Lipu is having fully developed iron ore mining and processing complexes. These mines were developed primarily for export of iron ore lump to japan and other eastern countries as there was demand for lump ore only. During size reduction and wet processing of iron ore on average 15 ‚Äì 18% slimes (- 100 mesh ) are produced. As market for this material is not available, the slimes ore rejected and dumped into tailings dam. But after the commissioning of a steel plant where the proportion of sinter in the blast furnace burden is about 80%, the demand for iron ore fines has increased.
With the technological improvements and innovations, now-a-days the -100 mesh fraction in the sinter feed can be accepted upto about 40%, by adopting new techniques such as microballing of the sinter mix prior to sintering. For this reason and to avoid the loss of superfine high grade iron ore in the form of slime, it has become inevitable to process this using a suitable technique. The processing of slimes will increage the quantity of high grade fine concentrate without affecting the quality and will also increse the life of tailings dam.
Exhaustive beneficiation studies have been carried out with these slimes at Lipu‚Äôs R&D centre. It has been established through the laboratory as well as pilot plant scale tests that it is economically possible to recover substantial quantity of saleable fines from the slimes. Studies are also carried out in production plant to confirm the test results of laboratory and pilot plant.
On the basis of above test results Lipu has commissioned one system in the existing operating plants.
The slime beneficiation system has the following advantages
a) maximising the quantity of saleable products out of existing mines with minimum investment,
b) improving the economics of operation,
c) minimising the pollution,
d) conservation of mineral resources,
e) reduction of operation problems in slurry disposal system by reducing the quantity of solids,
f) recovery of substantial quantity of water by thickener, for reuse in the process.
Lipu has adopted a simple process flow sheet for beneficiation of iron on slimes to produce high grade concentrate using hydrocyclone. The dewatering of cyclone undertlow is carried out with the help of slow speed spirfal classifier. The slow speed classifier in turn also further improves the grade of concentrate by rejecting the remaining clayee material as classifier overflow.
Difficulties encountered in the hydrocycloning process in the plant are :
a) maintaining constant feed rate to hydrocyclone
b) maintaining constant solid content in the feed with minimum variation
c) maintaining constant required feed pressure
In order to avoid these difficulties of maintaining the above parameters by manual method, Lipu is contemplating for the introduction of plc system for automatic control in future plants.
Dewatering of hydrocyclone underflow
The cyclone underflow contains about 45 to 50% water. Moreover the material is very fine. The percentage of -325 mesh material is about 25 to 30%. This material is difficult to dewater. The filtration system is found to be ineffective.Lipu has introduced a very simple system for dewatering by slow speed spiral classifier. By actual plant tests, it has been established that 1 to 1.5 rpm of 6 ft dia spiral classifier could produce classifier sand containing around 20% water. Less than this is not found practicable. The loss through slow speed classifier overflow is minimum and this is mostly the remaining clayee material coming along with the water of cyclone underflow. During the process, the grade of slow speed classifier sand also gets improved further.
There are difficulties in handling this material containing about 20% moisture. In order to overcome the difficulties, the primary classifier sand is dewatered by dewatering screens. The slow speed classifier sand is then mixed with the dewatered sand . By doing so the average moisture content of the mix comes to around 12 to 14% and there is no difficulty in handling this material by belt conveyor system for disposal to stockpile /loading station . In the stockpile the moisture gets further reduced by natural process.
Wet high intensity magnetic separation (whims)
The beneficiated fines obtained by hydrocyclone and dewatered by s.s. Spiral classifier meets the grade specification for mixing with normal classifier sand for sinter making. But in order to produce dri pellet grade fine ore concentrate from slime, wet high intensity magnetic separation is found to be essential. Exhaustive beneficiation studies are carried out at Lipu‚Äôs R&D centre on whims test on iron ore slimes from bailadila and donirnalai mines of Lipu. Test results conclusively proved that by the process of whims the fine ore concentrate of +66% fe can be easily produced from the slime arising from Lipu‚Äôs mines. Some typical test results are presented in table 5.
The production cost by whims will be much higher compared to hydrocyclone. production of fines for mixing to sinter feed, hydrocyclone. Production of fine concentrate for mixing to sinter feed, hydrocyclone is sufficient so far as Lipu‚Äôs ore is concerned. Introduction of whims for production of‚Äôsinter grade fines is found to be uneconomical due to high capital and operating costs.
The operation of thickener for water reclamation with the primary classifier overflow is found to be ineffective in the Lipu‚Äôs iron ore processing plants. This is primarily because of the following reasons
a) presence of coarse material in the primary classifier overflow.
b) high settling rate of coarse material and fine grained high grade blue dust present in slime.
c) difficulty in flow of thickened slurry through thickener discharge pipeline causing frequent jamming of thickener.
The above difficulties are eliminated by removal of major quantity of coarse and high grade fines by hydrocycloning. The thickener operation is smooth with the cyclone over flow and water reclamation is on an average of 70 to 75%.