Beneficiation and Mineral Processing of Mineral Sands

Beneficiation and Mineral Processing of Mineral Sands Hassan Z. Harraz [email protected]

Taken from the earth  Given back to the earth 1

OUTLİNE OF LECTURE 5: Examples Mineral processing:  Sand and Silica Sand ➢Processing Sand ➢Sand into Silicon-Silicon carbide  Heavy Mineral Sand ➢ Separation of Heavy Minerals from Black Sand/Sand ➢ Zircon to Zirconium ➢ Ti-Bearing Minerals

Beneficiation and Mineral Processing of Mineral Sands

2

‫منجم كوارتز ‪ -‬منطقة ام هجليج ‪ -‬الشركة المصرية للسبائك الحديدية ‪ -‬طريق ادفو مرسى علم‬

‫‪3‬‬

‫‪Beneficiation and Mineral Processing of Mineral Sands‬‬

6) MINERAL SANDS  Silicon (Si) is the 2nd most abundant element in earth’s crust.  Commonly found in its oxidized form (SiO2).  Sand is a naturally occurring granular material comprised of finely divided rock and mineral particles.  Sand is transported by wind and water and deposited in the form of beaches, dunes, sand spits, sand bars (placer deposits) ….etc.  Sand constituents of sands are silica (SiO2), usually in the form of quartz, and iron oxides.  The term "Mineral Sands" normally refers to concentrations of Heavy Minerals (HM) in an alluvial (old beach or river system) environment. Occasionally these deposits are referred to as "Beach sands". However mineral sands are also found in large Aeolian sand systems or “Dunal sands”.  Mineral sands orebodies essentially fall into two categories based on the mode of deposition: alluvial or aeolian. Alluvial deposits are further split into marine beach placers and lacustrine heavy mineral (HM) accumulations.  The principal Valuable Heavy Minerals (VHM) include ilmenite, leucoxene, rutile and zircon.  Variations of other titanium minerals occur between the end members of ilmenite and rutile, including pseudo rutile and anatase.

Mineral Sand  Mineral sands are the term given to a group of minerals commonly found and mined together from water or wind concentrated deposits. The principal valuable minerals include ilmenite (Fe.TiO3), leucoxene (FeTiO3.TiO2), rutile (TiO2), zircon (ZrSiO4) and monazite (Ce, La, Th, Nd, Y)PO4. (Table 1). In recent times, however, monazite has not been regularly sold as a product and stockpiling or returning to pit void, is common. Smaller volumes of garnet and staurolite are sold as rich products for specialized use. Variations of economically exploitable titanium minerals occur between the end members of ilmenite and rutile, including pseudo rutile and leucoxene, as well as the other polymorphs of TiO2, anatase and (more rarely) brookite.  The components of mineral sands deposits all have high specific gravity (greater than 2.85 g/cm3) and tend to lag or concentrate during storms when lighter components, such as quartz, are carried offshore or along shore by strong littoral drift.  HM accumulation occurs during periods of fair weather beach building and it is this HM that provides the basis for the thicker HM strandlines formed during major storm events.

Table 1: Common Mineral Sands, their physical properties and chemistry

 Mineral sands represent less than one percent of the value of the global resources sector.  The mineral sands industry consists of two principal product streams: ➢Titanium dioxide minerals: in the form of rutile, ilmenite and leucoxene. Ilmenite is also used to manufacture titanium slag and synthetic rutile products; and ➢Zircon.

Table 2: Typical TMF fractions for separating HM species Magnetic SG - 3.85

Mineral Trash

- 3.85 + 4.05

Magnetic Leucoxene

- 3.79 + 4.05

Leucoxene

- 4.05 + 4.38

Altered Ilmenite

- 4.05 + 4.38

Rutile

- 4.38 + 4.9

Primary Ilmenite

- 4.38 + 4.9

Monazite

+ 4.9

+ 4.9

6

Non-Magnetic


SG - 3.79

Mineral Quartz, trash

Zircon

‫محاجر الرمال الصفراء الناعمة‪ -‬جبل البستان ‪ -‬مركز الدلنجات ‪ -‬البحيرة‪ ،‬يبلغ سمك طبقة‬ ‫الرمال من ‪ 15‬الى ‪ 20‬متر‪.‬‬

