Hydraulic Transportation of Solids in Flotation Circuits: Pump Classification and Design Requirements

1. PUMPS IN FLOTATION PUMPS IN FLOTATION CIRCUITS CIRCUITS

2. Hydaulic Transportation of Solids Hydaulic Transportation of Solids Hydraulic Transportation of solids is moving the solids forward between different stages of solid/liquid mixing, solid/solid separation, solid/liquid separation, etc.. •Limitations in flow: from 1 m3/h up to 20 000 m3/h •Limitations for solid: maximum particle size 200 mm. This can be up to 350 mm for special applications. •Difference between clean liquid and slurry pumps is the permanent service requirement of slurry pumps.

3. Classification of Slurry Pumps Classification of Slurry Pumps BY DUTY BY APPLICATION •Pumps for particles upto 2 mm • Mud/clay • Silt • Sand, fine • Sand, medium 100-500 microns • Sand, coarse •Froth pumps -2 microns 2-50 microns 50-100 microns •Carbon transfer pumps (CIP and CIL circuit) •Sump pumps (pumps from floor sumps, submerged pump houses but with dry bearings) 500-2000 microns •Submersible pumps (the entire unit is submersed) •Sand and Gravel pump 2-8 mm •Gravel pumps up to 50 mm •Dredge pumps >50 mm

4. DESIGN REQUIREMENTS OF HORIZONTAL CENTRIFUGAL PUMPS DESIGN REQUIREMENTS OF HORIZONTAL CENTRIFUGAL PUMPS • The ability to pump high density abrasive slurries with adequate wear life . • The ability to pass large diameter solids. •The ability to handle air entrained and/or viscous fluids with reliability and minimal performance corrections. Because of these factors, a slurry pump is always larger than its liquid counterparts.

5. Wear in Pumps Wear in Pumps • Slurry in flotation plant is abrasive and causes wear •Wear can be minimized with; • appropriate pump design, • proper material selection • proper pump application To maximize wear life, thick casting sections are provided; ◦ on the impeller, ◦ the casing of the unlined pump, ◦ the wear liners of the fully lined pump •The unlined pump may offer the lower initial capital cost, •The lined version; • have longer wear life and lower replacement spares cost • safer from a pressure containment standpoint. • large clearances are provided within the impeller and casing to allow for the passage of large diameter solids, • reduce internal velocities and corresponding wear

6. Unlined and Lined Pumps Unlined and Lined Pumps Wear Components on Fully Lined Centrifugal Slurry Pump Wear Components on Unlined Centrifugal Slurry Pump

7. Drives Drives •On low horsepower applications (below 300 kW, 400 hp), belt drives are the most popular means of achieving the required speed for the duty point. •Belt drives are inherently quiet and also allow for relatively easy speed changes. •For higher horsepower applications, gearboxes are commonly used to meet the desired speed and duty point. •On applications where variable flow is required, variable frequency drives are used to provide the necessary continual speed changes.

8. Impeller Design Impeller Design To combat wear and allow for the passage of large diameter solids, heavy-duty slurry pump impellers feature thicker main pumping vanes and fewer of them. Both of these factors further contribute to a reduction in efficiency when compared with a clear liquid counterpart. While a clear liquid impeller usually has five to nine vanes, most slurry pump impellers have two to five, with four and five vane designs being the most common. Two and three vane designs are usually reserved for very large particle passing, as required in dredging applications.

9. Materials Materials •Hard metals: High hardness values to combat erosion •Elastomers: Ability to absorb the energy of the impacting particles, outperform hard metals, in terms of erosion resistance (fine particles <250 microns) •Ceramics: High hardness values to combat erosion

10. Hard Metals Hard Metals The three basic types of metals used to combat erosion in centrifugal pumps fall under ASTM A532 (class I, II, and Hl). •Martensitic White Irons (class I), •The Chromium-Molybdenum White Irons (class II), •High-Chrome Irons (class III). •These materials consist of hard carbides within a supporting ferrous matrix. •Lowering the carbon, and raising the chromium results in lower hardness, but retains additional chromium in the matrix for added corrosion resistance. •Increasing the chromium content provides further gains in corrosion resistance.

