Pelletisation of ferromanganese ore with particle sizes less than 4mm – an introduction TC Kruger and JD Steenkamp

Introduction  Ferromanganese is produced in a SAF  SAF requires a permeable burden  Pellets are preferred  higher porosity, uniform size and uniform shape  Up to 30 per cent of ore produced is smaller than 3mm (fines)  Large fines dumps are common both at mines and at smelter plants  One process to utilise fines is pelletisation

 This paper gives an overview of:  the factors relevant to pelletisation in general  the pelletisation of manganese ore fines specifically  reports on initial pelletisation test work conducted on -4mm manganese ore fines

Principles  Wetting and nucleation  a binder liquid is added to the feed  individual wet particles ‘stick together’ to form nuclei

Principles  Consolidation and growth  nuclei are joined by more fines through collision and further ‘sticking together’ to form a micro pellet  two micro pellets - surrounded by liquid films - are brought into contact with each other  A seed pellet is formed that capture dry and wet particles until the desired pellet size is obtained  Pellet porosity decreases due to the continual impact

Principles  Breakage and attrition  Breakage occurs when:  Equilibrium size is reached, and  Binding force can no longer maintain the load.  The surface of the pellet is smoothened by attrition of the sharp edges

Pellet growth  Growth pellets is controlled by two properties:  plasticity of the green pellet and  the viscosity of the superficial water layer  Plasticity is controlled by moisture content  The minimum plasticity defines the minimum moisture content required  The minimum moisture content value is material specific

Pellet growth  Binder liquid is ‘squeezed’ to the pellet surface  The viscosity of the binder liquid influences the rate  The viscosity has to be low enough for colliding pellets to combine within the time available during collision (uncontrolled growth if too low)  Viscosity of binder liquid is influenced by:  Binder dosage;  Temperature;  Material properties of the binder  Process parameters

Processes  Pressure  Briquetting, compaction, tableting  Tumbling  Drum, disc, cone and pin agglomerator  Extrusion  Screw and gear pelletiser as well as pellet mills  Thermal  Sintering, prilling, pastillating and flaking processes

Binders  Binders accomplish two important functions in pelletisation, namely:  Makes the moist ore plastic; and  During drying and sintering, the binder holds the particles in the pellets together

Binders  Types of binders  Bentonite (0.25 to 2.5 per cent by mass)  Cement (5 per cent by mass)  Lime (5 per cent by mass)  Cane molasses (3 per cent by mass)  Calcium chloride  Silicate or fluorosilicate of sodium

Characterisation  Pellet size distribution  Pellet shape  Pellet hardness  Pellet solubility in a liquid i.e. slag  Pellet dispersability in a liquid i.e. slag  Binder addition requirements  Pellet impact strength  Pellet abrasion strength  Pellet attrition index  Pellet compression strength  Pellet reducibility  Pellet porosity

Impact strength (drop strength)  Represents its ability to survive multiple drops in material handling systems  Is determined by repeatedly dropping a pellet onto an iron surface from a fixed height until the pellet fractures or chips  Is quantified as the number of drops that a pellet survived before fracture.  A typical value for green pellets is between 5 and 20 drops

Compression strength (crushing strength)  The compression strength of a pellet represents its ability to resist compressive forces without breaking  Is determined by placing pellets between two steel plates and evenly applying a measured pressure until the pellet fractures  The compression strength is expressed as the applied pressure in Newton or kilogram per pellet  A typical value for green pellets is between 0.5 and 5 kg per pellet

Equipment  Disc pelletiser  Drum pelletiser  Extruder  Pin agglomerator  Briquette making machines  Sintering machines  High-intensity mixers

Case studies  Mexico  Purpose of Study  Material Pelletised  Particle Sizes  Equipment / Process  Binder and quantity  Moisture and quantity  Curing / Drying  Firing  Testing  Drop Tests  Cold crushing strength  Tumble index

Case studies  Brazil (INCOMI)  Purpose of Study  Material Pelletised  Particle Sizes  Equipment / Process  Binder and quantity  Moisture and quantity  Curing / Drying  Firing  Testing  Drop Tests  Cold crushing strength  Tumble index

Case studies  Brazil (University of Sao Paulo)  Purpose of Study  Material Pelletised  Particle Sizes  Equipment / Process  Binder and quantity  Moisture and quantity  Curing / Drying  Firing  Testing  Drop Tests  Cold crushing strength  Tumble index

