High Pressure Grinding Rolls

RPBM wet grinding circuit of Jhamarkotra Phosphate concentrator


RK Meena and RC Kumawat


The incorporation of roller press in the wet grinding ball mill circuit of Jhamarkotra phosphate concentrator has resulted into doubling of the plant capacity from 1500 TPD to 3000 TPD and reduced power consumption substantially bringing the concentrator out of financial losses. The roller press operates in tandem with a wet grinding ball mill.


Introduction


Low grade phosphate ore of Jhamarkotra analyzing 16% to 18% P2O5 is upgraded to 31.5% P2O5 by two stage flotation process after size reducing the ore to 90% passing through 200 mesh using conventional equipment such as jaw crusher, secondary and tertiary cone crushers to get a product of -12.5 mm which is the feed to ball mills. Cyclone over flow from ball mill circuit is fed to secondary de sliming cyclones the under flow of which is fed to bulk flotation where silica and coarse particles are rejected as tailing. The froth of the bulk circuit is taken to acid floation circuit. The secondary cyclones over flow is also fed to the acid floation circuit. During the first stage flotation both dolomite and phosphate minerals are collected into froth using sodium oleate leaving silicates, silica and coarse particles into the tailings. The bulk concentrate thus obtained plus the secondary cyclones over flow is conditioned with phosphoric acid at pH 5 using sulfuric acid as pH modulator. The phosphate concentrate is collected as chamber product while dolomite the froth product is discarded as tailing after two cleanings. 1500 TPD capacity was achieved through two ball mills each driven by motors of 600 kW. The grinding circuit and other down stream operations are estimated to consume 56 kWh per ton of ore milled as norm.

Comminution


The most power consuming operations in mineral processing plants are grinding machines. Gaudin1 remarked, “why comminuting efficiencies of apparently effective machines should be so low”. The energy efficiency of grinding (ball mills) may be 10% or still low. It is reported2 that 50% of the costs incurred in base metal concentrators are on grinding. Prof. K. Schonert designed a comminuting machine (commercial machines are known as roller presses or high pressure grinding rolls) which is based on the principle of “crushing a mineral particle between other mineral particles instead of grinding elements”. This is achieved by choke feeding the roller press that has two rolls of which one is fixed and the other is sliding. Both rolls are driven by separate motors and communition pressure is applied hydraulically to the sliding roll. These machines are highly energy efficient and are successful in cement industry. However these machines were not successful in mineral (hard rock with high silica) industry3 mainly due to wear problems of grinding surfaces.


Capacity enhancement of the plant


After successful operation of the plant it is observed that the flotation circuit has excess capacity. Merely by increasing the flow rate of air into the Denver cells the through put into each line was increased by double. With this success the management decided to increase the capacity of grinding circuit from 1500 TPD to 300 TPD. This could be done by adding two more ball mills or by incorporating a roller press in the crusher plant. However the team of plant engineers proposed a grinding circuit incorporating a roller press that works in tandem with one of the existing ball mills and prepared a techno economic feasibility report that promised less capital cost, huge reduction in power consumption and profitability. This came to be known as third option while the first two options were proposed by the consulting company. The third option was referred to the consultants (KHD Humboldt Wedag, Germany) which was approved as valid and the best. The third option was implemented and the new system became operational from 27/3/2003. Conceptual flow sheets of the original grinding circuit of 1500 TPD and the new 3000 TPD4 are illustrated in figures 1 and 2.


Reengineering of the grinding circuit


The average Bond’s Work Index (BWI) of the Jhamarkotra ore is 7 kWh per short ton. KHD Humboldt Wedag (Germany) tested the Jhamarkotra ore on a pilot sized roller press with stud lining on the grinding surfaces, pressing the material at 5N/ mm2. After removing 125 micron size from the pressed product, the coarse fraction was crushed to -3 mm size and BWI of this material was observed to be at 5.73 kWh per short ton. Further the specific power consumption by the roller press was noted to be 1.9 kWh per metric ton. The roller presses is sized by after taking design margins which has rolls of 120 cm diameter and 63 width with each roll to be driven by 350 kW motors. The machine is of KHD Humboldt Wedag. The machine was incorporated in the grinding circuit to work in tandem with one of the existing ball mills the other is stand by.


