Lumps and blockages in cement silos, caused by unfavourable operating conditions of the cement mill, are an ongoing issue. Optimal mill operating conditions need to be specified to produce high-quality and silo-safe cement.
What is silo-safe cement?
Silo-safe cement is cement that does not hydrate in the silos and therefore does not form lumps and blockages. As water is always present during cement production, either through injection during grinding, as vapour in gases, or from the feed materials (such as gypsum, slag, trass, etc.), it is vital to ensure that the premature reaction of cement with water (prehydration) is avoided.
Prehydration in the mill may result in lower cement strength, while prehydration in the silo may result in lumps and blockages as well as a decrease in cement strength.
Testing for optimal conditions
According to research by FLSmidth coupled with decades of cement grinding experience, a low level of gypsum dehydration in the cement mill combined with high silo temperatures and lengthy storage times are generally the reasons for such problems in cement silos.
Tests were carried out at cement plants with vertical roller mills (VRMs) grinding different cement feed materials at varying operating conditions in order to highlight the most important reactions and specify the optimal mill operating conditions.
The main reactions occurring during cement grinding are:
1) Gypsum dehydration
Dihydrate gypsum ► Hemihydrate/anhydrate + Water
This released water might then react and prehydrate the cement:
2) Cement prehydration
Cement + Water ► Cement hydrates
Optimising the grinding parameters, such as water injection and mill outlet temperature is an effective way to control these reactions in the mill and later in the silo.
Potential prehydration and gypsum dehydration reactions in the cement mill and in the cement silo.
Operating parameters for ball mills
The following simple guidelines for operation of the ball mill were developed more than 40 years ago:
1. Water cooling in the ball mill should only be used for temperatures above 100°C. Furthermore, the cooling water should be sprayed correctly into the mill.
If the water injection system is partly clogged or does not function properly, prehydration might occur.
2. A mill outlet temperature of approximately 120°C is recommended to ensure that the gypsum is sufficiently dehydrated in the mill, thereby avoiding further dehydration in the cement silo.
VRM grinding parameters
One of the VRM’s great advantages is its versatility. The mill can easily grind multiple types of cement under different operating parameters and with little to no transition time. To achieve the full benefits of the VRM without problems, we should understand the effect of the different operational modes on the cement properties and quality.
FLSmidth conducted tests with OKTM mills to evaluate the effect of varying mill temperatures and water injection rates, as shown in the table. The tests were performed over several weeks in two different cement plants.
PLANT 1 produces an Ordinary Portland Cement (OPC) with low gypsum and C3A content. Here there were no special requirements as to mill temperatures.
PLANT 2 produces an OPC with high gypsum and medium high C3A content. Here the plant wanted high temperatures to simulate ball mill cement, so only 115-125°C were of interest.
Lumps blocking the silo outlet.
Both cement plants are producing water sensitive cements with a surplus of alkali relative to sulfur.
Thermal analysis was used to measure prehydration as well as the water from gypsum. Wk is the weight loss from prehydration, while Wg is the weight loss of water from gypsum.
Plant data and experience
The test data clearly show the relationship between cement prehydration (Wk), gypsum dehydration (Wg), operating temperature, and water injected into the mill:
With increasing mill temperature and decreasing water injection, the gypsum dehydration increases (less resulting gypsum water Wg). The less gypsum water remaining, the lower is the risk of silo problems
• As was expected, the prehydration is more pronounced at low temperatures (75ºC) and high water injection
• Water injection rate is of less importance, when the mill temperatures are high (>100°C)
• In order to dehydrate high contents of added gypsum to levels similar to ball mill cement, high mill outlet temperatures of 120-125°C are necessary
• The data indicates that high grinding temperatures and low water injection results in the best 28-day compressive strength
• Plant experience has shown that it may create silo problems, if cement ground at different temperatures and different gypsum water are mixed in the same silo
• High free CaO in the clinker may prevent/delay prehydration and consequently silo problems, because the free CaO will react with water first.
Recommendations for VRM operation
Based on these results we recommend the following for producing silo-safe cement:
1. Operate the VRM at low temperatures
(i.e. ~<75°C) and low water injection.
Although resulting in low gypsum dehydration (high content of gypsum water), the cement temperature in the silo is so low that no further gypsum dehydration will occur, and no water will be released in the silo.
2. Operate at high mill temperatures
(120-125°C), resulting in high gypsum dehydration and high cement temperatures that simulate ball mill cement and will be silo-safe.
3. Operate at medium temperatures
(80-120°C). If the plant does not experience problems, has short storage times and/or or the plant has special conditions for cooling the cement (cement cooler), this range of temperatures
may be used.
High grinding temperatures and low water injection result in the best 28-day compressive strength.