down thrown side fault in Zone -2 is surveyed as 80m on the North-eastern side thereby pumiceous perlite cropped out at surface at lower level as compared to Zone-3. The carbonate and clay content in Zone -2 is more than the other zones. It may be due to accumulation of Tuff (Volcanic Ash) erupted through the openings of the fault zone in this zone.
The Perlite, being of geothermal\meteoric alteration, occur as a “cap rock “over the Rhyolite massif. It extends to a average depth of 12.4 m as per the BRGM report, as observed in the bore holes. Below the perlite layer Intermediate Rhyolite (Partly altered) is found to be occur above the microcrystalline Rhyolite, which in turn extruded in the pre-existing Basaltic rock.
The glassy top layer is altered into Perlite of various grades from the top to bottom by thermal or meteoric water alterations. The tuffaceous materials cause for enrichment of clay/carbonate content in the top layers.
Based on the regional phenomenon the whole perlite deposit is divided in to four zones as Zone 1-4.
Genesis of Perlite
The origin of perlite is one of the interesting subject for the geologist to understand the deposit, the high silica with high potash (K20) content silicic lava get quenched when contact with atmospheric temperature and quickly cooled to form various types of glasses (Polymorphs) such as Obsidian ( Water <2 wt%), Pitch stone (Water >5 wt%) and pumicite. The glassy layers occupies the top most portion of the rhyolitic lava extrusion on the continental mass.
In course of geological time, the glassy materials get hydrated due to the following reactions:
i) Magmatic water - Geothermal comes from deeper active volcanics
ii) Connate water (meteoric)
Both sources may be cause for the alteration of volcanic glass in to perlite. The unreacted portion of obsidian lye on the bottom layer, followed by partly altered Rhyolite (Intermediate Rhyolite).
During 1988-1990, BRGM carried out the geological and mine-assessment study of the Egeralayta Perlite deposit in the Republic of Djibouti with an objective to define the existence and true value of the deposit and the domains of industrial utilization for the different types of ore. Based on the earlier outstanding works, JB Mining Limited intends to carryout pre-mining exploratory work on the Egeralayta Perlite deposits to establish the qualitative and quantitative reserves of Perlite. The company will carry out pre-feasibility studies by digging exploratory trenches and drilling the different zones of the deposit.
The surface geological study carried out by BRGM suggests that the deposit is sub-circular, with a diameter of about 2 km, and is of flow-dome type. It forms only a smaller part covering approximately 15 km long of a large Egeralayta.
The silicic massif is emplaced on a basaltic substratum through a zone of intersection or convergence of the regional fractures affecting south Ghoubbet region.
The basic lithological units which are exposed in the Egeralayta massif are as follows:
• The Upper Superior Perlite Complex
• The Total Middle Rhyolite Layer
• The Lower Inferior Perlite Complex.
The upper superior Perlite complex comprises different types of Perlite –
a) Pumiceous Perlite
b) Standard Perlite
c) Standard Perlite with obsidian, and
d) Perlite with spherulitic structure.
The different types of Perlite differ in their physical properties also. At some places, the rhyolites intervene with the Perlites. The BRGM has done extensive work by preparing a geological Map and sections in 1: 2000 scale with close spaced contour at5m interval showing all lithological and structural details.
A total of 23 boreholes to study the quality of the Perlite and also to determine the lithological succession. The objective of their work was to study the resource and reserve and establish quality of the Perlites in Egeralayta.
In the western block, a total of 17 boreholes were put and their average depth is 12.07. In the eastern block, a total of 6 boreholes were put and their average depth is 15.60. The maximum depth that was reached during drilling was 21.5 m (on Boreholes S5 and S22). The borehole data suggests the following lithostratigraphic units on the downthrown (Western) block:
- An upper layer (1 to 7.5 m) of Perlite with a more or less pumiceous (fine) texture.
- A middle layer (2-19 m) of standard light grey to dark grey Perlite layer passes in continuity and is often granulated. It is mostly brittle (crumby) or resistant as in the past mainly (often) granular Perlite.
- At the lower level, the Perlite is grey and is often very brittle. It comprises reddish sphreulites along with some obsidian. Its thickness is often more than 2 m.
BRGM has analyzed 13 samples; 11 standard expansion tests with additional corresponding analyses done by BRGM at laboratory of la Societe Francaise de Cermique (SFC) at Paris. The samples were prepared in advance by mixing and homogenization of various small pilot samples of expanded Perlite showing near (neighbouring) qualities.
