ethiopia_diatomite_march2007

8/9/2019 ethiopia_diatomite_march2007

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IntrodUCtIon

Diatomite, a sedimentary rock composed o the ossilized siliceous skeletons o microscopicaquatic plants, is used as a lter aid in the production o beer, ood, juice, wine, water,sweeteners, lubricants and petroleum. It is also used as a ller in paints, plastic and rubber,as a carrier or ertilizers and pesticides, and or insulation. World production o diatomiteamounts to about two million tons per year, at least two-thirds o which is used or ltration.Major producing countries are the USA (about 650 000 tons/year), China (400 000 t/yr),Denmark (230 000 t/yr) and Japan (130,000 t/yr).

propertIes and applICatIonsDiatomite is chemically inert, lightweight, and has a large void volume which is retained even when compressed. It issupplied commercially in three orms: natural, calcined and fux-calcined. Initial crushing and drying o diatomite orebreaks down the diatoms and the resulting powder is termed natural diatomaceous earth. Calcining the natural productat about 1000oC uses the small broken particles into larger ones, which reduces the surace area and increases the poresize and ltration rate. Adding a fux such as sodium carbonate during calcination lowers the melting point o the diatomparticles, allowing larger agglomerations to orm without loss o structure. Calcined material is size-classied to giveproducts o dierent, but tightly-specied permeabilities or ltration applications, where their purpose is to act as aprecoat to remove and retain suspended solids and also as a body aid to maintain porosity through the lter. A growing useo diatomite is in ltering out microbial contaminants such as bacteria, protozoa and viruses rom public water systems.

Filler uses amount to 15–20% o world production. Diatomite is a common addition to paint products such as fatemulsions, varnishes and primers because it reduces gloss and sheen, extends the opacity o the primary pigment, andimproves adhesion o subsequent coats. In plastics, its major use is as an anti-blocking agent in polyethylene lm.

Properties such as large surace area and low bulking value make diatomite suitable as a delivery medium orinsecticides, pesticides and ertilizers. Its chemical inertness, high melting point and low thermal conductivity makediatomite a useul source o insulating bricks, either sawn rom blocks o the natural material or, more commonly, roman extruded diatomite/clay mix.

laboratorY testIng

Laboratory evaluation o a potential diatomite deposit is rst carried out using a variety o mineralogical and chemicaltechniques. This is usually ollowed by small-scale processing trials, and subsequent analysis o the products todetermine use-related properties.



Initial mineralogical analysis is carried out by X-raydiraction, which identies impurities, particularly claysand crystalline silica (quartz). Chemical analysis providesthe total silica content which, when the contributions

rom quartz and clay minerals are subtracted, gives anindication o the purity o the diatomite. Trace elementsthat may be deleterious to its use in ood production arealso identied at this stage.

Diatom morphology greatly infuences suitability orspecic applications, and the size, shape and structure othe assemblage o diatoms present are investigated usingeither optical or scanning electron microscopy. Particle-size distribution, surace area, bulk density and colourare other important use-related properties, and theseare usually determined both on the raw diatomite andproducts rom processing trials. Air-classication is theaccepted method o producing material o the requiredparticle-size distribution, as this technique replicates thatused at a larger scale by industry.

dIatomIte In ethIopIa

Diatomite deposits in Ethiopia occur mostly in the MainRit. All are o lacustrine origin and range in age romTertiary to Pleistocene. The high-grade deposits o Gade

Motta, Adami Tulu, Chee Jilla and Abiyata (see insetmap) together amount to more than 40 million tons.

Some o these deposits are already being worked, withproduction currently running at 1700 tons/year. However,this level o production is insucient to meet the needso local industries. In addition, because processing othe raw material is minimal, these industries can not be

diatomite particle

liquor particle

Liquor + Bodyfeed

Filter-Cake

Filter septum

Filtrate

Precoat

FILTE

R

32°0'0"E

32°0'0"E

36°0'0"E

36°0'0"E

40°0'0"E

40°0'0"E

44°0'0"E

44°0'0"E

6°0'0"N 6°0'

10°0'0"N 10°0

ADDIS ABABA

JIMA

AWASA

GOBA

NEKEMTE AWASH

Rift fault

Precambrian faults &

shear zones

Town

Known Diatomite

occurrence

The distribution o diatomite occurrences in Ethiopia.

gyfi u — a wire mesh or cloth screen upon which precoat is

deposited.

pc — a thin permeable bed o diatomite particles deposited on

lter septum prior to ltration.

fii— process or removing suspended particles rom a liquor.

liqu— industrial fuid containing particle in suspension.by — diatomite added to the liquor to maintain fowrate by

increasing porosity o the lter-cake.

fi-c— a layer comprised o diatomite and liquor particles

which builds up on the surace o the precoat during ltration.

fi— clear liquor that has passed through the precoat.

fw— volume o ltrate that passes through precoat with

respect to time.

Ciy— amount/or lack o particles present in ltrate.



Cc Geological Survey o Ethiopia

Industrial Minerals Exploration DivisionPO BOX 2302

Addis Ababa, Ethiopia

tel: [251 11] 646 33 25

ax: [251 11] 646 33 26

email: [email protected]

www.geology.gov.et

supplied with material o the required quality. These two actors combined mean that increasing amounts o diatomite are beingimported, at a cost o more than double the world price o US$ 250–400 per ton (see table).

Imports of dIatomIte 2000-2005

Y 2000 2001 2002 2003 2004 2005

t 125 290 280 244 365 510

Us $ 86 956 215 217 238 170 227 635 317 168 502 368

Us$ 696 743 851 934 868 986

Cuy ii

FranceKenyaUSAEcuadorUK

UKBelgiumNetherlandsGermanyKenya

FranceBelgiumKenya

UKBelgiumNetherlandsKenyaFrance

FranceUKKenyaBelgiumUSA

FranceNetherlandsBelgiumUSAKenya

regIonal markets and trade

O the countries in the immediate region, Kenya is the only signicant producer o diatomite and, as can be seen rom the tableabove, is a major supplier to Ethiopia. Major users within the region are South Arica and the Kingdom o Saudi Arabia, both owhich import more than 6000 tons per year. Egypt and Tanzania import about 500 tons/year, and Uganda up to 400 tons/year. Thereis thereore scope or exporting diatomite products within the region, provided that they meet the required specications and arepriced competitively.

fUrther soUrCes of InformatIon

Reports on geological exploration or diatomite in Ethiopia can be obtained ree o charge rom the Geoscience Inormation Centre

at the Geological Survey o Ethiopia. Sta o the GSE can also provide inormation on diatomite imports, users within Ethiopia, anddetailed specications or dierent uses. The Central Geological Laboratory o the GSE is able to carry out evaluation tests on specicmaterials and recommend suitable processing fowsheets or dierent applications.

ethiopia_diatomite_march2007

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