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CONTENTS Introduction Bond’s Work Index Different Grinding Equipment's Crushing Grinding Bond’s Work Index Different Grinding Equipment's Grinding Circuits Ball Mill Rod Mill & Ball Mill SAG Mill & Ball Mill HPGR & Ball Mill Case Study Results Conclusion References

INTRODUCTION

Stress Forces - 1 courtesy: metso Tensile

Compression

Stress Forces - 3 courtesy: metso Impaction

Stress Forces - 4 courtesy: metso Shearing

BOND’S WORK INDEX W= 10Wi ( 1/√P – 1/√F ) Bond’s Work Index (Wi) is the energy required, expressed as kWh/short ton to size reduce a material of infinitely large size particles to a product of 80% passing 100 microns. Following Equation is used to estimate specific power consumption for scale up purposes. W= 10Wi ( 1/√P – 1/√F )

DIFFERENT GRINDING EQUIPMENTS BALL MILL In general the length of the mill is slightly higher than the diameter though not a rule. The grinding media is spherical . The size dispersion of the product is high.

ROD MILL Length is higher than the diameter as a rule. The grinding media is rods. Mill product is coarse grind. ROD MILL

SEMI AUTOGENEOUS MILLS Diameter is very large compared to length. Grinding media is feed lumps plus 4 - 12 % ball charged. Less sensitive to feed size composition unlike AG mills. SEMI AUTOGENEOUS MILLS

HIGH PRESSURE GRINDING ROLLS (HPGR) Two Rotating Rolls one fixed & other sliding against the fixed roll under pressure. High power saving. Product discharged as cake.

GRINDING CIRCUITS Fig:1 BALL MILL CIRCUIT 1) Ore Bin 2) Ball Mill 3) Mill Discharge sump 4) Mill Discharge Pump to Hydrocyclone 5) Hydrocyclone Fig:1 BALL MILL CIRCUIT

COMBINATION OF ROD & BALL MILL F = feed, 1) Rod Mill 2) Mill Discharge Sump 3) Pump 4) Hydrocyclone 5) Ball Mill Fig: 2 COMBINATION OF ROD & BALL MILL

COMBINATION OF SAG & BALL MILL F = feed, 1) SAG Mill 2) Mill Discharge Sump 3) Pump 4) Hydrocyclone 5) Ball Mill Fig: 3 COMBINATION OF SAG & BALL MILL

Fig:4 COMBINATION OF HPGR & BALL MILL 1) Ore Bin 2) HPGR 3) HPGR Discharge Screen Under Size Sump 4) Pump 5) Hydrocyclone 6) Ball Mill 7) Pump 8) Hydrocyclone Fig:4 COMBINATION OF HPGR & BALL MILL

CASE STUDY- 1 1500 TPD Jharmarkotra Phosphate Concentrator is laid in two streams with two ball mills operating in parallel. Each ball mill is driven by a 600 kW motor (fig:1) giving a throughput of 750 TPD. Energy Consumption in the 1500 TPD plant (Grinding, Floatation, Thickening & Filtration) per ton of the Ore treated was 49 kWh/ton at the best operating conditions. A roller press (HPGR) with drive motors of 700 kW (350+350) was retro fitted into one of the grinding circuit (fig:4) such that the roller press works in tandem with the ball mill. This has resulted into increased throughput of 3000 TPD. The power consumption of 3000 TPD plant (Grinding, Floatation, Thickening & Filtration) reduced to 31 kWh/ton of the Ore treated at best operating conditions.

RESULTS: TABLE: 1 TABLE: 2

Case Study - 2 At Rampur- Agucha lead zinc mines of M/s Hindustan Zinc Ltd, grinding circuits with rod mill – ball mill and SAG mill – ball mill were installed depending on the changing hardness of ore from different mine pits. For harder ores SAG – ball mill combination is preferred.

Power Consumption of Rod/SAG mill Circuits. Historical record suggests that rod mill-ball mill circuit including secondary crusher consumed 22.06 kWh per ton of ore to achieve ground product of d80 at 63 microns. Whereas SAG mill- ball mill including primary crusher consumed 25.75 kWh per ton of ore to achieve ground product of d80 at 63 microns.

Salient features of SAG- Ball Mill circuit Primary crusher feed is 1.2 meters and the product size is -150 mm. It may be noted that entire secondary crushing plant is avoided in SAG- BM circuits. Cost of consumables is 30% higher for SAG – BM circuits compared to RM – BM circuits. Maintenance cost of RM- BM to SAG- BM circuits is 1: 1.1 .

CONCLUSION: It can be seen that the grinding circuit of HPGR – Ball Mill are highly energy efficient(1,3). Attempts are being made(4) to replace ball mill from HPGR – Ball Mill circuits with verti mills or stirred media grinding mills where very fine product sizes are acceptable/desired. RM- BM circuits consume less power, less grinding media/ other consumables and also maintenance cost is less. SAG – BM circuits do not need secondary crushing, can handle ores with varying characteristics and causes less dust pollution.

REFERENCES: Basics in Minerals Processing, Metso, 2012. (a) Sekhar, D M R., Jain, C L., Jhamarkotra Phosphate Ore Processing Plant, International Mineral Processing Congress, Turkey, 2006. (b) HPGR in the Grinding Circuit of Phosphate Concentrator, http://www.xanthatetechnologies.in/resourcesdata.php?catid=Comminution 3 Joachintiarder, A T., Minerals Processing, Volume 53, 07-08/2012. 4 Drozdiak, JA., Klein,B., Nadolski,S.,& Bamber, A., A Pilot Scale Examination of a High Pressure Grinding Roll & Stirred Mills Comminution Circuit., International Autogeneous, Semi Autogeneous Grinding & High Pressure Grinding Roll Technology, Canada, 2011.

REFERENCES 5. Srikath Reddi, J Balasubramanian and Laxman Shektawat, Evaluating the Performance of Different Grinding Circuits at Rampura Agucha Mines, MPT- 2014, Andhra University. http://mineralengineer.wikifoundry.com/page/Size+Reduction