Resources and Reserves
The estimated the geological reserves by Area Block method for the surface boulders and perlite as encountered in the bore holes by computerised methods as given below:
The possible reserves is estimated to be 21 million tonnes based on the map
A) Area of Float Ore = 1,154,000 m2
- Average Depth = 1m
- Bulk density = 1.30
- Rate of Recovery = 95%
- Mineable Reserves of Float = 1154000 x 1 x 1.3 x 95%
- = 1,436,153 metric tonne or
- = 1.44 Million tonne
B) Subsurface Reserves as per Table –II = 3,134,896 Metric tonne or
- = 3.13 Million Tonnes
- Total of (A) and (B) = 4,541,565 Metric tonne
- = 4.57 Million Tonnes
- (The Subsurface reserves is estimated by taking into account of the Area and average thickness of 23 bore holes.)
The reserves are classified under Probable category as it needs some more exploration to prove it under “ Proved or Measured” Category
Mining and Processing
A mechanised method of mining shall be adopted for exploitation of Perlite from this area. The float ore shall be collected by the hydraulic excavator and loaded in to the dumpers of 10-18 tonnes capacity and transported to the stacking yard for grade wise stacking based on its physical\mineralogical properties.
The oversize boulders shall be broken down to <30cm size and the usable\saleable perlite shall be transported to the Crushing plant for further processing for up gradation and size\grade wise sorting and screening in to various fractions from 6 mesh to 100 mesh depending upon its application in the industrial end use.
The overall mineral to waste ratio for the first five years as 1 : 0.17
The proposed development for the first five years is given as under:
The grade perlite includes standard perlite (Float) and Insitu Standard perlite and some percentage of friable perlite that occur below the carbonate layer. These products shall be mined with an object of covering cryogenic, foundry , horticulture, filter aid and construction markets.
The proposed rate of production when the mine is fully developed shall be 50,000 MT per annum. The life of the mine is estimated as 90 years for the Mineral. The year from which the full operation effected shall be from 2009.
The mineral raised out of the mine is used for captive purposes for manufacture of various grain size of perlite for supply to various industrial applications through the manufacturers having facility for expanding the perlite. The three major applications are:
I. Construction Applications
Because of perlite's outstanding insulating characteristics and light weight, it is widely used as a loosefill insulation in masonry construction. In this application, free-flowing perlite loose-fill masonry insulation is poured into the cavities of concrete block where it completely fills all cores, crevices, mortar areas and ear holes. In addition to providing thermal insulation, perlite enhances fire ratings, reduces noise transmission and it is rot, vermin and termite resistant.
Perlite is also ideal for insulating low temperature and cryogenic vessels. When perlite is used as an aggregate in concrete, a lightweight, fire resistant, insulating concrete is produced that is ideal for roof decks and other applications.
Perlite can also be used as an aggregate in Portland cement and gypsum plasters for exterior applications and. for the fire protection of beams and columns.
Other construction applications include under-floor insulation, chimney linings, paint texturing, gypsum boards, ceiling tiles and roof insulation boards.
ii. Horticultural Applications
In horticultural applications, perlite is used throughout the world as a component of soilless growing mixes where it provides aeration and optimum moisture retention for superior plant growth. For rooting cuttings, 100% perlite is used. Studies have shown that outstanding yields are achieved with perlite hydroponic systems. Other benefits of horticultural perlite are its neutral pH and the fact that it is sterile and weed-free. In addition, its light weight makes it ideal for use in container growing. Other horticultural applications for perlite are as a carrier for fertilizer, herbicides and pesticides and for pelletizing seed.
Horticultural perlite is as useful to the home gardener as it is to the commercial grower. It is used with equal success in greenhouse growing, landscaping applications and in the home in house plants where the perlite aerates soil and promotes water retention.
III. Industrial Applications
Industrial applications for perlite are the most diverse, ranging from high performance fillers for plastics to cements for petroleum, water and geothermal wells. Other applications include its use as a filter media for pharmaceuticals, food products, chemicals and water for municipal systems and swimming pools.
Additional applications include its use as an abrasive in soaps, cleaners and polishes and a variety of foundry applications utilizing perlite's insulating properties and high heat resistance. This same heat resistant property is taken advantage of when perlite is used in the manufacture of refractory bricks, mortars, and pipe insulation.
The perlite of this area is suitable for almost for all purposed as it is proved to be the best grade by the laboratory expansion test and recovery by screening test. Pilot scale study to test the perlite in the Industrial application is also to go ahead to find its suitability and enhance the production to meet the market demands globally.
The Run of Mine (ROM) shall be transported to the stacking yard to the mobile crusher or subjected to manual sizing at the mines itself before dispatching to the crushing and screening plant for sizing into various fractions. The only fine size materials from 6 mesh to 140 mesh shall be useful for various applications as given below.
The size of the material required by the various industries\ applications are briefed as under,