‫‪7‬‬

‫‪Beneficiation and Mineral‬‬

White Sand, Sinai-Egypt

Beneficiation and Mineral Processing of Mineral Sands2016 © Hassan Harraz

8

Cross-bedding and scour in a fine sandstone, Sinai


9

Outcrops of White SandstonesBeneficiation In New roadand of Mineral ZaafaranaAin El-Sukhna, Gulf of Suez, Egypt Processing of © Hassan Harraz 2016 Mineral Sands

10

Abu Darag White Sandstones Quarry In Zaafarana - Ain El-Sukhna Area, Gulf of Suez, Egypt

Beneficiation and Mineral Processing of Mineral Sands © Hassan Harraz 2016

11

White Sand, Sinai


12


13

INDUSTRIAL SAND APPLICATIONS Industry, Trade

Applications and uses

Glass industry, Fused silica industry

as glass sand for white container and flat glass, crystal glass, lead crystal, optical glass, special glass, technical glass, borosilicate glass, glass wool and fused silica bricks and tools

Foundry industry

foundry sand for molds and cores for metal castings

Construction materials and concrete as raw material for premixed dry mortar, polymer cement concrete, textured plaster, sandy industry limestone, tiles, bricks, artificial cement stones, design and industrial floors etc. Water purification

as filter sands and gravels according to EN 12904 : 1999

Blasting abrasives and substitutes

for mechanical surface treatment

Ceramic industry, Extenders

for cement and resin bound masss, ceramic mass, filler, paints etc.

Iron processing and refractory industry

as raw material for filler sands (sliding bricks, silica ramming mixtures, repair systems etc.

Chemical industry

as raw material for sodium silicate (water glass) and SiC

Electronics industry

as fuse sands

gate

fillers),

fire-proof

silica

Domestic animals hygiene, Aquaristic as bird sand, parrot sand, chinchilla sand, terrarium sand and coloured quartz equipment Leisure and sports facilities

as special sand for playgrounds, riding-grounds, golf-courses, beach volleyball grounds, athletic grounds, artificial lawn etc.

Sanding and Spreading

as braking sand for trains and trams, as winter sand for snow and ice control (straight or mixed with salt) Beneficiation and Mineral Processing of Mineral Sands

14

Sand

15


Silica sand


16

World resources of Silica Sand • Silica sand resources is abundant on the world. • Its extraction is limited by ✓ geographic distribution ✓ quality requirements for some uses ✓ environmental restrictions • Extraction of theses resources is dependent on whether it is economic and are controlled by the location of population centers

http://minerals.er.usgs.gov/minerals/pubs/commodity/silica/780397.pdf


17


18

Purposes for the Utilize of Silica Sand Glass making, Foundry casting, specialist building applications, leisure ( e.g. golf course), filters in numerous products, Ceramics, Filtration, Plastics, the manufacture of chemicals, metal & Refractory, as additives in horticultural, & agricultural products & simulating oil production

19


Different Grade of Silica Sand

Determination of indicators of grain size, shape and grading distribution commonly used for descriptions of silica sand

Different grade of Silica sand 20


6.1) Processing Silica Sand What the analysed alternatives have in common is the mining and basic processing operation. The main assumptions for the first seven alternatives were as follows: surface mining using bulldozers; transportation from a mining site to the processing plant by dumpers; then washing and sizing in the processing plant which included sieve washing, attrition scrubbing, hydro-cyclone classifying and dewatering. The glass-grade silica sand production corresponded to alternatives, named as follows: 1) Basic processing by washing and sizing : i) Basic/wet; and ii) Basic/dry; 2) Electrostatic/dry separation; 3) Gravity concentration i) Gravity /wet; and ii) Gravity /dry; 4) Flotation concentration : i) Flotation /wet; and ii) Flotation /dry; (note: Flotation was abandoned because of plant destruction Environment)


21


22

Raw Material Black Sands / Heavy Mineral Sand

Zircon (ZrSiO4, SG. 4.6 -4.7) Quartz (SiO2, S.G. 2.6) Rutile (TiO2, S.G. 4.2 – 4.5) Ilmenite (FeTiO3, S.G. 4.7 – 4.8) Monazite ({(Ce,La,Th)PO4}, S.G. 5.0 -5.3)

Silica Sand/ Quartz

Ilmenite (Monazite) Rutile

Gravity Separation

Electromagnetic Separation

Electrostatic Separation Zircon

23


a) Screening sand; b) Classified and Grading; sizing, using hydrocyclones, attrition scrubbing, ……etc c ) Washing Machines; d) Chemical/acid leaching to remove Fe minerals and stained quartz

▪ One way to get powder of desired composition: from minerals, simple physical separation, + chemical purification to get products. ▪ Purity: often not very high; used in conventional ceramic industry.

