Operational Problems Caused by Excessive Moisture in Raw Materials During Calcination

Moisture, seemingly an insignificant “supporting actor,” is in fact a critical variable in the calcination process that affects everything it touches. Once the moisture content of raw materials exceeds the allowable limit, a chain reaction of impacts strikes everything from the thermal balance to product quality, from equipment safety to production efficiency. Specifically, these can be summarized into the following major categories of operational problems:

I. Severe Disruption of Thermal Balance, Soaring Energy Consumption

After moisture enters the kiln, it first undergoes evaporation and vaporization—physical changes that absorb a large amount of heat. According to measured data from fluidized bed roasting furnaces, when ore moisture content is 5%, the heat of water evaporation accounts for approximately 3% of the reaction heat; when moisture reaches 10%, this proportion surges to 7%. In cement clinker burning systems, when the moisture of coal powder fed into the kiln rises from 3% to 20%, the system’s thermal consumption increases by nearly 30 kcal/kg·cl. Similarly, in rotary kiln calcination of petroleum coke, for every 1% increase in moisture content, the calcination yield decreases by 1%. A large amount of heat is “stolen” by the moisture, leaving seriously insufficient effective heat, making it difficult to raise the kiln temperature. The end result is—to calcine the same batch of material, you need to burn twice as much fuel.

II. Kiln Temperature Fluctuations, Unstable Operating Conditions

Moisture evaporation causes a sharp drop in kiln temperature. Moreover, when water transitions from liquid to gas, its volume expands dramatically, generating large amounts of water vapor that envelop the material and block fresh air from entering the combustion zone, resulting in incomplete combustion. In the use of semi-coke (Lancarb), when moisture exceeds 25%, obvious smoking, weak flames, and difficulty raising kiln temperature occur. In calcium carbide production, water reacts with red-hot carbon in a water-gas reaction (C + H₂O → CO + H₂), directly consuming the carbon raw material and further dragging down the kiln temperature. Kiln temperature fluctuates wildly, operators are overwhelmed, and operating conditions become extremely unstable.

III. Material Agglomeration and Caking, Deterioration of Permeability

High-moisture raw materials are extremely prone to agglomeration. In fluidized bed furnaces, when ore moisture exceeds 8%, the ore particles easily stick together into lumps; wet sulfur-bearing tailings with 10% moisture bond very tightly—even after being crushed by a crusher, they enter the furnace as dense masses, causing poor fluidization at the feed point and triggering accumulation and scabbing. In calcium carbide production, water reacts with quicklime inside the furnace to form calcium hydroxide, causing the material to cake into a hard layer, which not only increases safety accident risks but also worsens permeability and increases slagging. The carbon industry faces the same problem: moisture makes it difficult or even impossible to crush, screen, and grind petroleum coke.

IV. Severe Decline in Product Quality

The damage moisture inflicts on products is all-encompassing. In dense soda ash calcination, high moisture causes severe scaling, increased alkali return, reduced calciner capacity, and higher steam consumption. In calcium carbide production, water reacts with calcium carbide (CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂↑), directly destroying the active component of calcium carbide, causing product quality to plummet sharply. In carbon calcination, excessive moisture leads to decreased carbon quality, substandard fixed carbon content, insufficient strength, and the calcined products develop blisters and cracks, causing the reject rate to soar.

V. Abnormal System Pressure, Prominent Safety Hazards

Moisture evaporation increases the volume of furnace gas. With high moisture, the furnace tends to develop positive pressure, worsening operating conditions and the environment. Sudden vaporization of water can cause violent fluctuations in furnace pressure and temperature, potentially triggering spattering or even explosions. In rotary kilns, if the kiln outlet negative pressure is not properly controlled (the reasonable range is -70 Pa to -90 Pa, while many domestic plants operate at -180 Pa to -200 Pa), wet material is shattered by high-temperature gas flow, increasing the amount of fine coke drawn out and intensifying burn-off losses.

VI. Reduced Yield, Raw Material Waste

Taking petroleum coke calcination as an example, high raw material fines, high moisture, and high volatile content all lead to low calcination yield. For every 1% increase in moisture, the yield drops by 1%—meaning that with the same raw material input, less output is obtained, directly hurting economic returns. At the same time, high moisture also increases return material and aggravates sealing problems, forming a vicious cycle.

In summary: Moisture is the most easily overlooked yet most destructive “invisible killer” in the calcination process. Controlling raw material moisture is the first line of defense for maintaining thermal efficiency, stable kiln conditions, and product quality. Generally speaking, the moisture content of incoming raw materials should be strictly controlled below 6%. The carbon industry requires calcined product moisture not to exceed 0.3%, and high-end semi-coke needs to be dried to around 2%—for every one percentage point reduction in moisture, production becomes more stable and costs are saved.