The main scale constituents are calcium salts, silicates and sometimes sugar carbon. Other inorganic scale constituents are of minor importance or rather special scenarios. In addition to the chemical composition, the removal of scale is influenced by its structure. Scale built up from layers (see figure) is often more difficult to remove and may require repeated chemical treatment.

Calcium oxalate occurs in beet and cane sugar factories and is formed in the crops by various reactions from precursor substances. It remains the most important scale constituent in the sugar industry and limits uptime if the juice is not treated. Calcium oxalate-containing scales exhibit a wide variation of colors and structures. Examples are shown in Figures 2 and 3.

In the second, acidic step, the calcium carbonate formed is dissolved with an acid containing a corrosion inhibitor to protect the system.

In practice, formic, sulfamic or hydrochloric acid are used. The formation of calcium oxalate deposits is no longer considered a problem in the European sugar industry, as their formation can be well prevented by thin juice softening, the use of polyacrylate-based antiscalant, or a combination of both techniques.

Calcium phosphate is the main compound that occurs in cane sugar factories in the initial stage and limits their operation time. It can be removed by acid cleaning with inhibited formic or hydrochloric acid. Most calcium phosphate enters the evaporators as particles rather than as dissolved ions. Calcium-phosphate-containing scales often have a less dense but thick structure (Fig. 5) and a dark color, indicating that they have originated from sludge particles rather than from crystallization of ions. Therefore, the most effective prevention is the optimization of the juice purification process, especially in sedimentation and filtration. Calcium phosphate scales, which are later formed by crystallization, can be prevented by using polyacrylate-based antiscalant.

Calcium aconitate is one of the essential scale types in South America. If the juice is not treated, it can cause significant operational problems with late effects.

Calcium aconite scales are very difficult to remove, especially mechanically. They can be easily removed chemically with formic acid and an inhibitor. Calcium aconitate formation can be prevented through the use of polyacrylate-based antiscalants.

When citric acid is used to chemically clean evaporators, calcium citrate scale can form because the soluble calcium citrate complex known from low-temperature cleaning applications is not stable at high temperatures. Therefore, citric acid is not recommended for cleaning evaporators at high temperatures.

Silicates are found in beet and cane sugar factories. While silicates in the cane sugar industry come from sugar cane, the main source of silicates in the beet sugar industry is limestone. In scale, silicate is usually not present as simple silicate. Silicates in scale have complex polymer structures and often occur as aluminum, magnesium and calcium silicates, especially in beet sugar factories.

• Prevention: The silicate content increases with decreasing pH at the point of use, therefore: do not use fresh milk of lime for second carbonation.
• Prevention for cane sugar factories: Use of polyacrylate-based antiscalant

Sugar charcoal and caramelized sugar are formed when sugar is burned on hot surfaces. This often happens as an accident during a power interruption or other unexpected malfunction. This type of encrustation shows an increasing tendency, mainly because the risk of accidents increases with the duration of the campaign. Outages caused by internal and external problems with power grids are being reported from a growing number of factories. In addition, the success of falling film and plate evaporators in the sugar industry has increased the risk for such scales, as these types of evaporators are more susceptible to the formation of sugar charcoal in the event of a failure (Fig.).

If the pipes or the space between the plates are not completely clogged, the sugar charcoal can be removed with an alkaline solution containing suspending agents and wetting agents. The suspending agent is important because the solution often removes the scale from the surface, but does not dissolve it 100%. Modern evaporators have an emergency system for water supply in the event of a malfunction to avoid the formation of sugar carbon. In cases where sugar caramelizes during standard operation, optimizing the juice distribution of the juice distribution system can reduce the problem.

Antiscalants for use in sugar juice are based on polyacrylate, and this chemical is restricted by national and international regulations. A polyacrylate-based antiscalant prevents scale formation by stabilizing small, still invisible microcrystals of calcium salts that form as the juice evaporates and concentrates. Without antiscalant, the microcrystals grow, stick together and form scale in this way. The antiscalant is able to attach one end of its molecular chain to the surface of the microcrystals and prevent them from growing further. As a result, the microcrystals remain stable in the juice, so that from a technical point of view they behave as if they were still in solution.