11. Elastometers (1) Elastometers (1) •Typically used in all applications with particle diameters not greater than 10 mm. •Elastomers can be broadly broken into two categories: natural rubber and synthetic elastomers. •Natural rubber is more erosion resistance due to its significantly greater resilience and tear resistance. •Resilience is a measure of how high a ball of the material will bounce measured as a percentage of the initial drop height. Typically, this value ranges from 65 percent to 90 percent dependent on the rubber blend. •Tear initiation resistance for natural rubber is typically in the range of 30 to 110 N/mm, dependent upon blend. •For fine particles applications (less than 100 microns) resilience has been shown to be more important in combating wear. •For larger particles (greater than 500 micron) tear strength (resistance) is more important. •As mineral processing slurries have a mixture of particle size, the best performing natural rubber will be one with the optimum combination of resilience for fine particle wear resistance and tear resistance to prevent larger particle damage.

12. Elastometers (2) Elastometers (2) •Synthetic elastomers are used in small particle applications where natural rubber would be subject to chemical attack, causing swelling, hardening or reversion of the natural rubber. The most commonly used synthetic elastomers for wear materials and some of their typical applications are: • Nitrile: Generally used in fats, oils and waxes. Moderate erosion resistance. Limited resistance to acids and alkali environments. • Butyl: Hydrochloric acid, phosphoric acid and sodium hydroxide. Sulphuric acid causes degradation and chlorinated hydrocarbons cause swelling. • Hypalon: Primary use in acid conditions with some resistance to vegetable and mineral oils. Not recommended for use in ketones or chlorinated solvents. • Neoprene: Moderate resistance to oils, fats, grease and some hydrocarbons. Can also be used in some mild oxidizing acids. •Synthetic elastomers also have higher temperature limits than natural rubber. While natural rubber is limited to 75 to 85 degrees C, dependent upon blend, the above synthetic elastomers have temperature limits ranging from 95 degrees C for Nitrile to 110 degrees C for Hypalon.

13. Elastometers Elastometers (3) (3) •The superior mechanical properties of natural rubber indicate it should be used in preference to synthetic elastomers, unless temperature and/or chemical resistance are overriding factors. •Polyurethane is an elastomer that is used in applications where there is a good chance of large particle "tramp" damage, which would otherwise cut natural rubber and synthetic elastomers. •Polyurethane is generally limited in temperature to 70 degrees C, due to swelling from hydrolysis attack, although polyurethanes are currently being produced, which reportedly can operate at 110 degrees C with no degradation. •Generally speaking, in most small particle applications, where "tramp damage" is not a problem, natural rubber will outperform polyurethane.

14. Tip Speed (1) Tip Speed (1) •Another important factor to consider in the selection of materials is the impeller tip speed limit. •For elastomers, the concern is vibration or fibrillation of the material due to the relative motion of the impeller with respect to the side liners . •This vibration can lead to heat generation within the elastomer and a thermal breakdown of the material. •Tip speed problems on elastomers are easy to diagnose, as the damage is always most severe at areas with highest relative speed. • Typically, natural rubber has a tip speed limit of approximately 27.5 m/sec. • Highly wear resistant soft natural rubber can have a tip speed limit as low as 25 m/sec, • Proprietary blends with improved thermal conductivity can operate at 30 m/sec.

15. Tip Speed Tip Speed (2) (2) Approximate speed limits and corresponding approximate beast efficiency point (BEP) head limits for various materials are listed below:

16. Pump Wear (1) Pump Wear (1) Wear in a centrifugal pump will consist of various modes of abrasion and erosion. Abrasion, which is the forcing of hard particles against a (wear) surface, only takes place within a slurry pump on the shaft sleeve and between the tight tolerance wear ring section of the impeller and the suction side liner. Erosion is more commonly used to describe the progressive wear loss from the interaction or impingement of the fluid and particles against the wear components. The three primary modes of erosion and their wear locations; • Deformation wear: Direct impact to the leading edge of the impeller vanes, the back shroud of the impeller and the "protruding" cutwater within the volute. • Random impingement: Random impacts to the impeller shroud and trailing edge of the main pumping vanes. • Low angle impact: Wear from the tangential or near tangential movement of particles against the volute casing or vane surface.

