Industrial Minerals Laboratory Manual DIATOMITE · Mineralogy and Petrology Series Industrial...

British Geological Survey

TECHNICAL REPORT WG/92/39 Mineralogy and Petrology Series

Industrial Minerals Laboratory Manual DIATOMITE S D J Inglethorpe

Mineralogy and Petrology Group British Geological Survey

Keyworth Nottingham

United Kingdom NG12 5GG

British Geological Survey

TECHNICAL REPORT WG/92/39 Mineralogy and Petrology Series

Industrial Minerals Laboratory Manual

DIATOMITE S D J Inglethorpe

A Report prepared for the Overseas Development Administration under the ODA/BGS Technology Development and Research Programme, Project 91/1

ODA Classzjkation: Subsector: Others Subject: Geoscience Theme: Mineral resources Project title: Minerals for Development Reference number: R554 1

Bibliographic reference: Inglethorpe, S D J Industrial Minerals Laboratory Manual: Diatomite BGS Technical Report WG/92/39

Subject index: Industrial minerals, diatomite, laboratory techniques

Cover illustration: SEM photomicrograph of the Rio Chiquito De Las Nubes diatomite, Costa Rica. The presence of genus Melosira sp. diatoms indicates fresh, deep water environment. The predominance of whole diatoms suggests that accumulation was by low-energy pelagic sedimentation.

0 NERC 1993

Keyworth, Nottingham, British Geological Survey, 1993

CONTENTS

Page

1. INTRODUCTION 1

2. BIOLOGY AND ECOLOGY OF DIATOMS

3. GEOLOGICAL, OCCURRENCE

4. INDUSTRIAL APPLICATIONS

5. LABORATORY CHARACTERISATION

6. LABORATORY TESTS RELATING TO INDUSTRIAL APPLICATIONS

7. CASE STUDY

8. CONCLUSIONS

REFERENCES

2

3

6

9

15

22

24

25

APPENDICES:

1. Preparation of diatomite for examination by optical microscope 27

2. Measurement of specific gravity 28

3. Measurement of bulk density 30

4. Sample preparation and calcination

5. Filtration theory

6. Measurement of permeability and wet cake density

7. Determination of oil absorption values

31

33

34

36

Preface

Industrial mineral raw materials are essential for economic development. Infrastructure improvement and growth of the manufacturing sector requires a reliable supply of good quality construction minerals and a wide range of other industrial mineral raw materials.

Although many less developed countries have significant potential industrial mineral resources, some continue to import these materials to supply their industries. Indigenous resources may not be exploited (or are exploited ineffectively) because they do not meet industrial specifications, and facilities and expertise to carry out the necessary evaluation and testwork are unavailable. Unlike metallic and energy minerals, the suitability of industrial minerals generally depends on physical behaviour, as well as on chemical and mineralogical properties. Laboratory evaluation often involves detemination of a wide range of inter-related properties and must be carried out with knowledge of the requirements of consuming industries. Evaluation may also include investigation of likely processing required to enable the commodity to meet industry specifications.

Over the last 10 years, funding from the Overseas Development Administration has enabled the British Geological Survey to provide assistance to less developed counties in the evaluation of their industrial mineral resources. This series of laboratory manuals sets out experience gained during this period. The manuals are intended to be practical bench-top guides for use by organisations such as Geological Surveys and Mines Departments and are not exhaustive in their coverage of every test and specification. The following manuals have been published to date:

Limestone Flake Graphite Diatomite Kaolin Bentonite

A complementary series of Exploration Guides is also being produced. These are intended to provide ideas and advice for geoscientists involved in the identification and field evaluation of industrial minerals in the developing world. The following guide has been published to date:

Biogenic Sedimentary Rocks

A J Bloodworth Series Editor

D J Morgan Project Manager

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Industrial Minerals Laboratory Manual

Diatomite

1. INTRODUCTION

Diatomite is a pale coloured, soft, light-weight rock composed principally of the silica microfossils of aquatic unicellular algae known as diatoms. It has a unique combination of physical and chemical properties (high porosity, high permeability, small particle-size, large surface area, low thermal conductivity and chemical inertness) that make it suitable for a wide range of industrial applications. Its main uses are as a filter-aid in the processing of liquid foodstuffs and chemical fluids, as a filler in plastics and paints, and as a raw material for the production of insulation bricks.

This manual defines the petrographic, mineralogical, chemical and physical characteristics of diatomite and describes simple test procedures for determining industrial specifications. Experimental results from recent studies in Thailand and Central America are included as examples. This manual is one of a series produced as part of the BGS/ODA R&D Project ‘Minerals for Development’.

Mineralogy and Petrology Group, British Geological Survey 0 NERC 7992

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Diatomite

2. BIOLOGY AND ECOLOGY OF DIATOMS

Diatoms are classified within the Kingdom Protista which also includes protozoans, moulds and fungi (Brazier, 1990). They are single-celled organisms consisting of a soft body (cytoplasm, oil globules and chloroplasts) enclosed by an opaline exoskeleton. The exoskeleton or frustule is composed of two halves, the smaller half fitting inside the larger half. Frustules are either circular (centric) or elliptical (pennate) in form, and are ornamented with sieve-like perforations (punctae) and intricate rib structures (costae). Diatoms create their food by combining carbon (obtained fiom photosynthesis of carbon dioxide) with nutrients extracted from seawater. They form the foundation of the Oceanic food chain as they are the staple diet of krill (tiny shrimp-like crustaceans) which support many marine vertebrates.

Diatoms are adapted to a wide range of aquatic environments including marine, brackish and fresh waters. The organisms require suitable environmental conditions if they are to flourish including appropriate temperature and photic conditions, a narrow salinity and acidity range, and a stable supply of nutrients including silica, nitrogen, phosphorus, iron, oxygen and carbon dioxide. Diatoms inhabit the photic zone at depths down to 200m and thrive in cold waters in sub-polar and temperate regions. Recent studies have used diatom assemblages as environmental indicators in Quaternary and present-day freshwater lakes (Abella, 1988; Stager, 1988).


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Diatomite

3. GEOLOGICAL OCCURRENCE

At the end of their life-cycle, opaline diatom exoskeletons settle through the water column and accumulate as ooze on the floor of the depositional environment, a process known as pelagic sedimentation; Kadey (1983) suggests an average sedimentation rate of 1-4 mm per year. Diatomite, a sedimentary rock, forms by dewatering and compaction of this ooze.

