Pusher Centrifuge: Operation, Applications and Advantages

Abstract:
Pusher centrifuge is a continuous filtering type centrifuge used for solid-liquid separation in the chemical and mineral industries. Pushers have been used for more than 60 years for dewatering relatively large, free-draining crystals. The pusher centrifuge has a unique design that minimizes moisture, impurity and crystal breakage in discharged cake. In this paper, the operation of the pusher centrifuge will be discussed. Scale-up variables and design will be briefly addressed and applications and advantages of the pusher centrifuge will be described.

Centrifugal devices, often used to achieve quick and efficient solid-liquid separation, work on two basic principles: sedimentation and filtration. Sedimentation or settling involves allowing the heavy phase to fall through the light phase and collect on an imperforate surface and typically involves fine solids with long settling times. Examples are decanters, disk nozzle centrifuge, etc. On the other hand, filtration depends on the particle size being large enough to build a cake on a filter cloth or a screen. The cake must also be porous enough to allow mother liquor to flow through it. Examples are pusher, peeler, vibrating screen centrifuge, etc.

Various types of centrifuges are manufactured, and each type of centrifuge has certain advantages over a specific range of process variables. Figure 1 shows the range of variables where the pusher centrifuge is generally used. The pusher centrifuge feed has 25-65 wt.% solids concentration of large free draining crystals (typically 80% retained on 150 microns). For most applications, these particles are crystalline in nature, but non-crystalline materials have been successfully dewatered on the pusher centrifuge. These particles must be distinct, free draining and the liquid must not be chemically attached to the solids, i.e., waters of hydration.

Operation:
As seen in Figure 2, the slurry is fed through a stationary feed pipe at the centerline of the pusher basket. The slurry is accelerated in the rotating feed funnel and distributed uniformly at the back of the rotating screen. This feed funnel is anchored to the pusher plate. The feed funnel gently accelerates the slurry to the basket speed in order to form an even cake and minimize particle attrition.

As the pusher plate moves into its back position, a clean wedge slot screen surface is exposed. The accelerated slurry is deposited on the wedge slot screen, and cake is built as the mother liquor drains through the basket. Then, the pusher plate moves forward and pushes the cake forward, towards the solids discharge end. During this time, the incoming feed is held in the feed funnel, until the pusher plate moves backward and exposes fresh screen for another annular ring of cake to be built. The reciprocation of the pusher plate causes the cake to progress towards the solids discharge end. The cake, which is under centrifugal force, becomes drier as it progresses in the basket and ultimately is discharged from the pusher basket into the solids discharge housing. The hydraulic mechanism in the drive shaft reciprocates the pusher plate and the feed funnel. Typically, the frequency is 25-40 strokes per minute, and the stroke length is 5-15% of the basket length.

Mechanical Features:
The horizontal orientation of the pusher inherently increases stability by minimizing vibrations. This is complimented by a long shaft and widely spaced bearings, which makes the pusher more stable and decreases downtime. The process housing isolates the solid and liquid materials from each other, preventing cross contamination. The solids housing has a solids discharge ring that confines and directs the solids downward into the solids discharge chute. The liquor housing contains internal baffles that provide segregation of mother liquor and wash. Back crystallization can occur between the wedges on the screen, resulting in increased resistance of the drainage of liquor. A series of backwash nozzles are installed on the liquor housing that rinse the back of the screen intermittently to wash out back crystallization.

The basket is a centrifugally cast design that has milled oblong slots for maximum drainage and improved backwash. The basket is fitted with a wedge slot screen that has a standard screen width of 0.010"-0.015" (254-380 microns).

The base supports the pusher and provides a reservoir for the hydraulic oil. The hydraulic oil flows through a suction filter into a vane-type constant delivery pump. The hydraulic oil flows to a pressure relief valve, and then into a stationary housing and a transfer bushing. A cooling coil is submerged in the oil to dissipate heat generated due to friction. The reciprocation of the hydraulic mechanism is triggered by a unique internally controlled operational valve. This valve is actuated internally by the pilot piston. This design works on low oil pressure and a higher flow rate, so that pump requirements are reduced for a similar size machine. In addition, lower oil pressure translates to lower wear on the hydraulic parts.

Both the hydraulic pump and the pusher hollow shaft are driven via sheaves and V-belts from a single motor. The hollow shaft drives the basket as well as the pusher shaft through a sliding key connection. The hollow shaft is supported by bearings at the front of the base and just ahead of the transfer bushing.