Raw Black (Heavy) Sand Water Water

Scrubbing

General Flow chart for Beneficiation of Black Sand

Screening, 600 m

Grit (>+600 m)

-600 m

Water

Water

Attrition Glass Grade Sand (Grade 1) -600 +300 m

Screening, 300 m -300 m

Water

Screening, 125 m Magnetic Separation

Impurities Iron Impurities

Silica Sand

Heavy Minerals

Spiral Classifier Dewatering + Hydrocyclones

Zircon Rutile Ilmenite Monazite

Water

Silica Sand Product (for Glass making/ Special Grade) 24


Electromagnetic Separation

Electrostatic Separation

Ilmenite (Monazite)

Zircon Rutile

(a)

(b)

6.2) Processing Sand The glass sand requires 9 processing as follows:

Stripping and extracting Silica sand

Transportation system; (d)

(c)

Screening silica sand

Classified and grading (f)

(e)

Silica sand washing Machines

(a) Stripping and extracting Silica sand; (b)Transportation system; (c )Screening silica sand; (d) Classified and Grading; sizing, using hydrocyclones, attrition scrubbing, ……etc (e ) Washing Machines; (f) Chemical/acid leaching to remove Fe minerals and stained quartz (g) Gravity (spirals, shaking tables, Reichert cones), Spiralling and/or tabling to remove heavy minerals. (h) Magnetic (low/high intensity dry/wet) and high tension separation methods (i) Dewatering.

Spirals for removing heavy minerals

(g)

Magnetic Separation


25

http://www.outokumpu.com http://www.tiomin.com/

Zircon (ZrSiO4, SG.4.6 -4.7) Quartz (SiO2, S.G. 2.6) Rutile (TiO2, S.G. 4.2 – 4.5) Ilmenite (FeTiO3, S.G. 4.7 – 4.8) Monazite ({(Ce,La,Th)PO4}, S.G. 5.0 -5.3) Beneficiation and Mineral Processing of Mineral Sands

Mineral sand slurry moving down the spiral gravity separator with increased concentration of the heavy minerals in the center of the spiral.

26

Zirconium Sponge

27



28

92%...0.125-0.5 mm

Typical grading curves for

processed glass sand


moulding sand

29

Grading curves of filtration sands

Grading envelope for building sand and silica sand


30

Recommended grading limits for

sand used for golf and bowling greens (STRI)

sports turf sand (STRI)


31

Specification of silica sand raw material Chemistry

32


Heavy minerals

Every sand deposit has small amounts heavy minerals (minerals that are more than 2.8 times as heavy as an equal volume of water). Many heavy minerals are resistant to physical weathering and erosion, can therefore withstand sediment reworking.

Some heavy minerals also have variable properties that can accurately indicate their ultimate source. In the lab, heavy minerals are separated from light minerals in heavy liquids and Beneficiation and Mineral Processing of 33 mounted on glass slides for examination. Mineral Sands

The ratio of light to heavy minerals was determined and the heavy ones examined more closely. 300 heavy mineral grains were counted.

Heavy minerals included hornblende, biotite, zircon, epidote and opaque minerals such as one would expect in potting soil from weathered granite. 34

Example of diversity of heavy mineral grains in sand


6.3) SAND INTO SILICONSILICON CARBIDE (Si-SiC) AND SILICONE What makes silicon unique and its chemistry the choice of a wide range of environmentally friendly industrial applications ?  High natural abundance and easy availability of starting material (Sand) (silicon is the second most abundant element on the earth)

 An easy purification process to pure silicon.  A simple and cost efficient method for synthesis of organochlorosilanes and their polymerization to silicone polymers.  Highly environmentally friendly end products with a diverse range of proven applications.