Diatoms first appears in the geological record about one hundred million years ago during the Upper Cretaceous, but most economic deposits are of Miocene-Pleistocene age. Diatomite deposits are frequently associated with volcanic activity, with air-fall ash, run-off waters and spring waters providing a source of dissolved silica to replenish that extracted by diatoms (Talliaferro, 1933). The purity of a diatomite is chiefly controlled by clastic and/or volcanogenic input during deposition. Calcareous micro-organisms which co-exist with diatoms in equatorial marine environments also affect purity.

At the present day, diatom-rich marine sediments accumulate in ocean basins in regions associated with the upwelling of nutrients, such as the zone of ocean current divergence in the sub-antarctic (Figure 1). Ocean floor areas are free from strong water movements which would introduce temgenous sediment, and are also deep enough to allow the dissolution of calcareous microfossils. However, the extensive marine Miocene-Pliocene deposits of California are known to have formed in a shallow-water neritic environment. Conger (1942) suggested that freshwater diatomite is likely to accumulate in mountain or crater lakes, free of sediment-bearing streams or rivers, where water inflow is by seepage (Figure 2). Most world deposits are of lacustrine origin. Marine deposits, although less common, are generally larger in size. Gradual, prolonged subsidence of marine and freshwater basins is necessary to allow development of thick accumulations of diatomite.

The effect of post-depositional processes on diatomite deposits is poorly understood. Diagenetic alteration is likely to influence the porosity and degree of cementation through dissolution, mobilisation and re- precipitation of diatomaceous silica, and in extreme cases is likely to convert diatomite to chert (Mathers, 1989). Diatomaceous ooze has been reported to contain a substantial proportion of organic matter derived from diatom soft bodies and so is a possible source rocks for petroleum deposits (Talliafen-o,1933). Conger (1942) reports that uplift and exposure of diatomite under good drainage conditions removes organic impurities by oxidation and leaching.


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Diatomite

. I . - . . . . . . . . . . . . . . . .

. . . . . .

Figure 1. Concentration of diatoms in surface sediments of oceans at the present day (after Brazier, 1991).

Plumes of sediment-rich river water e n t e r i n g l a k e as under , in ter and overf lows, depending on d e n s i t y relative t o l a k e w a t e r

Figure 2. Sedimentological model for accumulation of diatomite in a caldera lake environment (after Mathers, 1989)


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Diatomite

3.1 Major economic deposits

The following countries are major producers of diatomite and are responsible for 98% of world production: USA (approx. 33%), France, Romania, former USSR (CIS), Spain, Denmark, Korea, Mexico, Gemany, Italy, Iceland, Brazil (Harben, 1992). World production for 1988 was estimated at 2 Mt. with reserves of 2000 Mt. A brief survey of some commercial deposits is made below. A more comprehensive review of world production of diatomite is given in Anon. (1987).

3.1.1 USA

The most extensive deposits of commercial diatomite are the late Miocene-early Pliocene marine diatomites of the Sisquoc Formation located in the foothills of the Santa Ynez mountains south of Lompoc valley, western Santa Barbera County, California, USA (Taylor, 1981; Burnett, 1991). Commercial deposits over 1000 ft. thick are extracted by the Johns-Manville Corp. and Grefco Inc. (who market the M i t e and DicaZite range of products respectively) for filter-aid and fiier production. In mid-1986 Grefco also began to extract a high-quality Miocene-Pleistocene freshwater diatomite at Lake Britton, northeast of Burney, Shasta County, for filter-aid production.

3.1.2 Romania

Romania is the largest producer in Europe with lacustrine deposits located at Adamclisis Constanta, Patiragele Buxau and Minis-Arad. Diatomite products in Romania (and in the former USSR) are almost exclusively consumed by the construction industry.

3.1.3 France

‘Carbonisation de Actifs SA’ (Ceca) produces high-quality filter-aid grade diatomite from underground mining of a Miocene lacustrine deposit at Rioms-les-Montagnes. Ceca also operate an opencast diatomite mine at St. Bauzille on the east side of the Andance mountains. ‘Manville de France SA’ are France’s second largest producer with a mine at Murat, Cantal.


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Diatomite

4. INDUSTRIAL APPLICATIONS

In the United States in 1985,66% of diatomite produced was used in filtration, 21% for fillers, 1 % for insulation and 12% for miscellaneous uses (Anon, 1987). The world market for diatomite is increasing, although there is competition from perlite, calcined kaolin, talc, and other forms of silica (Harben, 1992).

4.1 Industrial processing

Raw diatomite is processed using the following methods:

Primary crushing After extraction the diatomite is crushed to aggregate-size pieces in roller or hammer mills.

Simultarleous drying and milling Crude diatomite (typically containing in excess of 40% moisture) is carried by a stream of hot gases produced by an air-blower coupled to a furnace, and is simultaneously dried and milled.

Air-classification The filter-aid and filler markets require powdered diatomite in different particle-size grades. These are produced by air- classification. For example, Lompoc diatomite is air classified to produce various grades of calcined filter-aids with different particle- size characteristics. A proportion of the dried, milled, air-classified powder is packaged and sold as ‘natural’ diatomite mainly for the filler market.

Calcination Following air classification, diatomite is calcined at 870- 1 100°C in a rotary kiln. The process transforms amorphous silica to crystalline a-cristobalite, reduces surface area, increases mean particle-size and increases hardness from 4.5-5.0 to 5.5-6.0 (Mohs’ scale).The main purpose of calcination is to increase filter-aid flowrate, improve the strength of fillers and increase abrasive properties. Greater change in physical properties is obtained by adding ‘soda ash’ [Na2C03] as a flux. Calcined and flux-calcined end-products are marketed both as filter-aids and as fillers.

4.2 Filtration

Diatomite is used as a filter-aid to process a wide range of industrial and non-industrial fluids including inorganic/organic chemicals, pharmaceuticals, beer, wine, whisky, fruit juices, vegetable juices, fuels, oils, dry cleaning solvents and swimming pool waters. A filter- aid is essentially a permeable bed of diatomite whose purpose is to


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Diatomite

remove fine-grained particles from a suspension and produce a clear filtrate at high flowrates.

4.3 Fillers

Diatomite is used as a filler in paints, plastics, rubber, pharmaceuticals, toothpastes and polishes. Diatomite fillers are marketed both as natural grades (e.g. Celite 266) and as flux-calcined grades (e.g. Celite 499, 281, White Mist). In certain applications diatomite is added to modify the physical properties of the product and is therefore referred to as a ‘functional’ filler.