Discussion of Process Variables:
The performance of the pusher centrifuge is a function of many parameters. Some of the important variables and their effect on performance are explained below:

Particle size – For a pusher centrifuge to be completely effective and produce maximum throughput, the particles should be as large as practically possible. The performance of the pusher is a function of the crystal size as well as shape. As the crystal size increases, the surface area per unit mass decreases, and there is less surface area for the moisture to bind to, providing a drier cake. In addition, the surface area is a function of the shape of the crystal as well.

Viscosity – In addition to porosity, drainage rate is a function of the viscosity of the mother liquor. As the viscosity increases, there is an added resistance to the separation of the liquid from the slurry. Consequently, the throughput of the pusher is de-rated according to the viscosity of the mother liquor.

Solids Concentration – In most cases, the solids discharge capacity or the hydraulic capacity is not the limiting factor. The usual limitation for the pusher centrifuge is the drainage rate. Hence, as the feed slurry concentration is increased, more solids can be processed for a given amount of feed slurry. Figure 4 depicts how more throughput can be achieved through the pusher as feed concentration increases. This holds true for drainage limited processes, which constitute a majority of applications.

Cake Quality–Cake quality is measured mainly in two aspects: purity and volatile matter. A wash liquid is intro-duced on the cake in order to displace the mother liquor along with the impurities. The amount of volatile matter present in the discharged cake is a function of the centrifugal force (Xg) and the residence time at that force. Increasing Xg force increases the sep-aration force and hence favors drainage. Figure 4 also points out that as more solids are processed through a given pusher, the residence time of the solids on the basket decreases, which increases volatile matter in discharged cake.

Another parameter for cake quality is particle attrition. The movement of the pusher plate along with the acceleration in the feed funnel causes some of the particles to break. In addition, if the particles are too fragile, Xg force will cause breakage and compaction, and volatile matter in discharged cake will increase. Attrition has been found to be a function of the Xg force. The gentle conveying of cake in the low Xg, single stage, long basket design of pusher centrifuges results in low particle attrition.

Scale-up:
In general, scale-up is based upon providing the amount of screen area, centrifugal force, and residence time to produce the desired cake throughput and quality. In most cases, it is most reliable to scale-up from pilot equipment because some of the factors influencing performance are difficult to define precisely, such as the effective friction factor of the cake against the screen. Hence, previous experience becomes critical in predicting the performance of pushers.

The two most important parameters explored for a new application are maximum drainage rate and volatile matter in discharged cake. Drainage rate varies from slurry to slurry depending on particle shape and size, and the viscosity of the mother liquor. The relationship between Xg force and volatile matter, drainage rate, throughput and fines loss are investigated. Variables like feed rate and concentration are investigated and cake thickness, bulk density and other observations are recorded. Using these data along with previous applications, the manufacturer is able to size the equipment required for the commercial application.

Pusher centrifuges are manufactured in a wide range of sizes, with diameter ranging from 20 cm. to 120 cm., and basket length varying from 25cm. to 75 cm.. The largest machines can produce up to 100 STPH of discharged solids. Normal operating Xg force is around 250-400 Xg. Pusher centrifuges have been made in a variety of series 300 stainless steels, Alloy 20 stainless steel, Monel, Inconel, Hastelloy and Titanium.

Features and New Developments:
The pusher centrifuge has many design features that offer unique processing capabilities. The pusher has also been modified over the years in order to enhance performance and to increase the applicability of pushers to a wider range of applications. These features and modifications include:

Push Hesitation: Most of the solids loss to filtrate occurs in the swept area. These particles pass through the wedge slots before the gap is bridged. Push hesitation is a modification that holds the pusher plate in the back stroke, allowing the cake to build up on itself. The cake acts as the filtering media and has an increased efficiency of capturing the finer solids. Although this modification reduces capacity, it has helped improve the solids capture efficiency and make the pusher applicable to smaller particles.

Horizontally Split Process Housing: The split process housing is designed to allow the removal of the rotating assembly without disassembling the basket and pusher from the shafting assembly.

Integral Hydraulic System: The hydraulic control is automatic and self-contained. It requires no external timers or mechanical controls.

Bearings: The rotating assembly is supported by two sets of widely spaced bearings, with the basket and screen assembly as a cantilever load. The calculated bearing life is in excess of 50,000 hours L10.

Seals: Several options are available to isolate process fluids from atmosphere, including a centrifugal liquid-ring seal and a non-contacting inert gas purged labyrinth seal that allows zero seal leakage. A variety of pusher shaft seals exist to prevent cross contamination between the hydraulic and process ends. Bearing isolators are located on the hydraulic reservoir to prevent oil leakage to the atmosphere.