Applications in steel refining & Semiconductor industry Inorganic polymer industry based on Silicones Industry based on piezo electricity of quartz Industry based on silicates: from bricks, glass, cement to crockery


35

Unique Properties of Silicon  Silicon mostly forms tetravalent compounds and to a minor extent divalent compounds  Silicon forms non-toxic organo derivatives and therefore finds use in a wide range of applications  Si-O bond is one of the strongest bonds based on silicon and silicone polymers having Si-O back bone are the most widely used inorganic polymers  Silicon forms multiple bonds very rarely and to stabilize such bonds sterically bulky groups are required  Silicon is a semiconductor and finds use in solar cell fabrication and electronics  Unlike carbon, silicon does not form stable double bonds with oxygen ( till 2014).  Quartz (silicon dioxide) shows piezo electric properties.  Silicon carbide has been the first LED and is still used Beneficiation and Mineral Processing of Mineral Sands

36

Gorilla glass


37

Silicon: Properties and Uses PROPERTY Durability and low cost of the material High sublimation temperature of SiC High thermal conductivity, and high maximum current density High voltage-dependent resistance

USE Grinding, honing, water-jet cutting Bearings and furnace parts. Semiconductor material in electronics. Lightning arresters


38

About the element  Silicon is a grey colored brittle solid having a melting point of 1414oC and boiling point of 3265oC.  It is a semiconductor and has a negative temperature coefficient of resistance as the number of charge carriers increases with temperature.  Naturally occurring silicon is composed of three stable isotopes, silicon-28, silicon-29, and silicon-30, with silicon-28 being the most abundant (92% natural abundance)

39


6.3.1) Isolation and Purification of the Silicon  The first step in making pure silicon is reduction of silica to an impure form of silicon known as ferrosilicon.  Ferrosilicon is an iron-silicon alloy that contains varying ratios of elemental silicon and iron.

>2200oC

SiO2 (Sand) + 2C (Coke) →

Si(Fe) + CO2 (g)

(Ferrosilicon or Metallurgical-grade Si)  Ferrosilicon accounts for ~ 80% of the world's production of elemental silicon. Silicon has a high propensity to form bonds with oxygen and Ferrosilicon is primarily used by the steel industry to remove dissolved oxygen in steel melt.

Ultrapure silicon for solar cells and electronics  For solar cell fabrication one requires silicon of much higher purity (+99.9%).  For electronics grade the purity required is even higher (99.9999999% ; known as 9 nines).  Although molten salt electrolysis of SiO2 or zone refining of metallurgy grade silicon can be carried out for purification a more well known method and cost efficient is to convert the ferrosilicon to SiCl4 or HSiCl3. These two relatively low boiling liquids can be purified to a very high level of purity by repeated distillation: Si(Fe) + Cl2 → SiCl4 (Liquid B.P.80oC) + FeCl3 (Solid)  Trichlorosilane is produced by treating powdered ferrosilicon with blowing hydrogen chloride at 300°C Si(Fe) + 3HCl (300oC) → HSiCl3 (Trichlorosilane: Liquid B.P. 31.8 C) + FeCl3 (Solid) + H2  A convenient method to make ultrapure silicon has been the “Siemens Process” in which highly pure silicon rods are exposed at high temperatures to trichlorosilane. Polycrystalline silicon gets deposited on the silicon rod. o

High Purity Si -Rod

2HSiCl3

1150oC

→

Si + 3HCl + SiCl4


40

Flow Diagram for obtaining Isolation and Purification of the Silicon

3 2

1

Hydroclorination of MGS: Si(Fe) + 3HCl (300oC) → HSiCl3 + FeCl3 (Solid) + H2

5

Distillation Si(Fe) + Cl2 → SiCl4 (Liquid B.P.80oC) + FeCl3 (Solid)

4

2HSiCl3 (1150oC) → Si (High Purity Si -Rod) + 3HCl + SiCl4

HCl and H2 Recovery

SiO2 (Sand) + 2C (Coke) {>2200oC} → Si(Fe) + CO2 (g)

Vaporization and CVD of HSiCl3


41

Silicon tetrachloride: A useful precursor


42

6.3.2) Fumed Silica (Pyrogenic Silica) from SiCl4 : A Useful Filler

Fumed silica is made mostly from flame pyrolysis of silicon tetrachloride

Fumed silica serves as a universal thickening agent and an anticaking agent (free-flow agent) in powders. Like silica gel, it serves as a desiccant. ➢It is used in cosmetics for its light-diffusing properties. ➢It is used as a light abrasive, in products like toothpaste. ➢Other uses include filler in silicone elastomer and viscosity adjustment in paints, coatings, printing inks, adhesives and unsaturated polyester resins. Beneficiation and Mineral Processing of Mineral Sands

43

Synthesis of Organosilanes The Direct synthesis: Rochow-Mueller Process (1940) The economically viable alternative for Grignard method: Modern synthesis of organochlorosilanes


44

Types of silicone polymers


45

Industrial Uses of Silicones

First human foot print on moon made with silicone soled shoes Moon temperature (-153 to +121°C)


46


47

SiC is also known as Carborundum. SiC is known under trade names Carborundum, Crystalon, and Carbolon, including black and green silicon carbide both with a shape of hex crystal. The black silicon carbide is classified into coke-made and coal-made black silicon carbide depending on different raw materials. The material is extremely hard and sharp, with excellent chemical properties. The hardness is between Diamond and Fused Alumina, but the mechanism hardness is higher than Fused Alumina. The micro hardness is in the range of 2840-3320kg/mm². Its , hardness is 13 Mohos scale.