Diatomite is commonly added to paint products such as flat emulsions, varnishes and primers because it has excellent ‘flatting’ properties (i.e. it reduce gloss and sheen), extends the ‘hiding power’ (opacity) of the primary pigment, improves adhesion of subsequent coats (‘good tooth’) and has good sanding properties. Moreland (1987) estimates that 15% of the total industrial usage of diatomite is as a filler for plastics. Its major use is as an ‘anti-blocking’ agent in production of low-density polyethylene film; diatomite is typically added at 500-3000 ppm concentration and prevents self-adhesion of the film.

4.4 Insulation, absorbent powders and granules

The Danish companies Skamol and Damolin produce insulation bricks from a raw material known as ‘moler’ - a natural mixture of diatomaceous silica and 20-25% ‘plastic’ clay (Griffiths, 1990). Insulation bricks are produced as follows: (1) crude moler is fed from a hopper to a primary crusher and reduced to a particle-size of 2-3 mm; (2) up to 50% pinewood sawdust is added to increase the porosity of the final product through gases generated during firing; (3) the moler/sawdust blend is then mixed with steam to improve plasticity, extruded and wire cut into lengths; (4) these ‘green’ bricks are dried at 90°C for 2-5 days and then fired.

Moler is also used to produce kiln-dried absorbent powders/granules (uses: industrial absorption, animal feed, pesticides, cat litter) and calcined powders (uses: explosives, seed coating, chemicals industry). It is likely that current uses for moler were developed because it is not sufficiently pure for filtration and filler applications (Lefond, 1983).

4.5 Miscellaneous

Diatomite is also used as a mild abrasive (toothpaste and metal polish) anti-caking agent in fertilizer prills, pozzolana/concrete additive to


8 . .

Diatomite

improves plastic and hardened properties, stabilising agent in explosives, chromatography support media, pitch control in paper manufacture, and as a catalyst carrier (for nickel catalyst in vegetable oil hydrogenation, for vanadium in sulphuric acid manufacture and for phosphoric acid in petroleum refining).


9 . -

Diatomite

5. LABORATORY CHARACTERISATION

This section describes the mineralogical, chemical and physical properties of diatomite. Appropriate methods for the laboratory assessment of diatomite are outlined in Figure 3. Diatomite has distinctive chemical and physical characteristics, including high silica content in combination with a low SG. An optical or scanning electron microscope is usually required to detect the presence of diatoms, and XRD analysis is necessary to identify diatomaceous silica (opal A).

5.1 Chemistry

The prime criterion for judging potential industrial uses is chemical composition. Typical chemical analyses of diatomites used by industry are shown in Table 1. Diatomites used in filtration and filler applications generally contain >85% silica. Theune & Bellet (1988) define maximum levels for several chemical constituents of filter-aids: (1) 4 % CaO to prevent turbidity in beer filtration resulting from oxalic acid formation and precipitation of tartaric acid crystals in the filter bed during wine filtration; (2) < 1.5% Fe203 to prevent formation of deleterious iron tannates in wine filtration; and (3) d o 0 ppm soluble salts to prevent chemical contamination of foodstuff liquors.

The British Standard Specifications for paint extenders (BS 17951976) lists two chemical categories for diatomite fillers: 70-80% silica (Si02) for ‘type 1’ diatomite fders, and a minimum of 80% silica for the ‘type 2’ classlfication. In contrast, diatomite extracted in Denmark for production of insulation bricks and kiln-dridcalcined powders and granules has a relatively low silica, high alumina and iron.

5.2 Mineralogy

The type of silica present in diatomite is a hydrous form of opaline silica which contains between 3-8% structural water. Several types of opaline silica were defined by Jones & Segnit (1 97 l), principally by their different X-ray diffraction (XRD) characteristics. These include opal C (well-ordered a-cristobalite); opal CT (disordered cristobalite / tridymite); and opal A (poorly-ordered, almost amorphous). Diatomite is composed of biogenic opal A silica which is indicated from XRD analysis by a broad peak in the vicinity of the a-cristobalite spacing at 4.05 A (Figure 4). The qualitative mineral content of diatomite can also be determined by XRD analysis. Common impurities include quartz silt and sand, volcanic ash and pumice, clay minerals, carbonate minerals, iron oxides/oxyhydroxides, hydrothermal minerals (e.g. alunite group) and organic matter.


I INITIAL CHARACTERISATION I

I

TESTS RELATING TO P O K E

Figure 3. Laboratory evaluation of diatomite

I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

I FILLERS FOR PLASTICS AND PAINTS I

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Diatomite

0.84 - 0.01 -

02 e cO-K a 1 I

0.0 10 -0 20.0 30.0 40.0 sD.0

Figure 4. Whole-rock XRD trace of a high-purity freshwater diatomite from Burney, California. The broad hump in the vicinity of the a-cristobalite peak (4.05 A) is typical of opal A type silica.

Figure 5. SEM photomicrograph of the Rio Chiquito De Las Nubes diatomite, Costa Rica. The presence of genus Melosira sp. diatoms indicates a fresh, deep water environment. The predominance of whole diatoms suggests that accumulation was by low-energy pelagic sedimentation.


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Diatomite

5.3 Microscopy

Diatoms are microfossils and so are not generally visible to the naked eye although larger diatoms (30-40 pm) may be resolved by a hand- lens. Optical or scanning electron microscope (SEM) methods are therefore usually necessary to identify their presence.

Inspection by binocular microscope is a useful rudimentary method for investigating the micropalaeontology and particle morphology of larger diatoms, and for estimating the proportions of coarse-grained mineral impurities (quartz, feldspar, heavy minerals etc.) present. A preparation procedure for diatomite prior to binocular microscopy is given in Appendix 1.

Secondary electron mode SEM analysis is carried out on fragments of rock coated under vacuum with gold or carbon. It is a high-resolution method (conventionally used for detailed petrographic work) capable of providing comprehensive data on the morphology of diatoms and their micropalaeontology. Semi-quantitative chemical analysis of individual mineral grains can generally be determined using an energy dispersive detector. A high Sflow Al spectra is typical of diatom particles. An SEM photomicrograph of a diatomite from Costa Rica is shown in Figure 5.

Diatomite composed predominantly of whole diatoms is likely to have accumulated by pelagic sedimentation in a relatively low-energy environment. However, if a large proportion of diatom fragments is present this suggest a relatively high-energy environment where attrition resulted from sediment transport and reworking. Micropalaeontological studies of diatom taxa can reveal whether a deposit was formed in a freshwater, brackish or marine environment, and whether deposition occurred in deep or shallow water.

