Automated press filters

With regard to the main objective of this improvement project, which was to increase the global zinc recovery, it can be concluded from the data in Table IV that we are very close to the minimum expected value of 94.5%. It can also be seen that, in comparison with the years previous to 1998, the total amount of residue produced is higher. This means that the soluble zinc recovery has been substantially improved with the operation of the automated press filters. 

The automation of filter presses has affected several other advantages and developments. Plate shifting mechanisms have been developed, allowing the cloths to be vibrated filter cloth washing, on both sides, has been incorporated to counteract clogging from the expression and downtimes have been reduced with automation, thus increasing capacities. 

The SulFerox sulfur-cake matrix tends to be quite compressible and reportedly lends itself well to filtration via plate-and-fiame filters or automated batch filter presses with recessed membranes for post-filtration sulfur-cake squeezing. It is claimed that only small amounts of wash water are needed to displace the residual process solution from the filtercake. Rotary drum vacuum filters were used in all early SulFerox applications, but variations in feed slurry characteristics due to differing inlet gas contaminants and required additive levels made their performance inconsistent. It is repotted that filtration by pressurized-feed filter presses has eliminated the problem of cake quality variation and has substantially reduced iron chelate losses (Anon., 1994). The sulfur filtercake from pressurized-feed filter presses is reported to contain 10 to 23 wt% moisture (Allen, 1995). 

Both flush plates and recessed plates can be specified. Recessed plates obviate the need for the frames but are tougher on filter cloths due to the strain around the edges. These presses are more suitable for automation because of the difficulty of the automatic removal of residual cake from the frames in a plate-and-frame press. Recessed plates with no frames limit the chamber width to less than 32 mm to limit the strain on the cloth, whereas plate-and-frame presses allow this to be more than 40 mm if necessary. 

The vertical recessed plate automatic press, shown schematically in Figure 15 and described previously, is another example of a horizontal belt pressure filter. Cycle times ate short, typically between 10 and 30 minutes, and the operation is fully automated. The maximum cake thickness is about 35 mm washing and dewatering (by air displacement) of cakes is possible. Apphcations include treatment of mineral slurries, sugar, sewage sludge, and fillers like talc, clay, and whiting. 

Significant improvements were made in the 1980s and early 1990s in high capacity, automated variable volume filters that incorporate automatic pressure filtration, expression, washing, and air displacement. Some of the large plate-and-frame automatic presses can operate at up to 2 MPa (ca 285 psig), with up to 100 chambers (25,26). 

Friedland, B. (1969). Treatment of bias in recursive filtering. IEEE Trans. Autom. Control AC-14,359-367. Gelb, A. (1974). Applied Optimal Estimation. MIT Academic Press, Cambridge, MA. 

Various filter designs are available to the industry, including plate and frame filter presses, horizontal tank/ vertical leaf pressure filters with retractable bundles or shells, and vertical tank/vertical leaf pressure filters. The latter (Figure 29) has been accepted by the industry as perhaps the best overall choice in terms of price, performance, space requirements, and ease of automation. Filter sizes up to 85-m filter area/filter are available with this model. However, the horizontal tank/vertical leaf model with either retractable bundle or retractable shell also has its proponents. Commonly, a major factor in deciding on this type filter is the fact that these units are available in sizes up to 200-m filter area/filter. 

Filter-Tape Method. A filter-tape system may be envisaged for automating even the 50-fxL method. An automatic tape system for collection of aerosols on filter paper was discussed earlier. The concept could be used here to transport samples to a drying chamber and then to an electrochemical cell where the sample is remoistened with a known volume of ionic-strength adjuster and measured by pressing the opposed electrodes together as shown in Figure 5. The paper must be pretreated by addition of hydro-phobic barriers to confine the sample to the desired area of the paper. All of these steps could be automated under control of the microprocessor. 

With equipment now available, it is possible to completely automate a filtration station. By simply pressing a button, the filter can be... 

This unit is a fully automated, totally enclosed filter press. The core of the system is a pressure chamber. It can be connected to peripheral equipment, such as a dryer or bin, for a totally contained system. 

FIG. 18-126 Automated recessed-plate filter press used in mineral applications. (Dorr-Oliver EIMCO.)... 

Traditionally, either plate and frame filters or pressure leaf filters have been used for bleach-clay removal. The sequence of change in usage was approximately as follows plate and frame filters, pressure leaf filters, self-cleaning closed filters, and automated filters. Pressure leaf filters began to replace plate and frame presses for several... 

Once the workstation PC reaches a ready status, press inject to remove the lit LED on the Dionex Automation Interface. Fill the 50-pL injection loop which is located at the sample port on the chromatography/SP with a filtered, aqueous sample. Press the inject and immediately proceed to the 900 interface box (PE-Nelson) and press start. ... 

Basically, this unit is a filter press with an automatic cake-discharge system. With the conventional filter press, hours of work are required, especially for cake discharge. In order to minimize these shortcomings, yet to maintain and further develop the advantages, filter press automation was pursued by the following three means ... 

To justify the substitution of the installed vacuum rotary filters by more efficient equipment, a pilot filter press was installed close to the lead-silver and jarosite residues underflow lines. Tests performed with the press on-line indicated that both residues were easily filtered with high soluble displacement efficiencies. On this basis, two new automated filter presses with 85 recessed 1500 x 1500 mm plates were bought. These filters substituted six 4.2 x 4.9 m vacuum rotary filters. 

At the end of the filtration cycle, the dewatered filter cake must be removed from the fabric in preparation for the next cycle. It is important that the cake is effectively discharged at this point since any delays will lead to extended filtration cycle times and therefore reduced process efficiency. This is particularly apt in filter press operations, where manual intervention may be necessary to remove sticky cakes. As a consequence, in addition to longer cycle times, the cost of the operator must also be considered. To some extent this topic may be linked to the cake moisture content because, broadly speaking, wetter cakes will adhere more tenaciously to the cloth. This problem has been partly addressed by the equipment manufacturers with the incorporation of high pressure wash jets and brush cleaning devices, and the filter media producers also continue to pursue the development of fabrics that will facilitate the ultimate goal of perfect, unassisted cake release and hence the achievement of a fully automated operation. 

These include the cost of labour, the cost of replacing components and the rent and rates on the area of land. A comparison of the latter is very unfavourable to diaphragm cells where almost twice as much land is necessary because of the low current density and the need for evaporation plant. Membrane and mercury cells have similar land requirements because the former are packed together closely in a filter press and to some extent the electrodes scale vertically. All the processes are largely automated and hence the difference in labour costs is likely to arise because of a variable incidence of component and pipework failure. Certainly the need to replace diaphragms on a routine cycle increases the labour costs for the diaphragm cell process. 

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