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Mine machinery

1. CHAPTER 6 Slurry Transport DR GEORGE AGYEI UNIVERSITY OF MINES AND TECHNOLOGY, TARKWA

2. Slurry Transport Watery mixture of insoluble matter is called slurry. Hydraulic Transportation or slurry transportation involves transporting solid material in the form of slurry by mixing it with water after appropriate size reduction. Using water as carrier one can have two  forms of slurry transportation: open channel transport and pipe line transport. Slurry pipe line can be used to transport coal, oil sands, phosphate concentrates limestone , copper, iron, lead, zinc, nickel as well as their ores, bauxite. 2

3. Slurry Transport Slurry transportation find application in: Dredging Mineral Transport (Syncrude, the world's largest producer of crude oil from oil sands, has refined hydro-transport technology for mining oil sands.) Coal in slurry form in Coal Washery through the washing process 3

4. Slurry Transport World’s longest slurry pipe line is operated by Alegria iron ore mine, Brazil.  The slurry flows at an average speed of 6km/h with a flow rate of 1,200m³/h. 4

5. Advantages of Slurry Transport Aesthetic – pipes are buried and out of site with only few surface structures The land is then used for agriculture or other uses. No noise, dust pollution 5

6. Slurry Transport Slurry is a mixture of solids and liquid. The density of a slurry can be calculated as ρ m = 100 / ( c w / ρ s   + (100 - c w ) / ρ l ) where ρ m = density of slurry (kg/ m3 ) c w   = concentration of solids by weight in the slurry (%) ρ s =  density of the solids (kg/ m3 ) ρ l =  density of liquid without solids (kg/m 3 ) The slurry concentration by weight can be measured by evaporating a known weight of slurry and measuring the weight of dried solids. 6

7. Slurry Transport The basic components of a slurry transport system are: Slurry preparation unit having Solid crusher and concentrator and Mixer and storage tank Feeding the slurry to the pipe line Slurry pumps (high pressure pump) Pipe line Intermediate pumping stations Dewatering unit 7

8. Slurry Transport Figure 1 Schematic Diagram of a Slurry Pipeline System 8

9. Slurry Transport Positive displacement, either plunger or piston type, depending upon the application, are the most common for use in conventional slurry systems today because of their high pressure capability. In order to obtain equivalent pressures using single stage centrifugal pumps, each station would require eight to ten of them installed in series. The resulting multiple passes through so many centrifugal pumps would alter the slurry properties because of particle attrition by the action of the impellers. 9

10. Slurry Transport Factors affecting slurry transportation that should be carefully considered during design of the slurry pipe line Size and concentration of the solid particles  are: Abrasivity of the slurry Pumping pressures Pipe diameter Reactivity between the solids and the liquid and the surfaces Viscosity of the liquid Critical Velocity 10

11. Slurry Transport The velocity of the slurry in a pipeline needs to be above a certain critical value to prevent settling of solids. This velocity is called critical velocity. It depends on: Particle Size ( d s ) Volumetric Concentration of Solids ( Cv ) Pipe Diameter (D) Viscocity of Fluid (µ f ) Fluid Density ( ρ f ) Solid Settling Velocity ( w m ) Solid Density ( ρ s ) 11

12. Trolley Assisted Dump Truck A complete trolley assisted dump truck system comprises of an overhead line, transportable transformer rectifier substations, and modified trucks with pantograph and hybrid engines.  In this system mega trucks of total weight of about 250t and tire diameter up to 3m can operate with external electric power supply while driving uphill on the steep ramps and return to the diesel electric   operation while driving downhill. The vehicle changes over to full elelctric drive as soon as the raised pantograph make contact with the centenary.  12

13. Trolley Assisted Dump Truck Figure 2 A Trolley Truck System of 1940 Palabora Trolley Assisted Truck 1980 13

14. Trolley Assisted Dump Truck Figure 3 Trolley Assisted Dump Truck 14

15. Trolley Assisted Dump Truck The alternating supply of power through the diesel electric unit and the overhead line results in fuel saving of about 30% and the speed on the steep ramp increases from 12 km/h to about 23 km/h. This accelerates turnaround times of the trucks and increases haulage. The principal characteristic of trolley assist operation is that the power available for the traction motors is greatly increased. This allows the truck to move up the grade faster, reduce the time needed to transport the mineral and, thus, increases production rates. 15

16. Advantages and Disadvantages Advantages of this system are saving in diesel consumption, higher productivity of truck fleet as numbers of trucks deployed get reduced, longer life of the truck. However it has the following drawbacks: Inflexibility in road alignment and development High cost of installing due to requirement of electrical systems More safety precautions and hazard minimization initiatives are required Truck size is limited by the size of the electric traction motor Certain topography may be unsuitable for operating with this system. 16

17. Trolley Assisted Dump Truck The heat buildup is proportional to the steepness and length of the grade and rolling resistance of the road. Thus cooling system is very important for such trucks. The electro wheels can tolerate substantial overload for short period of time. A fully loaded truck moving up a steep ramp at slow speed operates at a lower electrical efficiency.  Thus only internal diesel –electric system suffers from higher fuel consumption. Thus the concept of external electricity supply and trolley assisted truck system was developed. For the trolley assisted system the road should be wide enough to permit overtaking. 17

18. AC Drive Haul Truck An advantage that ac drive haul trucks have over dc drive haul trucks when operating on trolley is that the ac inverters decouple the traction motor voltage and speed from the dc line voltage. This is very important because it ensures maximum utilization of the equipment. The ac-motored truck, on the other hand, can take the line at any speed and move on the line at any speed independent of the dc-link voltage. 18

19. AC Drive Haul Truck In either case, trolley assist enables mine operators to increase on-grade speeds up to 80% and reduce overall cycle times by as much as 20%. The use of ac drives for trolley assist provides more reliable performance than dc drives, especially when the trucks are overloaded. 19

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