Case studies  India (Visvesvaraya regional college of Engineering)  Purpose of Study  Material Pelletised  Particle Sizes  Equipment / Process  Binder and quantity  Moisture and quantity  Curing / Drying  Firing  Testing  Drop Tests  Cold crushing strength  Tumble index

Case studies - comments Mexico 2 Brazil (INCOMI)3Brazil (University of Sao Paulo) 4 India (Visvesvaraya regional college of Engineering) 5 CommentsWider size distribution of feed materials yields higher pellet strength. This was achieved by mixing the ore with off gas dust in a ratio of 2:1.  The first commercial plant in the world to successfully produce pellets from manganese ores.  Bentonite increased the drop test values (to 30) and the dry compression values by 100%. Notes were made of the effect of particle sizes on the results obtained:  In general, the smaller particles performed better than the larger particles;  Larger particles required more water for pelletising; and  Larger particles are more suitable to bentonite but have on average lower strengths.  Increasing the bentonite content improved pellet properties (commercial limit: 1.5 mass per cent).  Slip of material indicated the optimum moisture content.  Pellets opening due to centrifugal forces indicated insufficient moisture content.  Particle size distribution was the dominating factor for good strength. Large contact surfaces (small particle sizes) and capillary forces were required for high wet and dry strength and required less moisture resulting in green pellets with lower porosity and moisture content produced in shorter time intervals.

Trials  Aim  To produce pellets from South African manganese ore fines with sufficient strength to be used in major processing units i.e. in sintering and SAF operations

Experimental design  porosity was used as design control variable  literature was used as reference for aim porosity  less than 30 per cent  controlling pellet porosity by controlling the content of very fine material in the mix resulted in high strength pellets  bentonite as a binder  pellets characterised by measuring their compression strength (aim of 5 kg per pellet) and impact strength (a minimum of 5 drops for a green pellet and 20 drops for a dried pellet)

Samples  Sample 1 consisted of material smaller than 4mm which represented fines screened from ore at the mines prior to transportation and at smelter plants prior to processing.  Sample 2 consisted of material smaller than 1400 microns; and  Sample 3 consisted of material smaller than 250 microns

Porosity  Bulk porosity of the ore was measured using the method of volume displacement Manganese Sample % Porosity Sample 1 (–4mm) Sample 2 (–1.4mm) Sample 3 (–250 µm) 32.2%33.3%30.6%

Binder Where:  b=bentonite in grams  P =aim porosity in ml  SV b =Swelling Volume of bentonite = (ml/2g)23  m= mass of material to be pelletised in grams  Calculated bentonite content of each pelletising mix was thus calculated as:  Sample 1 – 0.53 mass per cent;  Sample 2 – 0.57 mass per cent; and  Sample 3 – 0.55 mass per cent.

Pelletising  5 kilogram sample of each size fraction  Measured bentonite  Placed in the Eirich RV02 high intensity mixer and mixed for 60 seconds  1.2m diameter Radicon disc pelletiser  Angle of 35°and a rotation speed of 75 rpm  Pellet diameter was controlled between 10 and 12.5mm

Testing  Compression strength:  30 balls of each sample  Instron Technologies crushing strength machine, model 1011  Impact strength:  30 balls of each sample  Drop height of 450mm

Compression strength SampleAverageMinMax Std Dev Sample Sample Sample

Impact strength SampleAverageMinMax Std Dev Sample Sample Sample

Conclusion  Larger particles can be pelletised  Characteristics improved with increase in top size  Results show that pellets produced may have adequate strength for sintering processes  Further work is required to produce pellets suitable for SAF operations

Recommendations  Increasing the size range of particles  Increasing the -250 micron material content of pellets in increments;  Expanding the range of binders;  Increasing the range of the quantity of binder added;  Using a single, experienced operator to produce all pellets in the test program;  Characterising pellets by microscopic analyses;  Studying the effect of bulk porosity of pellets on the strength of pellets

Acknowledgements  Mr. L Lourens, Manager, Technology and IP, Exxaro Resources, Alloystream  Mr. A Dippenaar; Kumba Iron Ore, R&D Raw Material Technology  Dr. A-M Bonthuys, Independent Contractor (Editor, Translator, Proof reader, Writer)  Mr B Allison, contractor, Exxaro Resources, Alloystream

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