Understanding filtration › 5. Processes by sector

CHAPTER 5.2

Sugar plant lime cake

To obtain white sugar from beet, the juice must first be cleared of its impurities. This is done with lime, and the resulting precipitate is then filtered: the carbonatation lime cake (scums). This filtration runs without interruption throughout the campaign, and it cannot afford to break down.

Nicolas Ract · 7 min read · October 2026

Where separation sits in the process

The beets, washed and cut into cossettes, pass through a diffuser, where hot water extracts the sugar. The resulting raw juice contains, in addition to sugar, non-sugar substances that must be removed before evaporation and crystallisation. This is the role of lime–carbonatation juice purification [2]:

• liming: milk of lime, Ca(OH)₂, is added to the heated juice; some processes include pre-liming followed by main liming [2];
• first carbonatation: carbon dioxide is bubbled through, precipitating the lime as fine calcium carbonate crystals. These crystals trap part of the impurities. pH and temperature must be held to obtain a stable floc [1];
• separation of the lime cake, by settling and filtration [1][2];
• second carbonatation: a further pass with carbon dioxide, at lower pH, removes the remaining lime by forming large crystals that are easy to filter [2], followed by another filtration.

The lime and carbon dioxide are produced on site, in a lime kiln [2]. The quantities involved are large: according to a specialist source, a beet sugar plant consumes around 1 to 3 % lime (CaO) relative to beet weight, far more than a cane sugar mill [1]. The lime cake therefore represents a substantial tonnage to filter every day of the campaign.

The equipment

Lime cake filtration is a historical application of the filter press: a sugar-making treatise from the early 20^th century already devotes an entire chapter to lime cake filter presses, their methodical washing and the composition of the wash waters [3]. The equipment found today falls into several families:

• clarifiers (settlers), which continuously separate a clear juice from a concentrated mud [1];
• leaf or candle filters, which clarify the juice: the floc builds up on the elements until the pressure drop requires them to be discharged [1] (see chapter 2.5);
• filter presses and vacuum filters, which dewater and wash the concentrated mud to produce a handleable lime cake (see chapters 2.2 and 2.3). Field experience: both recessed chamber filter presses and membrane filter presses are found, and the choice between these technologies and their layout varies widely from one plant to another.

Washing the lime cake

The lime cake retains sweet juice. So as not to lose this sugar, it is washed: it is resuspended in water or washed in the filter, then filtered again; the recovered sweet water goes back into the process [1]. Filter cake washing is therefore a direct economic issue here, not a simple rinse.

What wears or blinds the cloths

• General guidance, scale deposits: the medium is rich in calcium; calcium salts can precipitate in the fabric and form a crust that water alone does not remove (see chapter 4.4).
• General guidance, fine organic matter from the beet (pectins, proteins), trapped with the carbonate, which causes deep blinding.
• Temperature: the juice is treated hot, around 80 to 85 °C before first carbonatation according to one process description [2]; this is close to the continuous-service limit of polypropylene (see chapter 3.1).
• Duration: thousands of consecutive cycles, with no possibility of a long stop, wear the edges and seams (see chapter 4.2).
• Field experience: poorly controlled carbonatation (pH, temperature) gives a finer, more gelatinous precipitate that filters badly and blinds quickly. The problem then shows on the cloth, but it is solved upstream.

What matters to the operator

• Continuity during the campaign: the plant runs day and night for several months; a stoppage of lime cake filtration can slow down the whole line.
• Juice clarity: cloudy juice fouls the evaporators and degrades sugar quality.
• Sugar lost in the lime cake: this is what washing is for.
• Dry solids content of the lime cake: according to the source cited, refiltered lime cake reaches a moisture content of 50 % or less [1]. The drier it is, the less it costs to store and transport.
• The inter-campaign shutdown: this is the only real maintenance window. The cloths must have been ordered in time.

Making use of the lime cake

Lime cake is not ordinary waste: rich in calcium carbonate, it is spread on fields as lime [1]. In France, it is used as a calcium soil improver, with reference to the NF U 44-001 standard (French standard on basic mineral soil improvers) [4]. However, according to a study by the Institut français du pétrole, its high water content rules out transporting it over long distances [5]. Field experience: the dry solids content of the lime cake, and therefore filtration performance, carries through into the logistics of spreading it.

Indicators to track

The "Sugar plant" typical case in chapter 4.5 provides the framework. In practice:

• Service-life unit: the campaign, and the cycles within the campaign.
• During the campaign: the number of cloths changed outside the schedule, their position and reason; cycle time at the start and end of the campaign; turbidity of the filtered juice, as measured by the site.
• On the lime cake: dry solids content and sugar content, as measured by the plant laboratory.
• Key indicator: the number of unplanned replacements during the campaign, the target being zero.

Choosing the medium: points to watch

• Durability over a whole campaign: service life is judged in campaigns, not months. A cloth that lasts a whole campaign is worth more than a cheaper cloth that has to be changed halfway through.
• Resistance to scale blinding and suitability for acid washing, checking that the washing product is compatible with the fibre (see chapters 3.1 and 4.4).
• Fibre and temperature: the margin relative to the actual temperature of the juice and of the washes.
• Cake discharge: on a filter press, sticky lime cake lengthens every opening; a smooth surface helps (see chapter 3.2).
• Filter cake washing: the cloth must let the wash water through evenly in order to desweeten the lime cake.
• Logistics: manufacturing lead times and safety stock for the campaign. Field experience: the order is decided on the basis of the previous campaign's history, well before the inter-campaign shutdown.