Crystal Structure of Silicon Carbide

Structure and properties Polymorphs of silicon carbide SiC exists in ~250 crystalline forms. The polymorphism of SiC include various amorphous phases observed in thin films and fibers, as well as a large family of similar crystalline structures called Polytypes. They are variations of the same chemical compound that are identical in two dimensions and differ in the third. Thus, they can be viewed as layers stacked in a certain sequence. α-SiC is the most commonly encountered polymorph; it is formed at temperatures greater than 1700oC and has a hexagonal crystal structure (similar to Wurtzite). β-SiC, with a zinc blende crystal structure (similar to diamond), is formed at temperatures below 1700oC. Until recently, the beta form has had relatively few commercial uses, although there is now increasing interest in its use as a support for heterogeneous catalysts, owing to its higher surface area compared to the alpha form. Pure SiC is colorless. The brown to black color of industrial product results from iron impurities. The rainbow-like luster of the crystals is caused by a passivation layer of silicon dioxide that forms on the surface. The high sublimation temperature of SiC (approximately 2700oC) makes it useful for bearings and furnace parts. SiC does not melt at any known pressure. It is also highly inert chemically. There is currently much interest in its use as a semiconductor material in electronics, where its high thermal conductivity, high electric field breakdown strength and high maximum current density make it more promising than silicon for high-powered devices. SiC also has a very low coefficient of thermal expansion and experiences no phase transitions that would cause discontinuities in thermal expansion.

49


α-SiC 6H-SiC

Si atom C atom β-SiC α-SiC 2H-SiC

α-SiC 4H-SiC

3C-SiC

For example the α-SiC can also be called 2H-, 4H- or 6H-SiC, depending on the unit cell, while β-SiC can also be called 3C-SiC because of the ABC stacking ... Beneficiation and Mineral Processing of Mineral Sands

50

Crystal Structure of Silicon Carbide

51


SiC Properties SiC is sharp but fragile with good heat-resistance, heat-conductibility, can be antacid and antalkali, and lower dilatability.

SiC has:  high hardness  high thermal consistency Moissanite ring natural light  very good resistance at high temperatures  low thermal expansion  electrical conductivity  is a semiconductor  non linear electrical resistance Substantial space heritage exists: ▪Space science applications ▪Military applications ▪Structures and reflecting optics  Si and C as alloying additive - Silicon Carbide dissociates in molten iron and the silicon reacts with the metal oxides in the melt. This reaction is of use in the metallurgy of iron and steel.

Beneficiation and Mineral Processing of

52


53

Benefits of SiC as a de-oxidizing agent  Silicon and Carbon are released from SiC as charged atoms.  Carbon works as a de-oxidiser removing free oxygen and reducing unstable oxides (e.g. FeO and MnO), typically: SiC + FeO → Si + Fe + CO(g)

 Removing these elements to the slag and increasing the life of furnace linings


54

SiC Production Process a) Raw Materials Preparation

Furnace mixes are calculated to obtain a specified SiO2 /C ratio, usually ~1.70. A furnace mix prepared to strict stoichiometry for the overall reaction will state tha150 t of SiO2 are to be mixed with 90t of carbon (e.g. 50% silica, 40% coke, 7% sawdust, and 3% salt) to produce 100 t of SiC and 140t t of CO.


