A current project examines how to avoid lumps and blockages in silos with vertical mill cement.
Lumps and blockages in cement silos have long been an issue at some cement plants. In the 1970s, FLSmidth conducted research on ball mills and cement storage that revealed the cause to be an unfortunate combination of insufficient gypsum dehydration in the mill, high silo storage temperature and lengthy storage time. When stored at high temperatures, the gypsum continues to dehydrate. The water released from the gypsum prehydrates the cement in the silo, and lumps begin to form.
In more recent developments, some plants have begun to report issues in silos storing cement produced in vertical mills. Similar to the past, these issues are the result of a combination of factors such as lower degree of gypsum dehydration or high rates of water injection such that the moisture content of the cement stored in silos is increased.
Close collaboration with cement plants is an important element of FLSmidth research. It ensures that the complexities of industrial operations are considered from the outset and that the research can be applied in practice. Titan Cement is a valuable collaboration partner in this project. The FLSmidth Research and Process departments were therefore pleased to be invited to carry out tests at Titan Zlatna Panega in Bulgaria. The plant produces OPC cement with 4% limestone and max. 3.5% gypsum using an OK™ Mill 30-4.
Read more about our OK™ Vertical Roller Mill here>>
The field tests were a critical part of a study that also included laboratory tests and special analyses. The results showed that the cement storage stability can be controlled by adjusting the OK Mill temperature and water injection (if water is used). Optimum mill operation depends on specific plant conditions and constraints, including clinker temperature, availability of hot gas, climatic conditions, quality of the feed materials (clinker composition, amount of additive materials such as gypsum, limestone, slag and their water content), legislative requirements, and storage time in the cement silo.
The OK Mill field tests were performed over several weeks. Complete quality analyses were performed on cement product samples taken with the mill in operation with different process conditions. The mill outlet temperature was varied from 75 to 120 degrees Celsius and water injection was varied from zero to approximately 2.3 percent (mass) of the total production in the mill.
Mill temperature tests
Within the temperature range of the tests, the effects on gypsum dehydration and prehydration were clear.
Low mill outlet temperature
• Cement produced at low mill temperatures (app. 75-80 degrees Celsius) had a relatively low level of gypsum dehydration compared to typical mill cement. However, as the cement temperature is low, the gypsum in the silo does not dehydrate. Thus making the cement silo-safe. This supports the conventional rule-of-thumb that cement temperatures below 70 degrees Celsius are silo-safe for longer periods of time (weeks).
• At low mill temperature excessive water injection should be avoided.
• Vertical Roller Mill (VRM) cement produced with low mill outlet temperature should not be stored in a silo together with high temperature cement (>70 degree Celsius).
High mill outlet temperature
• Cement produced at high outlet temperatures (>110 degrees Celsius) had significantly dehydrated gypsum. As a result, high temperatures in the silo do not cause problems.
• High mill temperature can produce cement with gypsum dehydration similar to ball mill cement, but water injection must be low to medium.
The graph illustrating the water content from gypsum in cement shows the impact of different VRM operating conditions evaluated in the study on gypsum dehydration.
Water injection tests
The impact of water injection is explained below.
In general water injected on the mill table acts as a grinding aid to increase production and lower power consumption. Water also helps reduce mill vibrations, especially with dusty clinker. Ideally, in this case it is better to study and improve the factors impacting clinker granulometry rather than use increased water injection rates.
Water content from gypsum in cement as function of mill outlet temperature and water injection
• If water injection rates are maintained within recommended safe ranges, silo-safe cement with good quality can be produced.
• High water injection can cause prehydration, especially at low mill temperatures, resulting in lower cement strength and an increase in the total water fed to the cement silo
• Excessively high water injection rates may be problematic even at high temperatures and should be avoided.
• OK mill operation with no or minimal water injection is best for producing silo-safe cement