5.4 Physical properties

k e diatomite is composed essentially of diatoms and diatom fragments in the size range 63-2 pm, but the presence of mineral impurities can alter particle-size distribution. The substantial >63 pm fraction of the Nan Jo diatomite (Table 2) is due to cementation of particles by goethite, and its large Q pm fraction is a result of a high clay mineral content.

Pure diatomite has an SG value of 2.0-2.1, >80% porosity (total void volume) and a bulk density between 0.2-0.4 gJcm3. The presence of mineral impurities increases SG and bulk density values and lowers porosity. SG is generally the most reliable indicator of purity, since bulk density and porosity are both influenced by particle-size


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Diatomite

distribution as well as mineral content. Procedures for the determination of specific gravity and bulk density are given in Appendices 2 and 3.

High-purity diatomites are typically white, buff or grey in colour (in the natural state and after calcination) as illustrated by the Burney diatomite (Table 3). The colour of calcined diatomite is particularly affected by iron content; for example, the low brightness values of calcined Lorna Camastro diatomite are due to a pink colouration imparted by hematite (formed from thermal decomposition of alunite-group minerals). Addition of a ‘soda ash’ flux may improve the colour of the calcined product. A procedure for sample preparation and calcination of diatomite for physical property testing is given in Appendix 4.

Table 1. Chemical composition of natural diatomites.

(A) (C) @) Element (%) Bumey Natural Celite 266 Damolin Skamol

91.39 0.10 1.97 0.48 0.002 0.04 0.5 1 NA 0.07 0.00 4.18

86.9 0.18 3.1 1.1 NA 0.65 0.4 1

3 o.88 0.15 3.8

68-80 NA 8- 10 5-7 NA 1-1.5 1-3.5

3 2-3 NA 0.5-3.5

74 1 .o 11 7 .O NA 2.0 1 .o 0.5 1.5 NA 1 .o

(A) Burney diatomite from Shasta County, CA, USA, extracted for filter-aid production by Grefco Inc. (BGS analysis.) (B) Johns-Manville diatomite paint filler @emmers & Kranich, 1986). (C) & (D) Dansk Moler Industri and Skamol-Skarrehenge Moler-vaerk technical literature.


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Diatomite

Table 2. Physical properties of diatomite (BGS data).

(1)> 63 Cun (%I

(3) 1 0 - 2 p (%) (4) 2 lun (%I

(2) 63-10 (%)

(5) Graphic mean (w) (9 Bulk density (g/cm3) (7) SG (8) Porosity (%) (9) Surface area (m2/g)

0.8 9.9 68.5 20.8 4.8 0.298 2.0 1 85 13.1

1.2 5.0 21.7 72.1 1.5 0.250 NA NA 59.8

18.2 3.3 19.6 58.9 1.5 0.245 2.26 89 NA

0 7.0 88.0 5.0 5.3 0.296 NA 79 3.4

0 24.0 66.5 9 .o 6.8 0.407 NA NA 4.7

(A) Burney diatomite, Shasta County, CA, USA. High purity: extracted by Grefco Inc. for fiter-&d production. (B) Loma Comastro, diatomite, NE of Liberia, Costa Rica. Impure: - 22% clay mineral content plus alunite- group minerals (Inglethorpe, 1990). (C) Nan Jo diatomite, Lampang, Thailand. Impure: 3040% clay mineral content plus 12% quartz and 6% goethite (Inglethorpe, 1991). (D) & (E) show values for samples calcined at 1100°C for 1 hour. (1)-(4) Particle-size distribution by wet sieving and sedimentation. ( 5 ) Mean particle-size of e63 pn fraction. (6)-(9) Values for dry powdered samples.

Table 3. Brightness values of natural and calcined diatomites

Sample

602 filter 603 filter 605 fiiter 606 filter Blue Blue-green Yellow-green Yellow (4700 A) (4900 A) (5500 A) (4800 A)

~ ~~ ~ ~ ~~

Burney 69.1 69.8 72.1 74.2 Bumey, calcined 66.0 66.1 73.4 78.6 Loma Camastro 75.6 74.7 78.1 78.8 Loma Camastro, calcined 32.4 30.2 36.6 46.0


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Diatomite

6. LABORATORY TESTS RELATING TO INDUSTRIAL APPLICATIONS

6.1 Filtration

Diatomite filter-aids have two functions: (1) as a precoat - a thin bed of diatomite used to filter fluids by removing suspended particles, and (2) as a bodyfeed - diatomite added to the fluid to maintain the porosity of the filter. A schematic diagram of an industrial filtration process is shown in Figure 6.

The Brewing Research Foundation (BRF) Surrey, England, has developed a model of filtration characteristics using permeability and mean hydraulic radius as the principal parameters (Reed and Picksley, 1987; Picksley and Reed, 1989). These two properties control ‘flowrate’ and ‘clarity’ respectively . These are key properties referred to by manufacturers and consuming industries. An outline of filtration theory is given in Appendix 5.

Permeability is defined as ‘the ability of a granular material to allow the passage of a fluid’, and is the property which determines the rate of flow of a fluid through a filter-aid. Manufacturers and consuming industries often refer to permeability in terms of ‘ flowrate’ or ‘relative flowrate’. Mean hydraulic radius is the average void size within a filter- aid and is a measure of ‘cut-off size’ - the smallest size of particle that the filter-aid is able to remove from suspension. In industry this property is often referred to as ‘clarification ability’ or ‘filtrate clarity’.

6.1 .I Filtration properties

The physical properties of diatomite filter-aids are summarised in Table 4. In a preliminary investigation, it is not practical to determine all the parameters given in Table 4. Permeability and wet cake density will give a good indication of likely filtration performance. Parameters such as mean hydraulic radius and effective void volume are useful theoretical concepts (Appendix 5)’ but are not essential to a preliminary investigation of diatomite for filter aids. Table 4 shows that increasing the particle-size of a filter-aid raises ‘flowmte’ by increasing permeability, and lessens ‘clarity’ by increasing mean hydraulic radius.

Procedures for the determination of permeability and wet density values of calcined diatomites are given in Appendix 6. Theune & Bellet (1988) state that calcined diatomite filter-aids have permeability values 9.03 pm* and wet density values (the density of a filter-aid precoat) between 0.35-0.5 gJcm3. The authors suggest that high permeability values are required for fast flowrates, and that low wet density values are


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Diatomite

necessary to provide porosity for the retention of particles removed from fluid during filtration.