17. Pump Wear Pump Wear (2) (2) Deformation wear (direct impact) is the most severe and low angle impact is the least severe. The degree of wear is dependent upon the following: • The kinetic energy of the particle: panicle mass (specific gravity) and velocity. • The panicle shape: sharp particles have small contact area and high local stress, so wear is more severe than with rounded panicles. • The slurry concentration: higher percentages of solids result in more impacts for a given flow. Slurries can be classified according to the following criteria (reference): • Heavy duty; Cw>35%, d85>400 µm, SGs>2.0, sharp particles. • Medium duty: 20%<Cw<50%, 150 µm <d85<400 µm, SGs>1.4, angular particles. • Light duty: Cw<20%, d85<150 µm, SGs>1.4, rounded particles.

18. Pump Wear Pump Wear (3) (3) In addition to the general specific speed limits and the material tip speed limits, to maximize wear life, the following general impeller tip speed limits can be applied based on the severity of the duty: oHeavy duty; 25 m/sec max. 32 meters head @ BEP oMedium duty; 32 m/sec max. 52 meters head @ BEP oLight duty; 38 m/sec max. 74 meters head @BEP Further recommendations can be made with respect to impeller type and flow range: oHeavy duty: heavy duty impeller @ 0.60 to 0.80 BEP oMedium duty: heavy duty impeller @ 0.70 to 0.90 BEP oLight duty; high efficiency impeller @ 0.80 to 1.1 BEP

19. Applications (1) Applications (1) Grinding Circuits: •The most severely erosive of pump applications. •Typically, the solids size is relatively large, the density tends to be high and the particles being freshly ground have sharp edges. •Fortunately most grinding circuits are low head applications allowing for relatively low pump speed. •Because of the large particles, heavy duty impellers with low specific speed operating at 60 to 80% of BEP flow are recommended. •Hard metal impellers are required to handle the direct impact of the solids on the leading edge of the main pumping vanes. •High tear strength natural rubber is an option on the casing liners. •On higher head grinding applications metal volute liners or casings are recommended.

20. Applications (2) Applications (2) Tailing Circuits: •The high flow and high head of these applications dictates the need for a balance of wear and efficiency in these relatively high horsepower applications. •Typically pumps with Ns in the range of approximately 27 to 33 provide the optimum balance of these two factors. •To further optimize efficiency, operation closer to the best efficiency point (70 to 100% of BEP) and impellers with thinner and longer wrap vanes than the heavy duty design used in mill circuits are recommended •Hard metal is typically used on impellers and side liners, due to the high tip speeds. •Casing liners can be elastomer or metal depending upon particle size and the possibility of large particle "tramp" damage.

21. Applications Applications (3) (3) Thickeners: Metal or elastomer wear components are used depending upon the solids size handled by the thickener. In high yield stress applications there appears to be a benefit to staying away from low Ns designs. Low NPSHR designs have the ability to better handle low suction pressure conditions. On high yield stress applications open impellers with flow inducing vanes are more likely to provide stable operation. NPSHR: Net positive suction head required

22. Applications (4) Applications (4) Froth: Past recommendations: Froth factors as great as six being applied to the desired volume flow. This oversizing was thought to provide the benefits of an increased eye area to handle larger air volumes without losing prime, A larger diameter impeller to keep speeds and inlet velocities low to minimize separation of gas and fluid and a lower NPSHR. These recommendations resulted in the application of typical slurry pumps at least one size larger than normally required for an air free slurry. Recent designs; Froth handling have been produced with separate inducers or oversized inlets and flow inducer vanes to lower NPSHR and provide positive displacement characteristics to feed the main pumping vanes. These designs show the promise of providing pumps of lower initial capital cost due to reduced size while also offering improved efficiency.

23. Applications Applications (5) (5) From a system design standpoint, oversized sumps with baffles and additional retention time to assist in venting of the gas prior to it entering the pump are recommended.

24. Specific Speed (Ns) Specific Speed (Ns)