Understanding filtration › 2. The equipment

CHAPTER 2.5

Candle filters and tower filters

Two machines less widespread than the conventional filter press, but ones you come across in chemicals, pigments, hydrometallurgy or catalysts. One works in a closed vessel with vertical filter elements; the other stacks horizontal chambers through which a single endless cloth runs.

Nicolas Ract · 7 min read · October 2026

The candle filter

The principle

A closed vertical vessel contains cylindrical filter elements, the candles, suspended from manifolds. The slurry is pumped into the bottom of the vessel; the liquid passes through the wall of each candle from outside to inside, rises through a central tube and leaves the vessel. The solids are deposited on the outside of the candles [1].

A technical source describes each candle as a central filtrate outlet tube, a perforated core surrounded by rods, and a filter sock slipped over it. The rods create an annular space that limits pressure drop and allows the sock to inflate during cake discharge. The candles are fixed to horizontal manifolds with a seal that prevents any bypass; a vessel can hold more than 200 of them [1].

The cycle

• Filtration: a thin cake forms, of the order of 5 to 20 mm according to this source [1]. Filtration stops on one of these criteria: maximum pressure drop, target cake thickness, minimum flow rate or maximum time [1].
• Cake washing if required, then drying with pressurised gas or air [1].
• Cake discharge: low-pressure gas inflates the socks from the inside; the dry cake cracks and falls into the vessel cone. It can also be discharged as a concentrated slurry [1].

According to the same source, synthetic socks retain particles smaller than 1 µm, and retention improves towards 0.5 µm once the cake has formed; for even finer particles, a filter aid is added [1].

The media

• Textile socks: synthetic woven fabrics or felts, chosen like filter press cloths for retention, cake release and chemical resistance (see chapters 3.1 and 3.2). Field experience: the fit of the sock on the candle and the tightness of its fastening at the top matter as much as the fabric; a loose sock fatigues during inflation, a leaking fastening lets slurry through.
• Sintered metal elements: one manufacturer offers them for catalyst recovery (nickel, platinum, rhodium, palladium) in fine chemicals and pharmaceuticals. The circuit stays closed, which limits the catalyst's contact with air, and cleaning is done in place by gas-assisted backwashing, without disposable media [2].

Applications

The sources consulted cite the clarification of liquids with a low load of fine solids: flue gas scrubber water and settler effluents in refineries, gasification grey water, with a filtrate of a few ppm, or even less than 1 ppm, in the cases reported [1]; and catalyst recovery [2]. The same source also describes an arrangement in which the candles thicken the sludge, which a filter press then dewaters [1].

The continuous-belt tower filter

The principle

The tower filter (also called a vertical filter press or automatic pressure filter) stacks horizontal plates one above the other. Each space between two plates forms a chamber. A single endless filter cloth snakes through the whole stack, passing through each chamber [3][4]. The plates are fitted with membranes, like those in chapter 2.2 [3].

The cycle

One manufacturer describes the following cycle for its machine [3]:

• filling and filtration;
• first squeeze by the membranes, using water, which evens out and dewaters the cake;
• cake washing if the process requires it, under sustained squeeze to prevent cracks and channelling;
• final squeeze, up to 16 bar on this machine;
• air drying;
• discharge: the pack opens, the cloth advances and scrapers release the cake from each chamber, with no operator;
• cloth washing by spray bars as it advances, at every cycle.

Washing at every cycle is a major difference from the horizontal filter press; it maintains the permeability of the cloth (see also chapter 4.4).

Strengths and limitations

• Strengths: fully automatic cycle, suitability for fine particles, slow-filtering slurries, sticky cakes and demanding washes, according to the manufacturer [3].
• Limitations: an article in the trade press notes high cost, limited filter area and mechanical complexity [4].

Applications

The manufacturer cited mentions alumina and hydroxides, catalysts, fine chemicals, food and pharmaceuticals, hydrometallurgy, metal salts and pigments [3]. For pigments, see chapter 5.4.

The cloth of a tower filter

At every cycle, the cloth undergoes squeezing, advancing under tension over rollers, scraping and washing. Field experience:

• it must be dimensionally stable: a cloth that stretches or shrinks disrupts advancing and tracking;
• its joint is a sensitive point, as on belt filters (see chapter 2.3);
• the edges wear where they contact the frames and guides;
• cake release must be good, since the cake is removed by scraping the cloth.

The manufacturer cited claims a cloth change in about thirty minutes and the possibility of inspection during operation [3]. As always, only the site's own history will reveal the actual service life (see chapter 4.5).