‘Total void volume’ is calculated from SG and wet density values as follows:

Total void volume (% porosity)= [ 1-(wet density/SG)] * 100/1

Permeability can be measured with simple experimental apparatus using Darcy’s Law:

Permeability (13) [pm2] = p U L * 1012 AP

p = viscosity of water at 20 O C U = superficial velocity = (volumetric flowrate [m3/s] / cross-sectional area [m*]) AI? = pressure drop across bed L = depth of bed [m]

Diatomite should be prepared for these tests using the preparation and calcination procedure described in Appendix 4. During the test only volumetric flowrate (U) and depth of bed (L) are recorded, as the other parameters are constant. The apparatus used to cany out permeability and wet density determinations is illustrated in Figure 7. Procedures for particle size measurement of mineral powders are given elsewhere in this series of manuals.


17 . ..

Diatomite

diatomite particle

- liquor particle

Liquor + bodyfeed

Precoat

Filtrate M

Glossarv:

Filter septum - a wire mesh or cloth screen upon which precoat is deposited Precoat - a thin permeable bed of diatomite particles deposited on fiter septum prior to filtration Filtration - process for removing suspended particles from a liquor liquor - industrial fluid containing particles in suspension Body feed - diatomite added to the liquor to maintain flowrate by increasing porosity of the filter-cake. Filter-cake - a layer comprised of diatomite and liquor particles which builds up on the surface of the precoat during filtration Filtrate - clear liquor that has passed through the precoat Flowrate - volume of filtrate that passes through precoat with respect to time Clarity - amount/or lack of particles present in filtrate

Figure 6. Schematic diagram of an industrial filtration process


18

, .. ...... .. . .~ . , .. ..;. . . . . ,.. . . . . . ~ - . . . . ,

Diatomite

Table 4. Filtration properties of diatomite filter-aids (BGS experimental data and technical literature).

‘Flowrate’ ‘Clarity’ Mean Wet Total Effective Mean

Measured hydraulic cake void void particle permeability radius density volume volume size SG

Product m21 [ P I [g/cm31 [%I [%I [WI ~~ ~~~ ~

Dicalite 215 Bumey Superaid FPlW Celite 578 FP4 Standard Supercel

Hyflo Supercel speedflow

0.02 # 0.04 0.04 #

0.069 0 0.07 * 0.20 0 0.22 0 0.22 #

1.62 *

NA NA NA 0.3 0 NA 0.5 0 0.7 0 NA NA

NA 0.42 0.352 NA NA NA NA 0.368 #

NA

NA 79 84 NA 83 * NA 86 * 84 86 *

NA NA NA 17 0 27 * 25 0 35 * NA 58 *

5.8 2.25 5.3 2.01 5.9 2.25 NA NA 7.1 * NA NA NA 6.8 * NA 7.5 2.25 #I 10.7 * NA

Nan Jo (ax) 0.02 N A 0.59 73 N A 3.1 2.1 7

0 Reed & Freeman (1991). * Reed & Picksley (1987). # Manufacturer’s data.


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Diatomite

Figure 7. Laboratory filtration apparatus. Components are connected in series to a filter- pump as follows: (1) conical flask, (2) vacuum regulator unit, (3) bleed valve, (4) vacuum meter and (5) filtration unit.


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Diatomite

6.2. Fillers

General characteristics of fillers that influence usage include

colour oil absorption surface wetting and bonding properties chemical resistance strength/iability.

The physical properties of diatomite fillers are summarised in Table 5; chemical specifications are discussed in section 5.1. Remmers and Krannich (1986) cite high oil absorption, high silica content and low bulk density as important properties of diatomite fillers for paint. A procedure for determination of oil absorption by diatomite is given in Appendix 7.

Table 5. Physical property specifications of diatomite fillers for paints and plastics.

Celite:

Silver Frost K 5 * White trace NA NA NA NA 115 White Mist* White trace NA NA NA NA 160 266# Grey 0.5 96 92 NA 16 135

28 1# White 1.5 96 68 NA 3 110

supmoss* White 0.1 NA NA NA NA 120

499# White trace 99 82 NA 1 105

Type 1 diatomite* / 15 70 10 / / 60-100 Type 2 diatomite* / 5 75 20 5 / 60-100

9-10 1.48 NA 0.7-3.5 NA NA 9-10 1.48 NA 0.7-3.5 NA NA 9-10 1.48 NA 0.7-3.5 NA NA 6.5 1.42 0.128 NA NA NA 9.5 1.48 0.136 NA NA NA 9.5 1.48 0.136 NA NA NA

6.5-10 / / / 1 15 6.5-10 / / / 1 1

* Moreland (1987). #I Remmers & h i c h (1986). 5 BS 1795 (1976).


21

Diatomite

6.3. Insulation bricks, absorbent powders and granules

Insulation bricks are produced from a Danish clay-rich diatomite known as ‘moler’ which is characterised by a low Si02 content and high levels of Fe2O3 and A1203 compared to other commercial diatomites (Tablel). The physical specifications for insulation bricks are based on bulk density, porosity, cold crushing strength and thermal conductivity (Table 6).

The heat resistant properties (or low thermal conductivity) of diatomite bricks result from their high porosity (or low bulk density). The Danish company Skamol produce seven types of insulation brick (with a temperature resistance of 900-950°C) classified on the basis of bulk density; low bulk density bricks have good heat resistance but only moderate strength whereas high bulk density bricks have better strength but lower heat resistance. The main uses of these products are in kiln and furnace applications.

Table 6. Typical physical properties of diatomite insulation bricks.

ROTOL HTpoR(450)

Bulk density (kg/m3) Porosity (%) Cold crushing strength (kg/cm2) Modulus of rupture (kglcm2) Thermal conductivity (kcal/ m hr “C)

800 450 66 81 100 8 20 NA 0.17-0.2 1 0.09-0.16

Data source: Skamol-Skarrehenge Moler-vaerk technical literature. (Thermal conductivity values quoted are for temperature range 200-80O0C.)

~~ ~


22 ,. . .

Diatomite

7. CASE STUDY

7.1 Diatomite, Thailand

A 20 kg sample of diatomite was collected from Nan Jo quarry, Mae Tha, Lampang Province, Thailand. A laboratory study was planned to define the chemistry, mineralogy, petrography and physical properties of the diatomite, examine mineral processing techniques to reduce levels of mineral impurities, and evaluate the sample as a potential industrial raw material for filter-aid production (Inglethorpe, 1991). A flowsheet summarizing the laboratory investigation of this material is shown in Figure 8.