55

b) Manufacturing Processes for SiC  Silicon carbide is manufactured industrially by the electrochemical reaction of high purity silica or quartz sand with carbon in an electric resistance furnace (Acheson process): >2200oC SiO2 + 3C → SiC + 2CO(gas) The process is an endothermic reaction requiring between 8,000 – 10,000kWh per tonne of product.  Acheson process to produce α-SiC by reacting of ~50% high-purity silica sand or quartz, 40% finely ground low-sulfur coke, 7% sawdust, and 3% salt (Sodium Chloride: NaCl) in a resistance electric arc furnace for 36 hours at 2200-2400oC .  Preferred carbons are petroleum coke (pitch coke) and anthracite.  Addition of (3%) Sodium Chloride (NaCl) ensures the removal of trouble some impurities as volatile chlorides. The presence of Sawdust (7%) increases the porosity of the reaction mixture and eases outgassing. The process gives off carbon monoxide (CO).  There are four chemical reactions in the process that produces silicon carbide (SiC) at 2000-3000oC: C + SiO2 → SiO(g) + CO(g) SiO2 + CO (g) → SiO + CO2 (g) C + CO2 (g) → 2CO(g) 2C + SiO → α-SiC + CO(g)  The first light emitting diodes were produced using silicon carbide from the Acheson process. Beneficiation and Mineral Processing of

56

57


SiC Furnace Recovery


58

 The electric furnaces, in which this reaction is carried out, are ~1533 m3 in size and are lined with refractory material. Electrodes at opposite ends are connected to a graphite core. The furnace is filled round this core with the reaction mixture and electrically heated to 2200 to 2400°C. The heating up time is ~18 h and the reaction time a further ~18 h. After cooling, the sides of the furnaces are removed and the unreacted material on the edges removed.  The material formed in the Acheson furnace varies in purity, according to its distance from the graphite resistor heat source. Colorless, pale yellow and green crystals have the highest purity and are found closest to the resistor. The color changes to blue and black at greater distance from the resistor, and these darker crystals are less pure.  The SiC, which has formed round the graphite core, is broken up and separated into different qualities:  The purest SiC is bright green (99.8% SiC), the color changing with decreasing SiC-content from dark green (99.5% SiC) to black (99% SiC) to gray (90% SiC). Beneficiation and Mineral Processing of Mineral Sands

59

Inside a SiC Furnace

30-40% SiC 85% SiC Cross-sectional view of a typical cylinder, showing the interior cavity in which graphite is formed, the pure silicon carbide body and the less pure material in the reactant zone.

Less Pure Silicon Carbide

97%+ SiC

Pure Silicon Carbide Beneficiation and Mineral Processing of Mineral Sands

60

 70 t of raw mixture yields 8 to 14 t of high grade SiC and 6 to 12 kWh of energy are required to produce 1 kg of raw SiC.  The raw SiC is processed by crushing in jaw crushers or hammer mills and subsequent fine grinding in ball mills.  Very pure SiC qualities are obtained by chemical treatment with sulfuric acid, sodium hydroxide or hydrofluoric acid.  SiC is remarkable for its unusually large variety of different morphologies, which differ in their stacking sequences of hexagonal and rhombohedral layers. All hexagonal and rhombohedral forms are often simply described as α-SiC. The commercially available SiC produced by the Acheson process is α-SiC.  The manufacture of cubic β-SiC, which is favored at temperatures below 2000°C, or mixtures of α- and β-SiC is carried out by deposition from the gas phase (Chemical Vapor Deposition (CVD)).  β-SiC powder with good sintering properties and small crystallite size is {e.g. obtained by the thermal decomposition of alkyl silanes or alkyl dichlorosilanes in plasmas or flow reactors at temperatures above 1000°C: CH3SiCl3 (>1000oC) → β-SiC +3HCl  In the manufacture of synthetic graphite, the Acheson process is run for approximately 20 hours, with currents of 200 A, and voltages of 40,000–50,000 V (8–10 MW). The purity of graphite achievable using the process is 99.5%. Beneficiation and Mineral Processing of Mineral Sands

61

Lely process  To make Pure SiC crystal, in which SiC powder is sublimated into hightemperature species of silicon, carbon, silicon dicarbide (SiC2), and disilicon carbide (Si2C) in an argon gas ambient at 2500oC and redeposited into flake-like single crystals, sized up to 2×2 cm, at a slightly colder substrate. This process yields high-quality single crystals, mostly of 6H-SiC phase (because of high growth temperature). 3SiC (Argon (g), 2500oC) → Si2C + SiC2

A modified Lely process involving induction heating in graphite crucibles yields even larger single crystals of 10 cm in diameter.  Cubic β-SiC is usually grown by the more expensive process of chemical vapor deposition (CVD). Homoepitaxial and Heteroepitaxial SiC layers can be grown employing both gas and liquid phase approaches.


