Vacuum belt filters

The main features in which the Radford process differs from the batch operation are in thermal dehydration and compounding. Water-wet nitrocellulose on a continuous vacuum belt filter is vacuum-dried followed by hot air transfusion (80°C) to reduce the moisture to less than 2%. After cooling, alcohol is sprayed on the nitrocellulose to a concentration of 15—20%. The alcohol-wet nitrocellulose is then transferred from a surge feeder to a compounder by a continuous weigh-belt along with the other ingredients of the composition, which are also weighed and added automatically. 

An example of the concentration effect on the specific cake resistance is available (12) that reports results of some experiments with a laboratory horizontal vacuum belt filter. In spite of operational difficulties in keeping conditions constant, the effect of feed concentration on specific cake resistance is so strong that it swamps all other effects. 

Figure 7.17. Principle of the Pannevis vacuum belt filter...

Vanadium is stripped from the organic solution with 1.5 mol dm sulfuric acid to a concentration of about 50gdm V. After an iron removal step with strong sulfuric acid, the organic solution is washed with water and recycled. From the vanadium strip liquor, ammonium polyvanadate (APV) is precipitated by oxidation and addition of ammonia. The APV slurry is thickened and pumped to a vacuum belt filter, where the APV cake is carefully washed with fresh water. The APV filter cake is dried and then calcined to vanadium pentoxide. 

Solvent Fractionation—is the most efficient fractionation process. Crystallization is performed in the presence of a solvent, usually acetone, at a ratio of between 3 and 5 to 1 (solvent to oil). Separation is usually performed on a vacuum belt filter. The high separation efficiency and the purity and yield of the solid fraction are the main advantages of solvent fractionation. The high investment and operating costs prevented the adoption of this process for commodity products such as margarines and shortenings. 

Vacuum belt filters for installation in integrated test plants have been described [Maier 1990]. These compact filters allow the functions of filtration of the suspension with removal of the mother filtrate, one or more washing zones with recovery of the wash filtrates, drying of the cake, and washing of the recycled belt to be incorporated. The adjustable working length of the belt is about 1 m for a belt width of about 5-lOcm.The solids throughput is about 0.5-10 kg h . ... 

Although widely used in dust collection, nonwoven fabrics have found only limited use in liquid filtration because their thickness and density render them prone to blinding in many applications. One area where they have found some success, however, has been in the filtration of metal ore concentrates such as copper on horizontal vacuum belt filters. These applications tend to be very aggressive on the filter fabric, and hence a suitably designed and finished nonwoven fabric is often more cost effective than a considerably more expensive woven fabric, especially if required in lengths of around 80 m and widths up to 6 m. The solids which are captured in such applications quickly form a cake on the surface and, should some penetration occur, as with woollen spun yams, the bulky nature of the material provides scope for the particles to escape. For such arduous applications, nonwoven fabrics are generally in the area density range of 800-1000 gm- ... 

Once the reaction is complete, the reaction mixture is passed over a suitable separator such as a vacuum belt filter, rotary filter, filter press, or decanter to separate crystals from the mother liquor and washed free of excess reactants (see Figure 22.12). The hot deionized wash water may be evaporator condensate, which is recycled from the mother liquor concentration process. Volumes and, therefore, evaporation costs can be minimized by using a countercurrent washing process. The effluent wash water has a pH value between 9 and 12. The mother liquor filtrate and wash water contain excess of sodium hydroxide and possibly an excess of alumina. The complete utilization of the mother liquor is essential in the manufacturing of zeolites. Therefore, it is concentrated in... 

Coal Coal dewatering Rotary vacuum belt filter 1 Dimensional stability to ensure... 

Figure 6.25 Precoating of a horizontal vacuum belt filter with hydrocyclone underflow...

For those filters which allow variations in cake formation time within a fixed cycle time (such as the horizontal vacuum belt filter, for example), the advantage of pre-thickening is that for thicker feeds the cake formation time can be shortened, thus giving more time for dewatering, washing or other cake-processing operations. On the other hand, it can be shown (see Svarovsky or section 12.2.1.3) that in an optimum cycle time, at... 

