HPGR in the Grinding Circuit of Phosphate Concentrator

HPGR in the Grinding Circuit of Phosphate Concentrator

DMR Sekhar

Abstract

Reduction of energy consumption at Jhamarkotra Phosphate concentrator of M/s Rajasthan State Mines and Minerals Ltd was achieved in three stages (1) stabilizing the through put as per the norms of the detailed feasibility report of the project to achieve energy consumption to 53 - 56 kWh per ton of ore treated (2) increasing the through put of flotation circuit to double, utilizing in built additional capacity there by reducing energy consumption to 49 – 50.44 kWh per ton of ore treated (3) retrofitting Roller Press in the grinding circuit there by doubling the plant capacity from 1500 TPD to 3000 TPD to achieve weighted average of energy consumption to 33.68 kWh per ton during the period 2003 to 2012.

Introduction

Phosphate concentrator of 1500 TPD was commissioned at Jhamarkotra, Udaipur during the year 1992 to treat ore containing phosphorites (sedimentary crypto crystalline apatite), dolomite as major minerals and silicates (quartz, oligoclase, sillimanite etc.) as minor minerals. The ore contained 13 to 18% P2O5, 10 to 12% MgO and 4 to 6% SiO2. The ore is crushed to 12.5 mm size followed by grinding in ball mills. The ground ore is sent to first stage bulk flotation of phosphorite and dolomite in alkaline pH leaving silica and silicate minerals in the tailing. The bulk concentrate of phosphorite and dolomite is treated in the secondary reverse flotation circuit in acidic pH at 5. Sulfuric acid is used to reduce pH and phosphoric acid is used as depressant to phosphate minerals while floating dolomite.

Conventional crushing and grinding machines such as ball mills, jaw and cone crushers are not energy efficient 1. Size reduction machines known as High Pressure Grinding Rolls (HPGR) or as Roller Press (RP) designed by Prof. K. Schonert are based on the principle of “crushing a mineral particle between other mineral particles instead of grinding elements”. This is achieved by choking feeding HPGR that has two independently driven grinding rolls, a fixed one and the other sliding. The comminution pressure is applied hydraulically through the sliding roll. HPGRs became popular size reduction machines in cement industry and in size reducing diamond ores. The success of HPGR/ Roller Press in hard rock application was very limited 2 till 2003, mainly due to wear problems of the grinding surfaces. Rolls with studded grinding surfaces however overcame the wear problem as the ore forms an autogenous wear layer around the rolls. A roller press supplied by KHD Humboldt Wedag was retrofitted in the wet grinding ball mill circuit of Jhamarkotra plant which became operational 3 since 27th March, 2003. Rene Klimowsky reported 4 the introduction of several HPGRs manufactured by Polysius in hard rock applications since 2006. Joachim Harder 5 in his recent review writes that SAG and ball mills have reached their capacity limits which is not the case with High Pressure Grinding Rolls.

Crushing Circuit

The primary crusher is double toggle jaw crusher of 1500 mm X 1200 mm, driven by 190 kW motor to accept a feed size of 1000 mm, discharging a product size of 200 mm at the rate of 345 mtph. The primary crusher product is fed to a short head cone crusher of 1750 mm driven by a motor of 200 kW, discharging a product size of 60 mm. The tertiary crusher which accepts the feed size of 60 mm is of 1750 mm size, driven by a motor of 200 kW, discharging a product of 12.5 mm. This is conceptual description of the crusher plant.

Grinding Circuits

The detailed feasibility report of the 1500 TPD Jhamarkotra plant envisaged energy consumption @ 55 kWh per ton of ore treated. The actual energy consumption scenario at Jhamarkotra plant was discouraging initially. The two stage flotation and the grinding circuits are the main energy consuming areas of the plant. It took several years to reach the rated capacity of the plant due to mechanical maintenance problems in the mill circuit. The mechanical maintenance problems were fully resolved during the years 1999 – 2000 and 2000 – 01. The extra capacity of the flotation circuit was fully utilized 3 by blowing more air into the flotation cells leading to doubling the capacity of the flotation circuit. This has resulted into the reduced power consumption @ 49 to 50 kWh per ton of ore treated.

1500 TPD capacity of the Jhamarkotra plant was achieved through two ball mills of size 3.3 meters diameter X 4.2 meters length (shell to shell), each driven by a motor of 600 kW which may be noted from figure 1. The Bond’s work index of Jhamarkotra ore is 6.5 kWh to 7 kWh per short ton.

KHD Humboltd Wedag (Germany) tested Jhamarkotra ore in a pilot size roller press with studded lining on the rollers, applying a pressure of 5 N /mm 2. The roller press product after removing 125 microns material was size reduced to -3 mm and Bond’s work index of this material is determined to be 5.73 kWh per short ton. That is there is reduction in the work index of the ore after one pass through the Roller Press. Lab testing also indicated the specific power consumption is 1.9 kWh for RP. Thus RP motor rating for a feed rate of 125 tons per hour and a recirculating load of 175 tons is estimated = (125 + 175) X 1.9 = 570 kW, say 600 kW. That is each roller to be driven by 300 kW motors.

