As the plants in the steelworks require soft water, Salzgitter Flachstahl has a water treatment plant in Adersheim. It has a slow decarbonization installation for softening approx. 17 mill. m3/a drinking water to create process water.
Calcium hydroxide in the form of milk of lime is added to hard drinking water. This neutralizes hydrogen carbon ions and the free carbon dioxide, which are then precipitated as calcium carbonate. The calcium carbonate is deposited in five large reaction tanks at the water treatment plant in Adersheim. The tanks are cleaned at regular intervals. With the addition of drinking water, the lime sludge deposited is flushed by gravity into a monofill located below the water treatment plant. The lime sludge is deposited in the monofill in a matter of hours. Each year, some 3,800 to 4,500 m3 lime sludge are collected in the plant.
Lime sludge has very fine grains (grain size distribution between 0 and 20 µm) and thixotropic properties, i.e. lime sludge becomes liquid with the application of mechanical energy. In order to lose the thixotropic properties, the sludge has to be drained or conditioned. Only then can the sludge be transported by truck and scattered over agricultural areas by spreader vehicles. To date, more than 66,000 m3 lime sludge have been removed from the monofill and recycled. The next recycling campaign is planned for 2013 and aims to recycle approx. 15,000 m3 lime sludge.
Lime sludge is used as agricultural lime, which is distributed uniformly over farmland by large spreader vehicles.
This material offers many advantages in agriculture, both economical and ecological. Liming agricultural areas with agricultural lime creates an optimum arable soil. Plants use the calcium in the lime to build cell walls and grow roots, which results in stronger plant growth. Liming raises the pH value. This improves plants’ ability to absorb nutrients, leading to a more efficient use of commercial fertilizers containing ingredients such as nitrogen, potash and phosphorous. Furthermore, lime prevents the eluviation of these nutrients.
They are thus available to plants for longer. In addition to increasing the number of nutrients, liming also has a positive effect on the formation of a clay-humus layer. It supports the development of pores in the soil, so improving aeration, irrigation and water storage. This counteracts silting and facilitates mechanical processing.
Agricultural lime obtained when softening water offers a wide range of advantages and, as a result, less fertilizer is required.
The reduced use and improved availability of commercial fertilizers not only saves money but also results in less contamination of the groundwater by nitrogen, phosphorous and other fertilizers. The recycling of lime sludge thus helps to converse resources in a variety of different ways.