Svarovsky, L. and Walker, A. J., The effect of feed thickening on the performance of a horizontal vacuum belt filter , 1st World Congress Particle Technology, 331 Event of EFCE, Part IV, NMA mbH, Nuemhurg (1986)... 

The cake can, however, be washed in situ, without the need for reslurrying and repeated filtration. Instead, a whole washing train of, say, three stages can be placed on just one horizontal vacuum belt filter (or two stages on a high-duty vacuum drum filter ). 

Figure App. 15.1 illustrates the steps required to accomplish a three-stage countercurrent wash in a laboratory test. It is easiest to understand the principle behind this test by visualizing a continuous horizontal vacuum belt filter in operation (an indexing type is referred to here but the same applies to the models with continuous belt movement). The slurry enters the feed zone and the cake is formed (Step 1). The cloth indexes (follow Leaf C in the figure) and the formed cake move into the first wash area (Step 2). Another...

The new process scheme is shown in Figure 9-39. The diagram depicts much of the existing plant equipment used for the operation of the Stretford process prior to the LO-CAT conversion. The revamped process was based on the conventional LO-CAT two-step absorption cycle with a venturi scrubber followed by an absorber column. The sullur separation method took advantage of the available equipment and followed the standard Stretford technique of skimming off the sulfur froth in the second oxidizer tank. The sulfur slurry from the fioth tank, containing about 20%. solids, was pumped to a vacuum belt filter where most of the solution was reclaimed through a wash process. The solid sulfur (80% solids content) was then sent to waste disposal. 

From the final pond the concentrated brine (Table 1.3) with a density of about 1.25 g/cc was pumped nearly 4.8 km (3 mi 1.5 mi in 1967, Gadsby, 1967) to the processing plant in the town of Silver Peak. The plant had been converted from a silver ore cyanide-leach plant that had operated there from 1864-1961. In the conversion all of the tanks and settlers were rubber lined to reduce iron contamination in the product, and considerable new equipment was added. The solar pond brine was first reacted with lime to remove most of the residual magnesium and some of the sulfate and borate ions, and then a small amount of soda ash was added to precipitate most of the calcium from the lime reactions. The slurry from these operations was settled and filtered, and the overflow solution sent to storage tanks. From there the brine was pumped through filter presses to be totally clarified, and then heated to 93°C (200°F lithium carbonate has an inverse solubility) and reacted with dry soda ash and hot wash and make-up waters to precipitate the lithium carbonate product. Extra water was added to prevent salt from crystallizing, since the pond brine was samrated with salt. The lithium carbonate slurry was thickened in a bank of cyclones, and the underflow fed to a vacuum belt filter where it was washed and dewatered. The cyclone overflow and filtrate were... 

Gas-tight vacuum belt filters are a valuable development in belt filtration technology. They are required where the solid materials or liquids involved in the process give off fumes, which may be toxic or chemically explosive and/or just create... 

The filter press has been very successfully applied to the dewatering of a range of sludges, especially those from water and waste treatment processes, and competes strongly in this application with the vacuum belt filter and the decanter centrifuge. 

Small belt presses can be used as the final dewatering stage on a vacuum belt filter, or even on a rotary drum filter. When added to a belt filter (as in Figure 3.74), the vacuum dewatered cake is taken by the belt around a series of rollers and squeezed between a subsiding belt acting on top of the cake and a series of taut narrow belts acting on the underside of the main belt, with gaps between them for the liquid to... 

Figure 3.74 Vacuum belt filter with auxiliary belt press...

Rotary vacuum drum and vacuum belt filters are extensively used on many process filtration operations, to provide filtration, clarification, dewatering or concentration. Moulded thermoplastic vacuum drum filters are particularly suited to providing continuous... 

The impureties contained in the spray dried panicles (sulphates) are removed by successive filtra-tions and washing s of aqueous suspensions of these particles. These steps are simultaneously operated on a vacuum belt filter. 

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