Case Study

The global sugar sector is an increasingly competitive industry where cost reduction and increased productivity is of the essence. The ISSF reports that an assessment of the performance of basic stainless steel grades in the European sugar industry has shown that the use of these grades has led to a 50% drop in plant maintenance costs.

Corrosion and rapid wear of factory equipment are widely recognised as the major contributors to production costs and quality problems in the sugar industry. The root cause of this is the fact that the processing of sugar cane is highly corrosive and/or abrasive. In the early years, producers of sugar almost exclusively used carbon steel in the plants.

This design was based on the perceived low costs of the material. This was ultimately the incorrect decision since carbon steel does not have high resistance to either mechanism. The processing environment is created by a mixture of abrasive particles, moisture,heat, and acidity creating a very hostile  environment for regular materials. However, to the utility ferritic this is home.

BACKGROUND TO 3CR12

For historical reasons, users and potential users of stainless steel believed only austenitic grades with higher levels of chrome and nickel would be suitable for these types of environments. This is based on the false perception that nickel contributes mainly to corrosion resistance.

The truth is that nickel enhances weldability and ductility but does not contribute to corrosion resistance.

Nickel is also an expensive alloying element and contributes to the higher pricing of austenitic stainless steel (also known as the 300 series).

However, although ferritic grades offer a lower and more stable costing structure, not all ferritics are weldable to the required level for the industry. The locally developed utility ferritic known as 3CR12 would be the solution. 3CR12 is recognised as the original 12% chrome utility ferritic grade. Internationally 3CR12 is also designated as type 1.4003. Whilst ferritic grades are not readily weldable above 3mm gauge thickness, 3CR12 is a weldable ferritic utility stainless steel and is not normally prone to stress corrosion cracking as is the case with austenitic stainless steel. It is selected for its corrosion resistance, strength and toughness and is particularly suited to wet materials handling due to its ability to resist abrasion induced corrosion. 3CR12 was developed as a superior alternative to coated carbon steel, COR-TEN, and aluminium.

Examples of 3Cr12 in the sugar industry:

Figure 1

Figure 1 shows components of the main cane carrier. Note the reflectivity of 3CR12 sides polished by service since 1985. Originally designed from 10mm thick material, these sides still measure thicker than 9mm after 18 years of service. Records show that carbon steel would last a maximum of ten years in this application. This also illustrates the self-cleaning ability of 3CR12 and stainless steel in general.

Figure 2

The cane carrier, often described as the cane conveyor, is the moving apron that conveys the cane to the factory and assures the feed to the mills by transporting the cane from the yard to the crusher. Figure 2 shows the exit from the cane carrier with 3CR12 visible on the inside of the opened lid. 3CR12 is also used on related equipment such as cane levellers.

Figure 3

The juice heater consists of an assembly of tubes; the sugar cane juice circulates through the tubes and the vapour outside them. Suitable headers force the juice to pass a certain number of times from bottom to top and from top to bottom of the heater by restricting the juice each time to a few of the tubes.

A visual comparison between 3CR12 and mild steel on scalding juice heater covers is shown in Figure 3. Note the corroded condition of the mild steel of the unit at the back and the good condition of the 3CR12 cover in front. Both units have a similar life span.

In the sugar industry, a major component of the extraction process is a device known as a diffuser. Crushed and shredded cane is repeatedly washed by warm water as it travels along a conveyor, leaching the sugar from the cane. Similar evidence can be found at the diffusers where lids made from 3CR12, and mild steel can be compared over a similar service life. The two images in Figure 4 show the difference between the two grades. The picture on the left shows a 3CR12 lid on a diffusor after several years in service. On the right, is a lid made from mild steel after similar service life.

Figure 4

Figure 5

Centrifugation is a mechanical process, which has the function of separating or clarifying a mixture, from different densities of its components. The centrifuges used in the production of sugar are designed for processing massecuite, a mixture of sugar crystals and molasses, which is produced by the crystallisation phase of sugar refining. The centrifugal spins the massecuite in a perforated basket at speeds of up to 1200 rpm. 3CR12 is used for the centrifugal exterior and interior. Figure 5 depicts the 3CR12 interior of the centrifugal.

Figure 6

Effluent and wastewater handling suit the use of 3CR12. The remarkable difference in corrosion resistance offered by the utility ferritic is illustrated in Figure 6 showing a 3CR12 frame with a carbon steel grid. The frame and grid are the same functional age and while the grid is for all practical reasons destroyed, the frame remains in very good condition and even shines.

The two images in Figure 7 show in detail the different levels of corrosion for stainless steel (left) and carbon steel (right) grids installed at the same time and exposed to the same effluent.

Figure 7

CONCLUSION:

Important lessons are to be learned through the experience of the sugar industry. It shows us the importance of having technical staff and process engineers aware and informed on proper grade selection, the mechanisms for corrosion and Life Cycle Costing. The informed person knows that stainless steel is unbeatable in virtually all applications and is Simply Brilliant!