Nan Jo quarry is situated to the south of Lampang. Diatomite is currently extracted for use by the local paper-making industry. The geological map of this area is the Department of Mineral Resources (DMR) Changwat Lampang 1:250 000 scale sheet. The diatomite sequence is part of the Mae Mo Group which consists of freshwater sandstone, shale, carbonaceous shale and lignite beds of Miocene-Pliocene age. The sample examined in this study was collected from the basal bed of the 10 m high north face of the quarry.

Results indicated that the Nan Jo diatomite contains 30-40% clay minerals, 12% quartz and 6% goethite. These impurities are probably of detrital origin. The sample was deposited in a relatively high-energy freshwater environment. Attrition during sediment transport reduced whole diatoms to 10-1 pm size fragments. The sample has a low Si02 and high A1203 and Fe2O3 content compared to diatomites utilised by industry. Mineral processing trials were not effective in upgrading the sample.

The filtration performance of the Nan Jo diatomite is poor. Calcined samples are of low permeability (0.02 prnz), high filter-cake density (0.57-0.61 g/cm3) and small average particle-size (2.5-3.7 pm). The iron oxide content of this material is above levels acceptable for the processing of foodstuffs. The sample is therefore unlikely to be suitable for use in filter-aid applications. However, the chemistry and mineralogy of the Nan Jo sample are similar to commercially-mined diatomites used in the production of insulation bricks.


23

Diatomite

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ( ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (T) . . . . . . . . . (> . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . : .... : ,-Sample,,. : j:j PETROGRAPHY: I I I (I I i.i:. Polished' .:< . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ;;:: Rock-chip ': ;.::i.i; I I - I .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PETROGRAPHY: ii;;~insectio"i,:i:i : : : . : . . . . . . . . . . . . . ' . : . : ' :: : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .................... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - secondary electron mode - Optical microscopy

-Electron probe micro-analysis

(EPMA) element maps Crushed to

. Dispersed Sub-sampled

in deionised r Milled to by water

e2 pm fraction separated &

dried

MINERALOGY:

-Clay fraction

XRD analysis

I rime-splitting I

Gently disaggregated

to c63 pm

Calcined at 1 100°C for 1 hour

I

I I I I

scanning electron microscopy

(SEW

( ,,,,powder I: J . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - Whole-rock major-element

chemistry

- Whole-rock X-ray diffraction

(XRD) analysis

PHYSICAL PROPERTY TESTS:

- Permeability

- Wet-cake densiiy

- Total void volume

- SG

- Bulk density

- Particle-size analysis

Figure 8. Flowsheet showing laboratory procedures used to investigate the Nan Jo diatomite.


24

Diatomite

8. CONCLUSIONS

Diatomite is a pale coloured light-weight rock composed principally of the silica microfossils of aquatic unicellular algae known as diatoms. Its main uses are as a filter-aid to remove fine particles from industrial fluids, and as a filler in plastics and paints. Diatomite is also added to a range of insulation products to improve their thermal resistance.

The type of silica present in diatomite is a poorly-ordered opal A type which is recognised from a broad X-ray diffraction peak at 4.0581. Pure diatomite is characterised by high silica content (>85%), high porosity (>80%), low SG (2.0-2.1), low bulk density (0.2-0.4 9/cm3), and fine- particle size (typically 63-2 pm).

Diatomite used in industrial filtration is of high chemical purity with low calcium, low iron and low soluble salts content. The key property controlling filtration performance is permeability (porosity, wet cake density and particle-size are also important filtration characteristics). Diatomite fillers for paints and plastics are also chemically pure, as elements such as iron adversely affect colour. Key specifications for filler applications are good whiteness, high oil absorption and a fine- grained, narrow particle-size distribution. Danish ‘moler’ diatomite is used in insulation bricks and absorbent powders and granules. Its high A1203 and Fez03 content precludes its use in filter-aids, paints and plastics.


25

Diatomite

REFERENCES

Abella, S E B (1988) The effects of Mt. Manzama ashfall on the planktonic diatom community of Lake Washington. Limml. Oceanogr. 33(6), 1376-1385.

Anon. (1987) Diatomite. Ind. Miner. 236, 22-39.

Brazier, M D (1990) Microfossils. George Allen & Unwin, London: 39- 44.

BS 1795: 1976. British Standard Specifications for Extenders for Paints. Brit. Standards Inst. Publ.

Burnett, J L (1991) Diatoms - the forage of the sea. Calif. Geol. 44(4), 75-81

Conger, P S (1942) Accumulation of diatomaceous deposits. J . Sed. Petrol. 12(2), 55-66.

Griffiths, J (1990) Denmark’s industrial minerals. Ind. Miner. 279,50- 55.

Haben, P W (1992) Industrial Minerals Handybook. Metal Bulletin PLC, London: p 27.

Inglethorpe, S D J (1990) Mineralogical composition and filtration properties of a sample of diatomite from Loma Camastro, Costa Rica. Rep. Brit. Geol. Sum. WG/90/13R.

Inglethorpe, S D J (1991) Evaluation of a diatomite sample from Nan Jo quarry, Lampang Province, Thailand. Rep. Brit. Geol. Sum. WG/91/32R.

Jones, J B & Segnitt, E R (1971) The nature of opal: (i) Nomenclature and Classification. J . Geol. SOC. of Austr. 18 (l), 57-68.

Moreland (1987) in Handbook of Fillers for Plastics. (Katz, H S & Milewski, J V Eds.) Van Norstrand Reinhold Co., New York.

Lefond, S J (1983) Industrial Minerals and Rocks (5th Edn.). AIMMPE publication.

Mathers, S J (1989) Costa Kcan diatomite: a review of existing knowledge and future potential. Rev. Geol. Amer. Central. 10,3-17.

Picksley, M & Reed, R J R (1989) Characteristics of Filter-aids. J . Inst. Brewing. 95(3). 169-179.

Mineraloav and Petroloav Grow. British Geoloaical Survev 0 NERC 1992

26

Diatomite

Reed, R J R & Freeman, G J (1991) Filter-aid fundamentals - selection of clarification characteristics by use of ‘mean hydraulic radius’. J. Znst. Brewing. In press.

Reed, R J R & Picksley, M A (1987) The determination of the ‘true’ filtration characteristics of diatomaceous earth. J. American Sociew Brewing Chemists. 45 (2), 48-53.

Remmers, T E & Krannich, M A (1986) Pigment Handbook: Vol. 1 Properties and Economics, 2nd Edn. Lewis, P A (4). John Wiley & Sons.