62

6.4) ZIRCON TO ZIRCONIUM 6.4.1) Zircon Containing Raw Materials ▪ The principal economic source of zirconium (Zr) is the zirconium silicate mineral, zircon (ZrSiO4). ▪ Zircon is also the primary source of all hafnium (Hf). ▪ Zirconium and hafnium are contained in zircon at a ratio of about 50 to 1. ▪ Zircon is a coproduct or byproduct of the mining and processing of heavy-mineral sands

Murray Bay heavy mineral concentrate dominated by zircon (colorless, rounded grains) and rutile (deep yellow grains) (Image length 20mm)

Hawks Nest, a beach sand deposit within Murray Bay (Australia) operated by Mineral Deposits Ltd. They extract 21,147 t of concentrate from about 20’000’000 t of sand. This deposit has a very low grade (0.2 -0.3wt% heavy minerals), but large reserves. The sand is extracted by a dredge. http://www.mineraldeposits.com.au/ Beneficiation and Mineral Processing of Mineral Sands

63

6.4.2) Zirconium Extraction • •

The starting raw materials for the production of zirconia are the minerals Zircon (ZrSiO4) and, to a lesser extent, Baddeleyite (β- ZrO2). To extract zirconium (Zr), we first Chemically extraction of zirconia (ZrO2) from zircon (ZrSiO4) ore.

i) Chemical extraction of zirconia (ZrO2) from zircon (ZrSiO4) ore  There are two methods to make zirconia: thermal decomposition and precipitation: ❖ Thermal Decomposition Method: high temperature melting and decomposition (use arc furnace or plasma arc) >1750oC;  quench  use acid to dissolve ZrO2 or alkaline for SiO2 ❖ Precipitation Method: ▪ Zirconia (ZrO2) of 99% purity is obtained by the caustic fusion of zircon (ZrSiO4). ▪ zircon + NaOH  high temperature reaction  Na2ZrO3 + Na2SiO3  + water  filtration to remove Na2SiO3.nH2O  crude sodium zirconate  + HCl  filtration to remove SiO2 colloids  get ZrOCl2 – HCl solution  evaporative concentration  crystallization  filtration to remove impurities (Fe, Ti, Na, Al, HCl …etc)  get ZrOCl2.8H2O  repeat and secondary crystallization  high purity ZrOCl2.8H2O  calcination  zirconia (ZrO2)…{can be represented by the flow diagram: Fig 1}. Beneficiation and Mineral Processing of Mineral Sands

64

Zircon sand (zirconium silicate ZrSiO4) ZrSiO4 + 4NaOH (Fusion) → Na2ZrO3 + Na2SiO3 + 2H2O

fusion with sodium hydroxide (NaOH) at 870 K

Sodium silicate (Na2SiO3) and sodium zirconate (Na2ZrO3) Na2ZrO3 + 3H2O→ Zr(OH)4 + 2NaOH

washing with water

Zirconium hydroxide, Zr(OH)4, formed by hydrolysis. Sodium silicate removed Zr(OH)4 + 2HCl → ZrOCl2 + 3H2O

dissolution in hydrochloric acid (HCl)

Zirconyl chloride, ZrOCl2 precipitation of pure intermediates and reactions with amonia (NH4OH) or sodium hydroxide (NaOH) or sodium carbonate(Na2CO3) ZrOCl2.+ 4NaOH = Na2O +2NaCl + Zr(OH)4

Zirconium hydroxide, Zr(OH)4 Calcination Zirconia Powder (ZrO2)

ZrOCl2 + 2Na2CO3 = Na2O +2NaCl + Zr(CO3)2

Zirconium carbonate, Zr(CO3)2

Solutions of zirconium compounds Calcination Zirconia Powder (ZrO2)

Figure 1 : shows Flow Diagram for obtaining zirconia from zircon Beneficiation and Mineral Processing of Mineral Sands

65

ii) Chemical extraction of zirconium (Zr) from zirconia (ZrO2) ▪ The Kroll method is used for zirconium and involves the action of

chlorine and carbon . ▪ The resultant zirconium tetrachloride (ZrCl4) is separated from the iron trichloride (FeCl3), by fractional distillation. ▪ Finally, zirconium tetrachloride (ZrCl4) is reduced to metallic zirconium

by reduction with magnesium (Mg). ▪ Air is excluded so as to prevent contamination of the product with oxygen or nitrogen. ZrO2 + 2Cl2 + 2C (900°C) ZrCl4 + 2Mg (1100°C)