Pre-Drain Funnel: The pre-drain funnel removes a portion of the mother liquor through a perforated surface. This feature helps pre-concentrate the feed, which is especially important in drainage-limited applications. Crystals that tend to back-crystallize are not suitable for this feature, as the funnel cannot be backwashed.

Applications:
The pusher centrifuge has been applied to a wide variety of industries. Although typically pushers are used in the inorganics industry, it has also been used extensively in chemical industries like organic intermediates, plastics, food processing and rocket fuels. Notable pusher applications include inorganics like ammonium sulfate, soda ash, potash, sodium bicarbonate, sulfates and sulfides, borax, borates, chlorates and chromates.

Many organic intermediates have been successfully separated in the pusher, including paraxylene, adipic acid, oxalic acid, caprolactam, nitrocellulose, carboxymethylcellulose, etc. In food processing, the pusher has been employed for the production of monosodium glutamate, salt, lysine and saccharin. Plastics applications include PVC, polyethylene and polypropylene, and a number of resins classified under various trade names. In order to understand the performance of the pusher centrifuge in various applications, some applications are discussed as case-studies:

Soda Ash: Pusher centrifuges have been used for a wide variety of soda ash applications including sodium carbonate monohydrate, sodium carbonate sequicarbonate (trona), light soda ash and dense soda ash. Typical soda ash is 95% larger than 150 micron and 99% larger than 75 micron. Feed slurry usually has 50% solids by weight, and discharged cake has about 4% moisture.

High throughput continuous operation compliments the high volume manufacturing needs in this mining industry. Trouble free operation and reduced down times are key features as most of these plants are located in remote places.

Sodium Bicarbonate: Sodium bicarbonate is made by carbonating soda ash. The soda bicarb is crystallized and has to be separated from the mother liquor. Feed to the centrifuge contains about 40% solids by weight, and the crystals are 35-50% larger than 150 microns and 85-95% larger than 45 microns. Discharged cake contains 4-6% moisture. Modifications to the pusher centrifuge make it applicable for dewatering this fine crystal.

Paraxylene: Typical paraxylene crystals are 100-400 microns and are fed to the pusher as a frozen slurry. The single stage long basket design results in as high as 99.9% pure cake.

Many features have been added to the pusher centrifuge for this specialized application. Special measures are taken to prevent cross-contamination of product and oil. A series of lip seals and rod scrapers are used on the shaft seal to eliminate cross-contamination. The pre-drain funnel pre-concentrates the feed to the pusher centrifuge allowing higher capacities. The process housing has integral vents that assure proper gas movement within the process housing, which prevents downstream product contamination.

Adipic Acid: Adipic acid is manufactured in multiple steps of crystallization, centrifugation and remelt in order to reach the purity requirements. The adipic acid crystals are 90% larger than 150 microns, and the nitric acid concentration is decreased from 30% in the initial feed slurry down to 15 PPM in the discharged cake.

Nitric acid removal from adipic acid is critical for further processing. The pusher centrifuge has excellent wash characteristics, which is a significant advantage. In addition, a single stage design provides a more efficient backwash. This highly corrosive application demands a robust design.

Cotton Seed Delinting: Cotton planting seeds are treated with sulfuric acid to remove the lint to enable metered planting. The slurry is thick and non-flowing and is conveyed to the pusher using a screw conveyor. The acid treatment makes the lint brittle, so that the seed can be easily delinted in a subsequent tumbling operation. Discharged cake has about 10% moisture. This application shows that the pusher can be successfully used for non-crystalline products. A low speed design results in lower seed breakage, and the rugged design allows extended and reliable service in a seasonal application.

Conclusion:
Any centrifugal device offers the distinct advantage of a quick and efficient separation. Pushers, in particular, offer high throughput in a continuous fashion. Low moisture and high purity in discharged cake has made the pusher centrifuge an excellent choice for solid-liquid separation. Thousands of pushers have been sold in mineral and chemical processes for a wide variety of applications due to its high throughput capacity and reliability. Over time, the pusher centrifuge has proved to be an excellent choice to dewater a concentrated slurry of free draining particles. Design modifications have made the pusher centrifuge applicable to an even wider range of applications.

References:

1). Baumann, D. K., Todd, D. B., "WHEN TO USE A PUSHER CENTRIFUGE", Chemical Engineering Progress, September, 1973

2). Lueng, W. W. F., "INDUSTRIAL CENTRIFUGATION TECHNOLOGY", McGraw-Hill, 1998

3). Perry, R. H., Green, D. W., "PERRY'S CHEMICAL ENGINEER'S HANDBOOK", sixth edition, McGraw-Hill, 1984