Stager, J C (1988) Environmental changes at Lake Cheshi, Zambia since 40 000 years B.P. Quat. Research. 29,54-65.

Talliaferro, N L (1933) The relation of volcanism to diatomaceous and associated siliceous sediments. Geological Science [Univ Calif. Publ.] . 23, 1-55.

Taylor, G C (198 1) California’s diatomite industry. Calif. Geol. 34(9), 183-192.

Theme, C & Bellet, J (1988) Aptitude of diatomaceous ore for filter aid processing Proc. 8th Znd. Min. Con$ 223-229.

Mineralogy and Petrology Group, British Geological Survey 0 NERC 7 992

27

Diatomite

Appendix 1: Preparation of diatomite for examination by optical microscope

Use this technique to positively identify the presence of diatoms in a sample. Stages 5 to 8 are optional.

CAUTION: Handle CC4 with care. Stages 5-8 should be carried out in a fume cupboard, and safety glasses and protective gloves should also be worn.

Apparatus and chemicals

5 litre Plastic bucket Petri dish Hard bristled scrubbing brush Small glass funnel 45 pm/325 mesh sieve (20 cm diameter) ‘Fast’ filter paper Glass beaker Stirring rod Small stoppered vial/container

Carbon tetrachloride (CC4)

Method

1.

2.

3.

5 .

6 .

7.

8.

9.

10.

Immerse sample in bucket of water and gently scrub using hard- bristled scrubbing brush to liberate microfossils.

S t r a h resulting suspension through a 45 pm (325 mesh) sieve.

Transfer >45 pm residue into an evaporating dish and dry at 55OC.

Place dried sample residue in a beaker and add carbon tetrachloride (CC4, SG 1.58) at 2-3 times the sample volume.

Stir using a clean dry glass rod. Diatoms should float to the surface if present.

Decant ‘float’ into a fiiter-lined funnel and allow to air-dry. Repeat stages 5-7 if necessary.

Sprinkle a thin layer of dried diatoms onto a petri dish backed with black paper.

Illuminate and examine using a binocular microscope.


28

Diatomite

Appendix 2: Measurement of specific gravity

This procedure is used to determine the specific gravity of small mineral fragments or powdered minerals.

Apparatus/materials

Balance (0.01 g readability) Drying oven (105°C) Pycnometer bottle with stopper Aluminium dish or s m a l l evaporating basin Distilled water

Method

1.

2.

3.

4.

5.

6.

7.

8.

Weigh out approximately 10 g of powdered sample into an aluminium dish/small evaporating basin and place in 105OC oven overnight.

The following morning, place a clean pycnometer bottle with stopper in oven at 105°C for one hour.

Accurately weigh the pycnometer bottle with stopper to the nearest 0.01 g. Note reading as A = ..... g.

Remove stopper from pycnometer bottle and add approximately log of dry mineral powder. Replace stopper and weigh bottle plus contents to the nearest 0.0 1 g. Note reading as B = .... g.

Remove stopper and carefully add distilled water to the bottle to cover sample until pycnometer bottle is three quarters full. Gently agitate bottle (in an ultrasonic bath, if available). Continue agitation until sample is thoroughly wetted and all trapped air has been dispelled.

After agitation add more water to the pycnometer bottle until water level is just below the brim. Replace stopper until both the bottle and fine capillary opening in stopper are full of water.

Thoroughly dry outside of pycnometer bottle and weigh bottle plus contents. Note reading as C = .... g.

Thoroughly rinse out pycnometer bottle and then repeat steps 5- 7 but with water only filling the bottle. Note reading as D = .... g*


29 . .

Diatomite

Appendix 2 (continued)

Calculation

Specific Gravity (SG) = B - A ( B + D ) - ( A + C )


30

Diatomite

Appendix 3: Measurement of bulk density

This method is used to determine a nominal bulk density value for powdered minerals.

Apparatus/materials

Balance (0.01 g readability) Drying oven (105OC) Aluminium dish or smal l evaporating basin Measuring cylinder (10/20/50 ml)

Method

1. Weigh out in excess of 10 g powdered sample into an aluminium dish/small evaporating basin and place in a 105OC oven overnight.

2. Place weighing boat on balance and tare weight. Weigh out 10.00 g (+ 0.01 g) of sample.

3. Transfer sample from weighing boat to measuring cylinder (10, 20, or 50 ml size as appropriate).

4. Gently tamp measuring cylinder by gently knocking base of cylinder against flat surface. Repeat this action 10 times. Record volume of sample as V = .... ml.

Calculation

Bulk density (BD) [g/cm3] = 1O[g]/V [ d l


31

Diatomite

Appendix 4: Sample preparation and calcination

This is a procedure for the preparation of diatomite powders and calcined diatomite powders for physical property testing.

Apparatus

Dust masks Balance (0.1 g readability) Drying oven (55°C) Laboratory muffle furnace Jaw crusher Roller mill (or large automatic/manual mortar and pestle) 63 pm (240 BS mesh size) 20 cm diameter metal sieve, (preferably used in conjunction with vibratory or vacuum-assisted sieving apparatus) Furnace crucibles, tongs, heat-resistant gloves, heat-resistant face shield

Method

1. Dry a portion of the ‘as received’ sample in a 55°C oven overnight.

2. Coarsely crush to 4 mm diameter in size using a small jaw- crusher if available. (Note: this coarsely-crushed diatomite may be used as stock material for bulk chemical and XRD analyses.) Sub-sample by riffle splitting or coning-and-quartering.

4. Gently grind the coarsely-crushed sub-sample, preferably using - a roller mill at a narrow ‘gap’ setting. Alternatively, gently grind for approximately 5 minutes using a pestle and mortar.

5. Screen ground sample on a 63 pm aperture metal sieve. Vibratory or vacuum-assisted sieves are most effective. Weigh c63 pm sieve undersize obtained and transfer to sample bag.

6 . Re-grind any >63 pm sieve oversize by repeating steps 4-5.

7. The <63 pm sieve undersize material obtained can be used as powdered diatomite in physical property tests.

Note: pure diatomites are soft and friable and easily pass through a 63 pm sieve. However, diatomites containing clay minerals or iron oxides are often more cemented, and a significant proportion of these sample will remain in the >63 pm fraction even after repeating stages 4-5 several times. In these cases, the >63 pm material remaining should be weighed and retained for reference.

.. I

1. .. . ..


32

Diatomite


Calcination method

1. Re-heat laboratory muffle furnace to llOO°C.

2. Weigh out approximately 100 g of powdered diatomite and record weight to nearest 0.1 g. Transfer sample to a furnace crucible.