 ZrCl

4

+ 2CO

2

+ Zr

 2MgCl

▪ Excess magnesium and magnesium dichloride is removed from the product by treatment with water and hydrochloric acid to leave a zirconium "sponge". ▪ This can be melted under helium by electrical heating. Beneficiation and Mineral Processing of Mineral Sands

66

6.5) Titanium Dioxide 

Titanium dioxide minerals are used mainly as feedstock for the world’s titanium dioxide (TiO2) pigment industry. As a pure white, highly refractive and ultraviolet light absorbing product, titanium dioxide pigment is commonly used in architectural and automotive paints, plastics, paper, textiles and inks. Titanium dioxide feedstock is also used in the manufacture of welding electrodes. Titanium minerals are non-toxic, non-fibrogenic and biologically inert and they can be used safely in foodstuffs, pharmaceuticals and cosmetics (Iluka, 2008 and TZMI, 2008).



Titanium feedstocks supply different markets however world demand for ilmenite, leucoxene and rutile is determined by the demand for titanium oxide pigment. The pure white pigment is used as an opacifier in paints, plastics and paper, accounting for around 93 per cent of global titanium feedstock consumption (Figure 1).



Rutile, synthetic rutile and titanium slag can be used to produce titanium metal. Due to the combination of strength and lightness of titanium metal, it is used for advanced engineering applications, including architectural coatings, the aerospace and defence industries as well as a range of other applications, including sporting equipment and jewellery. Titanium metal is also used in desalination plants and corrosive chemical industries and its non-reactive properties make titanium metal one of the few materials that can be used in the human body for hip replacements and heart pacemakers.

Figure 1: Breakdown of titanium dioxide feedstock markets by end-use sector, 2006 (source TZMI and Iluka). 67


6.5) Titanium Dioxide Titanium dioxide (Titania (TiO2)) pigment is produced from Ti-bearing ore (i.e., ilmenite FeTiO3) by two alternative process routes: the chloride and sulphate processes.

i) Sulfate process: ▪In the sulfate process Ti-bearing ore (i.e., ilmenite FeTiO3) is treat with sulfuric acid at 150-180°C to from the soluble titanyl sulfate TiOSO4 FeTiO3+ 2H2SO4+ 5H2O→ FeSO4.7H2O +TiOSO4 ▪After removing undissolved solids and then the iron sulfate precipitate the titanyl sulfate is hydrolyzed at 90°C to precipitate the hydroxide TiO(OH)2 TiOSO4 + 2H2O → TiO(OH)2 + H2 ▪The titanyl hydroxide is calcined at about 1000oC to produce titania (TiO2). ii) Chloride process: ▪ In the chloride process, a high-grade titania (Ti-bearing) ore is chlorinate in the presence of Coke (carbon) at 900-1000°C and the chloride TiCl4 formed is subsequently oxidized to TiO2, as following: 2FeTiO3 + 3C + 6Cl2 → 2TiCl4 + 2FeCl2 + 3CO2 TiCl4 + 2O → TiO2 + 2Cl2↑


68

69


Chlorination: ▪ React Ti-bearing ore with chlorine (Cl2) and coke (carbon) at 900-1000°C to form TiCl4 and metal chloride: 2FeTiO3 + 3C + 6Cl2 → 2TiCl4 + 2FeCl2 + 3CO2↑ ▪ Separate TiCl4 from metal chlorides and purify TiCl4. Oxidation ▪ Convert TiCl4 to TiO2 particles by reacting with oxygen:. TiCl4 + 2O → TiO2 + 2Cl2↑ ▪ Recycle chlorine back to chlorination Finishing ▪ Coat TiO2 particles with additives to provide desired properties based on market application. ▪ Mill TiO2 to desirable particle size using steam.

Figure 1 : Flow Chart of Titanium Oxide Process


70

Waste Treatment

Ti + NaCl Ti NaCl

Vapour

Ti-bearing ore Chlorine (Cl2) Coke ( C)

Heat

Chlorination

TiCl4

Filter Ti

Na

Chlorine (Cl2)

NaCl

Liquid Sodium

Wash

Na

NaCl (aq)

NaCl (s)

electrolysis

Dry

Water

Ti NaCl (aq)

Filter

Ti Figure 2 : Flow Chart of Titanium metal Process


71

Follow me on Social Media

http://facebook.com/hzharraz

https://www.linkedin.com/in/hassan-harraz-3172b235

http://www.slideshare.net/hzharraz

72 Beneficiation and Mineral