Cyclones abrasion

Although performance curves are valuable in assessing classifier performance, frequently the cyclone overflow size analysis is used more than the d Q of the cyclone. In practice, clusters of cyclones (in parallel) are used to handle large capacities. Cyclones are manufactured in sizes ranging from 0.01 to 1.2 m in cyclone diameter, ie, the cylindrical section at the top (2,10). Capacities mn from 75 to 23,000 L/min. Materials of constmction vary widely. Rubber-lined or aH-polyurethane cyclones are used when abrasion is a problem. 

The dust has a broad size distribution, including particles under 10 to 15 [Lm as well as larger and possibly abrasive particles. A large low-velocity cyclone may be used to remove the coarse particles ahead of a unit vrim small-diameter multiple tubes. 

Current designs for venturi scrubbers generally use the vertical downflow of gas through the venturi contactor and incorporate three features (I) a wet-approach or flooded-waU entry sec tion, to avoid dust buildup at a wet-dry pmction (2) an adjustable throat for the venturi (or orifice), to provide for adjustment of the pressure drop and (3) a flooded elbow located below the venturi and ahead of the entrainment separator, to reduce wear by abrasive particles. The venturi throat is sometimes fitted with a refractoiy fining to resist abrasion by dust particles. The entrainment separator is commonly, but not invariably, of the cyclone type. An example of the standard form of venturi scrubber is shown in Fig. 17-48. The wet-approach entiy section has made practical the recirculation of slurries. Various forms of adjustable throats, which may be under manual or automatic control. 

Ceramic materials have found acceptance as cyclone liners for the apex orifice as well as other areas which exhibit severe abrasion. These include areas such as the lower cone liner and vortex finder. Nihard has also proven to be an acceptable wear material, especially for vortex finders and other areas which require strength as well as abrasion resistance. Other elastomer materials such as neoprene and nitrite are also utilized when hydrocarbons are present or when the temperature exceeds 60 °C. Urethane has found acceptance, especially in areas where the solids are relatively fine,... 

Vortex breakers 1. A device used to straighten out rotary flow in a duct a short distance after a fan. 2. A device found in a cyclone discharge fitted to reduce shell erosion by particulate abrasion. 

Within the range of their performance capabilities, cyclones are one of the least expensive dust-collection systems. Their major limitation is that, unless very small units are used, efficiency is low for particles smaller than five microns. Although cyclones may be used to collect particles larger than 200 microns, gravity-settling chambers or simple inertial separators are usually satisfactory and less subject to abrasion. 

Matsen (1985) pointed out a number of additional problem areas in scale-up such as consideration of particle size balances which change over time due to reaction, attrition and agglomeration. Erosion of cyclones, slide valves and other components due to abrasive particles are important design considerations for commercial units which may not be resolved in pilot plants. 

Attrition in fluidized bed systems leads primarily to a loss of bed material since the cyclones, which are mostly used for the collection of entrained material, are not able to keep the attrition-produced debris inside the fluidized bed system. The material loss through the cyclone is, therefore, usually taken as the attrition rate. This means that among the attrition modes discussed in Sec. 2, namely fragmentation and abrasion, it is abrasion which is the attrition mode of interest for fluidized bed systems. 

As is seen from this latter figure, an increase of the solids loading results in a decrease in the cyclone attrition rate. This may be due to a cushioning effect of the increased solids concentration which is well known in comminution processes. This cushioning effect may be interpreted as a decrease in the efficiency T] of the abrasion process. If we assume r] to be a function of jJ and in its simplest form to follow an exponential function,... 

In high-temperature processes, cyclones are often lined with refractory to both minimize heat loss and protect the metal surfaces from abrasion. These refractory surfaces are not as smooth as metal, but after a few days of operation, the refractory becomes smoother because of the abrasive action of the solids. 

Mining and Ore processing Abrasion resistant linings for hoppers, chutes, cyclones, slurry pipelines, separators etc. Also bolt-in rubber wear plates for chutes and hoppers and conveyor rollers. Soft natural rubber for abrasion resistance. Hardness range 40 to 70°A... 

The most important factor in any application is to determine the nature of the wear. Is the wear abrasive or is it erosive When polyurethanes are used in a dry screening application, the particles move through the air and are not blown along in a current of air. This application is abrasive wear. In a dry cyclone where the air blows the particles, it is an erosive wear situation. The basic layout of a cyclone is given in Figure 8.6. 

In the South African mining industry most processes for recovering precious metals, coal, minerals and diamonds are wet methods, involving the suspension of solids in water [1], In the control of such wet abrasive conditions in the pumping of fine particles size slurries, particle size separation in cyclones and in floatation cells, the choice of soft natural rubber lining is the most cost effective. 

Attrition of solids is known to take place during handling or on collection in gas cyclones, for example, but little is known of how it is related to particle properties, which particle sizes are most affected, and how attrition can be related to abrasion. Clearly, large particles are more likely to be affected by attrition finer fractions are generated by knocking off corners or by complete breakage of the larger particles. 

There is clearly a need to investigate the mechanism of attrition to relate it to the fracture properties of the solids, and to develop a realistic attrition index , similar to that used for abrasion in cyclones. Such an index would indicate the relative importance of operating conditions and design variables such as inlet velocity, feed solids concentration or cyclone diameter. This could then be used in scale-up to predict (or minimize) the effect of the shape, the particle size distribution or the hardness and strength of the feed solids, if known, may allow such predictions without any experimental tests. Generally, better understanding of attrition and its relation to abrasion may lead to better equipment design and operation. 

Despite their insubstantial nature, plant materials are also subject to contamination by sample containers and milling equipment. Paper sample bags may act as sources of contamination including boron (Standing Committee of Analysts 1986). Normally, vegetation is comminuted in high shear cyclone mills or blenders fitted with stainless steel or titanium-coated blades. As with other more resistant materials the amount of contamination from the mill can be related to the abrasiveness of the sample (Allan etal. 1999). 

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