3. Place crucible in furnace and calcine sample for 1 hour.

4. After 1 hour, remove crucible from furnace, place on heat resistant surface and allow to cool.

5. After cooling, record weight of sample to nearest 0.1 g.

6. Repeat steps 4-6 of sample preparation method to obtain powdered calcined diatomite.

Caution: Dust masks should be worn throughout both procedures. Protective heat-resistant clothing (lab coat, gloves, face shield) should be worn at all stages of handling material to and from furnace.


33

Diatomite

Appendix 5: Filtration theory

Diatomite porosity (‘total void volume’) has two separate components: (1) the internal porosity of particles, and (2) the external porosity between particles (‘effective porosity’). Permeability can be calculated theoretically from total void volume (et) as follows (Reed and Picksley, 1987):

o = (Ed3 5(6/d)2 (1 -

d = mean particlediameter

Permeability values obtained from equation 1 are far higher than experimental values, but by modifying this equation closer agreement is obtained:

De = 5(6/d$ (I - &e)’

de = effective mean particle diameter

= [(l- Ee)/(l - &dl113

In equation 2, effective permeability (Be) is calculated using only external porosity (E, - ‘effective void volume’). E, can be determined experimentally (6 is calculated using equation 3). The model defined by equations 2-3 suggests that the flowrate of diatomite filter-aids is primarily controlled by their external porosity, and that flow through the internal porosity of the filter-aid is neghgible. Because of the positive correlation between % external porosity and particle-size, filter-aids with a coarse particle size distribution should have higher permeability values than those with a f i e particle-size distribution. This is confirmed by experimental findings.

Mean hydraulic radius (m) can be calculated from effective void volume and effective permeability using the following equation:


34 1 . . .

Diatomite

Appendix 6: Measurement of permeability and wet cake density

This test is used to assess the filtration performance of calcined diatomite. Samples should be prepared using the procedure given in Appendix 4. Permeability and wet cake density values should be compared with data quoted by Theune and Bellet (1988) (see Section 6.2.1).

Apparatus

Balance (0.1 g readability) Vacuum gauge Vacuum regulator (optional) Filter-pump 5-10 m of ‘Tygon’/rubber vacuum tubing l O O m l glass beaker Glass stirring rod Stainless steel ruler ‘Fast’ filter discs (or ‘fast’ filter paper) Stopwatch 11 Nalgene filtration unit (or sintered glass funnel + 11 measuring cylinder)

Method

1.

2.

3.

4.

5.

6.

7.

8 .

Place beaker on balance. Record weight to within 0:l g and tare.

Add 10 g (k 0.1 g) of calcined powdered diatomite to beaker and record weight.

While constantly stirring, add the minimum amount of water for the sample to flow smoothly.

Turn on the filter-pump and place filter disc/paper on perforated septum/sintered glass of filter unit/funnel and moisten.

Stir diatomite slurry and transfer to filter unit/fimnel to form a smooth even cake on surface.

Dry beaker in oven at 55’. After drylng place on balance and record weight of remaining diatomite.

Set vacuum pressure to a constant value (typically 15-20 “Hg) for duration of test, using either a vacuum regulator or an air- bleed valve.

Simultaneously start stopwatch and add water to filter unit/hnnel, and record volume of filtrate at 1 minute intervals between 0- 15 minutes.


35 . ._

Diatomite


9. Stop recording volume of filtrate after 15 minute reading

10. Allow remaining water to drain through apparatus until surface of diatomite cake is dry.

1 1. Record depth of diatomite cake to within 0.5 mm using the steel ruler (average several depth readings if necessary).

Calculations

Permeability

Permeability is calculated using Darcy’s Law (See section 6.1.):

Permeability (f3) [pm2]

= p u L * 1012 AP

p = viscosity of water at 20 OC = 0.0010019 [kg/m/s] U = superficial velocity = (volumetric flowrate [m3/s] / cross-sectional area [m2]) volumetric flowrate [m3/s] = volume of filtrate after 15 minutes [m3/s]/ (time = 900 [SI) cross-sectional area = n; (0.5d)2 [m2] AP = pressure drop across bed (15 inches Hg = 50663 [Nm3]) L = depth of diatomite cake [ml\

Wet-cake density

1. Subtract weight recorded in step 7 from that recorded in step 2. This value equals the dry weight of diatomite used in the test (m).

2 . Wet density of the diatomitew) can the be calculated as follows:

WD [g/cm3]= mass of diatomite / volume of diatomite cake - - m n; (0.5d)21

m = dry weight of diatomite [g] d = diameter of filter [cm] 1 = depth of diatomite cake [cm]


36

Diatomite

Appendix 7: Determination of oil absorption values

This method is used for testing the oil absorption of pigments for paints (adapted from British Standards Institute method BS 3483: Part B7 British Standard method for testing pigments for paints).

Apparatusheagents

Balance (0.1 g readability) Refined linseed oil Ground glass plate (typically 300 mm by 400 mm) Small palette knife with tapered steel blade (typically 140-150 long by 20-25 mm wide) 10 ml capacity burette, 0.1 ml graduations

Method

1.

2.

3.

4.

5.

6.

7.

8.

Weigh out approximately lg of dry powdered sample and record value to nearest 0. lg. Note reading as M = ..... g. Fill burette to zero meniscus with linseed oil.

Transfer sample to glass plate.

Add linseed oil slowly (4-5 drops at a time) from the burette. After each addition rub the oil into the sample using the palette M e . Initially, oil added will be absorbed rapidly.

Continue additions of oil as in step 3 until a stage is reached when ‘conglomerates of oil and sample are formed’, i.e. prolonged mixing is required for absorption of oil.

From this point, continue to add oil one drop at a time and follow each addition by thoroughly rubbing with the palette knife.

Cease the addition of oil when a paste of smooth consistency has formed. This paste should just spread without cracking or crumbling and should only loosely adhere to glass plate.

Read burette to 0.1 ml to record volume of oil added. Note value as V = .... ml. The time taken to complete the operation should be 20-25 minutes. The operator should mix the oil and sample with maximum effort. If necessary, determine the oil absorption value of a known reference sample for comparison.


37

Diatomite


Calculation of oil absorption value

Mass/mass basis:

mass oil absorption per lOOg of sample [g/lOOg] = 93V/M

Volume/mass basis:

volume oil absorption value per lOOg of sample [ml/lOOg] = lOOV/M

Quote results to the nearest ml or g [/lOOg]


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