A Roller Press with roll sizes of 120 cm diameter X 63 cm width and each roller driven by a 350 kW were retrofitted into the ball mill grinding circuit such that the roller press operates in tandem with one of the existing ball mills to achieve a through put of 125 metric tons per hour (3000 TPD). May note 350 kW motors were installed instead of 300 kW, considering a design margin of 16.7%. The roller press is fed with 25 mm size ore. The operating pressure of the sliding roller is 135 – 140 bars and rollers rotate at 13.5 revolutions per minute. Schematic material flow diagram is shown in figure 2. The energy consumption of the Jhamarkotra plant during various stages of development may be noted from tables 1 to 3.

The power consumption against plant through put in million tons per annum are also shown graphically in figures 3 to 6.

Table. 1

Sl. No / Year / Through put in million metric tons per annum (Dry) / energy consumption in kWh/metric
ton.
1 / 1994-1995 / 0.195 / 67.73
2 / 1995-1996 / 0.352 / 62.33
3 / 1996-1997 / 0.255 / 66.07
4 / 1997-1998 / 0.292 / 64.84
5 / 1998-1999 / 0.453 / 53.72
6 / 1999-2000 / 0.411 / 56.08

Table. 2

Sl. No. / Year / Through put in million metric tons per annum (Dry) / energy consumption in kWh/metric
ton.
1 / 2000-2001 / 0.425 / 49.09
2 / 2001-2002 / 0.381 / 50.44
3 / 2002-2003 / 0.413 / 49.20

Fig. 3

Table. 3

Sl. No. / Year / Through put in million metric tons per annum (Dry) / energy consumption in kWh/metric
ton.
1 / 2003-2004 / 0.645 / 37.34
2 / 2004-2005 / 0.639 / 36.24
3 / 2005-2006 / 0.842 / 31.00
4 / 2006-2007 / 0.807 / 31.54
5 / 2007-2008 / 0.776 / 31.25
6 / 2008-2009 / 0.776 / 30.73
7 / 2009-2010 / 0.694 / 32.31
8 / 2010-2011 / 0.595 / 37.42
9 / 2011-2012 / 0.536 / 38.84

Fig. 4

Fig. 5

Fig. 6

Summary and closing remarks

[1] At various stages of development say stabilization of 1500 TPD plant (Table 1 and Figure 3), re – engineering of the flotation circuit (Table 2 and Figure 4) and retrofitting of roller press in the grinding circuit (Table 3 and Figure 5) we note that energy consumption per ton of ore treated is inversely proportional to the through put.

[2] Retrofitting HPGR/ Roller Press in the grinding circuit of Jhamarkotra Phosphate Concentrator greatly enhanced the financial viability of the plant.

Acknowledgements

The author is grateful to his colleagues and friends Abdul Mannan, RK Meena, CB Rajan, RC Kumawat, LS Rathore for providing 6 the recent data of Jhamarkotra plant. Mr. Rodolf Pich, Mr. Gerd Ehrentraut of KHD Humboldt Wedag (Germany) and Er. Goutam Basu (KHD) India, Er. SK Madhok of M/s Hyderabad Industries Ltd were keen to make this project a success. Prof. TC Rao and Er. BN Chaterjee provided guidance as individual consultants.

Epilogue

This author joined M/s RSMM Ltd during March, 1995 and served there as Head, Phosphate Concentrator in the capacity of chief manager. He conceptualized the roller press ball mill grinding circuit. The project was initiated by Sri KS Money the then Managing Director of M/s RSMM Ltd. The project became operational during 2003 – 2004 and will be completing ten years of operation during 2012 – 2013.

References.

(1)  Gaudin, A.M., Principles of Mineral Dressing, Tata Mc Graw-Hill Publishing Company Ltd., 1977, New Delhi.

(2)  Chris Morley, HPGR in hard rock applications, Mining Magazine, September, 2003.

(3)  Sekhar, DMR and Jain, CL., Jhamarkotra phosphate ore processing plant, https://www.researchgate.net/profile/Dmr_Sekhar2/contributions a version of this article is in the proceedings of IMPC – 2006, Turkey.

(4)  Rene Klimowsky, Mining Magazine Congress, 8 -9 October, 2009, Niagara on the Lake, Canada.

(5)  Joachim Harder, AT Mineral Processing, Volume 53, 07 -08/ 2012.

(6)  Abdul Mannan, RK Meena, CB Rajan, RC Kumawat and LS Rathore, “RPBM - Wet Grinding Circuit of Jhamarkotra Phosphate Concentrator”, article communicated to BMPC – 2013.