Understanding filtration › 2. The equipment

CHAPTER 2.4

Disc filters

Large discs half-immersed in a tank, turning slowly and emerging covered with a cake of ore: this is the tool for continuous dewatering of mineral concentrates. The filter medium takes the form of bags slipped over sectors, which are replaced one by one.

Nicolas Ract · 7 min read · October 2026

The principle

A vacuum disc filter consists of parallel discs mounted on a hollow horizontal shaft. Each disc is divided into independent, removable sectors, covered with filter medium [2]. The lower part of the discs is immersed in a tank where the slurry is kept agitated [1][2].

The vacuum is drawn through the hollow shaft: the filtrate passes through the medium and the sector, enters tubes in the shaft, then a separator where air and liquid are separated; the air goes to the vacuum pump, the filtrate to a pump [1]. A distributor valve head, at the shaft end, determines at every moment whether a sector is under vacuum or receives compressed air [2].

One revolution of the disc

• Cake formation: the sector dips in, the vacuum draws the liquid and the solids are deposited. One manufacturer indicates a formation time of 5 to 30 seconds [1].
• Drying: the sector leaves the tank, and the air drawn through the cake drives out some of the water until the vacuum is cut off [1]. Spray washing is possible on some filters [2].
• Cake discharge: a brief blast of compressed air (“snap blow”) inflates the bag and releases the cake, which falls past a scraper into a chute [1][2].

The operating settings cited by the sources: tank level, rotation speed, split of time between formation vacuum and drying vacuum, blow pressure, scraper position [1], as well as feed density, vacuum level and condition of the sectors [2]. Rotating faster shortens drying and gives a wetter cake [3].

Vacuum or pressure

Vacuum is limited by atmospheric pressure: the available pressure difference cannot exceed about one bar. To go further, the disc filter is enclosed in a pressure vessel. One manufacturer describes such a “hyperbaric” filter operating at a pressure difference of 2 to 5.8 bar, with cake discharge by air blow and cake removal through a device suited to the pressurised vessel [4]. The stated benefit: a drier cake and higher throughput on fine products (coal, bauxite, iron ore, copper concentrates, red mud) [4].

There are also ceramic disc filters, whose sectors are porous plates that draw water by capillary action, without a cloth; their proponents highlight much lower energy consumption, since air does not pass through the plates [5]. This chapter deals with filters using textile media.

Typical applications

The sources consulted place the disc filter in the minerals industries: copper, lead and zinc flotation concentrates, cyanidation tailings, coal concentrates, barite [2], preparation of iron concentrates for pelletising [3]. One manufacturer assigns it to dense slurries (specific gravity above 1.9) that are fairly coarse (a P80 size, below which 80 % of the particles pass, above 75 µm), giving cakes of 15 to 100 mm, and to products that are often abrasive [1].

The same manufacturer specifies that its filter is used for dewatering, without cake washing [1]; when washing is essential, the vacuum belt filter is often preferred (see chapter 2.3). For the specific features of minerals, see chapter 5.3.

After [2]. Capacity depends mainly on the proportion of fines and the solids content; these older figures serve to indicate orders of magnitude, not for sizing.

Filter bags

On most disc filters with textile media, the cloth takes the form of a bag (also called a “sock”) slipped over the sector and closed at its base, on the shaft side. The bag is inflated at each air blow, which also helps keep the cloth clean [2].

Field experience: the bag must fit the sector. Too loose, it flaps, folds and wears against the ribs; too tight, it is stretched during the air blow and fatigues at the seams. The fastening at the base of the sector must be leak-tight: if it leaks, the slurry bypasses filtration. Bag making-up follows the same rules as for filter press cloths (see chapter 3.4).

Wear and replacement

What wears

• The bag, through abrasion by the product and repeated flexing from the air blow. Field experience: wear is often concentrated on the sector edges, where the scraper makes contact, and at the seams.
• The filtrate tubes and the valve head: an article on disc filters in iron concentrate plants reports erosion caused by the high velocity of solids-laden air; polyurethane or rubber linings in the exposed areas markedly extended their service life [3].
• The scraper, set too close, which rubs the bag on every revolution.

The warning sign: the filtrate

The same source states that normal filtrate contains less than 1 % solids by weight, preferably 0.5 % or less; a hole in the medium can raise this figure markedly, and the solids carried over accelerate erosion of the tubes [3]. Regular monitoring of filtrate turbidity or solids content is therefore the best way to spot a holed bag.

Replacement

Since the sectors are independent and removable, changing a sector or a bag is described as quick [2]. Field experience: on a large filter, all the bags are rarely replaced at once; the holed ones are changed, and planned shutdowns are used to replace an entire disc. Record the position of each replaced bag: a disc or position that uses more bags than the others often points to a scraper, a damaged sector or poorly distributed agitation (see chapter 4.5).