The roller press discharge cake is wet screened over a screen of 6 meters long and 3.6 meters width with slots of 1.8 mm width and 20 mm long. The over size from the screen is re circulated to the roller press. The wet screen under flow is fed to a cluster of three hydro cyclones of 254 mm diameter (RP Cyclones) the under flow of which is fed to the ball mill and the over flow goes to secondary (de sliming cyclones) cyclones. The ball mill discharge is fed to a cluster of eight hydro cyclones of 254 mm diameter (BM Cyclones) the under flow of which is re circulated to the ball mill while the over flow goes to the secondary cyclones. The over flow of secondary cyclones goes to acid flotation circuit while the under flow goes to the bulk flotation circuit.


The crushing plant is adjusted to produce -25 mm product instead of -12.5 mm by increasing the set of cone crushers. The roller press is designed to accept -30 mm feed. M/s MECON the consultants to implement the project estimated the power consumption of the Roller Press Ball Mill (RPBM) grinding circuit with the down stream operations to be at 38.79 kWh per ton of ore milled. Table 1 shows the actual performance of the plant after RPBM scheme is commissioned.


Table 1 – Power Consumption of Plant after RPBM Circuit became operational

s.no Year RP Crushing Wet RP Crushing dry Consumption power Consumption power
MT MT kWh kWh/MT
1 2003-04 672090 645206.40 25096079.07 37.340
2 2004-05 665237 638627.52 24105249.69 36.236
3 2005-06 876994 841914.24 27185805.92 31.00
4 2006-07 841125 807480.00 26529869.75 31.54
5 2007-08 808280 775948.80 25254937.50 31.25
6 2008-09 808706 776357.76 24854506.25 30.73
7 2009-10 722721 693812.16 23349602.00 32.31
8 2010-11 619980 595180.80 23200281.92 37.42
9 2011-12 335638 322212.48 12918832.54 38.49 upto oct. 2011


It is evident from Table 1 that the performance of RPBM grinding circuit surpassed all estimates with respect to the power consumption.


Historical Events


The third option was developed by Dr. DMR Sekhar the then chief manager (OD) with the active assistance by an young team of 20 engineers working with him at the plant. Other general managers and group general managers were highly skeptical of RPBM project. Sri KS Money IAS, the then Managing Director of RSMML took a bold decision to implement the third option,the RPBM project which was unconventional at that point of time. Consultants estimated the cost of the third option to be at 50 crores of rupees. Technology Development Board, Government of India provided 5 crores of rupees as soft loan. The project was in fact implemented within a cost of around 35 crores (as estimated by the plant engineers). Dr. Sekhar was conferred mining engineering design award for designing the RPBM circuit and reengineering the flotation circuit. M/s Rajasthan State Mines and Minerals Ltd won Rajasthan Energy Conservation Award for the year 2008 – 2009 from the Department of Energy, Govt. of Rajasthan, Jaipur. This project already earned 1.5 crores from Carbon Credits and another 2.5 crores is being processed for payment.


RPBM Circuit Jhamakotra




Acknowledgements


The authors are thankful to Sri Ajitab Sharma the Managing Director of RSMML and Er. LS Rathod, Group General Manager (phosphate division). We are glad to be the members of the team of plant engineers.

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.



References

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


[2] Pradip, Scientific and Technological Challenges in Mineral Processing, Mineral Processing and Extractive Metallurgy Review, Vol. 10, (1992).


[3] Chris Morley, HPGR in hard rock applications, Mining Magazine, September 2003.


[4] Sekhar, DMR and Jain, CL, Jhamarkotra Phosphate Ore Processing Plant, International Mineral Processing Congress – 2006, Turkey.


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