In many industrial applications, fine particles are agglomerated to form larger particles — for sale to final consumers or for ease of material handling for downstream processing. Typically, the agglomeration process involves the addition of moisture and binder to form the larger particles. The additional moisture normally must be dried from the final particles.
For the typical mineral processing operation of a process plant, fines are generated during the drying and calcining of the ore. These fines are collected using cyclones or electrostatic precipitators (ESPs). Then, the fines are recycled back into the process or discarded in settling ponds. Unless agglomerated, the recycled fines will reduce the operating capacity of the process line.
Handling Fines During Mineral Processing in Rotary Kilns and Furnaces
While processing ores in rotary kilns and electric furnaces, the handling of associated fines can lead to bottlenecks in the process or issues involving storage or containment. Drying these agglomerated fines and introducing them back to the process offers benefits such as a decrease in recycled fines (and, therefore, an increase the process throughput) and improved energy efficiency.
Fines can be a major issue in the handling of ores. The fines contained in the mined ore can easily range from 7 to 14 percent. Fines are inherent in the ore as it arrives at the processing plant, and they are generated as the ore is processed through the kiln and electric furnace. As the ore is being processed, these fines collect in the exhaust gas streams. Recycling the fines can create a bottleneck in the kiln/furnace process because the fines continue to build up if they are continuously recycled. Collecting the fines and storing them in settling ponds is an option, but as the percentage of fines increase, there are financial incentives to recycle these fines back into the process to recover the metals.
Plants have investigated various methods of agglomeration such as extrusion or disk pelletizing. Both of these processes add moisture to the fines to aid in the agglomeration. The mechanical forming of these agglomerations will not add enough strength to survive in the kiln, which leads to a continuous recycle of the fines. The added moisture also will increase the thermal load on the kiln, which can affect the temperature profile and, therefore, the calcining performance.
Drying of the agglomerated fines prior to recycling back to the kiln addresses two concerns regarding the recycling of the fines:
- Drying can increase the mechanical strength.
- Drying can remove the added moisture to minimize the thermal impact on the kiln.
Although many types of drying designs exist, a conveyor dryer was chosen for testing because the design allows for a static bed of material, zonal temperature control, absolute control of retention time, and use of hot exhaust gases from other processes as the heat source for drying.
Design of an Industrial Conveyor Dryer for Fines Agglomeration
The conveyor dryer tested for fines agglomeration included several key features.
Conveyor Assembly. The conveyor assembly carries the product through the dryer. It must be strong enough to carry the load of the product and be able to run continuously for many years with minimal maintenance. It also must be open enough to allow air to pass through the product, but at the same time, the openings must be small enough to contain the product on the bed.
The conveyor bed itself is typically made of either mesh belting or perforated sheet metal bedplates. The mesh belting is supported by slider bars or a traveling frame that supports the load of the product. Bedplates often include integral stiffeners and are designed to support the weight of the product. The bedplates are typically 4 to 12 inches (100 to 300 mm) wide in the direction of travel and 3.3 to 13.5 feet (1 to 4 m) wide across the machine. The openings in the bedplates are either round holes or slots. Choosing the correct perforation size and percent-open area or mesh size is critical to the proper dryer operation. The openings must be small enough to contain the smallest product. A plugged bed will not allow air to flow through; therefore, drying rate and/or uniformity will suffer.
The conveyor bed can be actuated by either driving the conveyor bed directly or by driving bed chains attached to both sides of the conveyor. The arrangement with attached conveyor chains is the most common.
Because the product usually is stacked at a depth of 1 inch to 3.3 feet (25 mm to 1 m) on the conveyor bed, some means of containing the bed of product along the sides of the conveyor bed must be incorporated. This typically is accomplished by side guides. Travelling side guides attached to the bedplates contain the lower part of the bed and stationary side guides attached to the dryer frame contain the remainder of the product bed. The gap between the travelling and stationary side guides must be tight enough to contain the product and to minimize bypassing of process air.
Dryer Enclosure. The dryer enclosure usually is constructed of a steel frame with insulated panels and doors. The enclosure must contain the hot air and resist corrosion. Dryers that will be used to dry corrosive products or that will be washed down with water typically have a stainless steel enclosure. A properly designed dryer enclosure will allow thorough, easy access into the enclosure for cleaning, inspection and maintenance.
Air Fans. The dryer fans can be either integral to the dryer or externally mounted. Often, the fans that circulate the air through the bed are separate from the fans that supply or exhaust air from the dryer.
Proper fan design and fan specifications are critical to proper conveyor dryer operation. Most conveyor dryers will operate with a range of products and production rates. The fans must be selected to be as insensitive to these variations as possible. The most common type of fan used is the backward-inclined fan. This fan is chosen due to its non-overloading characteristics, high efficiency, low noise and relatively low cost.
Heat Source. Heat input to the conveyor dryer commonly is by combustion of natural gas or by indirect steam heat through steam coils. However, a number of other heat sources also are used. They include LP gas, fuel oil, thermal oil or electric heaters. The heat source normally is installed directly in line with the circulated air.
An alternative method for controlling the temperature of the drying zones is to blend hot makeup gases with the circulated air. This involves balancing the amount of makeup gas with exhaust gas to control the internal pressure in each drying zone. The amount of makeup gas required for each zone is a function of the evaporation load.
In the mining ore application described in this article, hot exhaust gas/air from the kilns was used as the source of heat rather than a direct heat source in the dryer.
Product Feeder/Spreader. One of the most critical components on the conveyor dryer is the product feeder or spreader. As in any convective drying system, it is imperative that airflow through the product be uniform. Convective heat and mass transfer is proportional to airflow velocity past the product.
Product can be loaded onto the conveyor in a number of ways. Oscillating feeders can consist of either an inclined spout or a conveyor (belted or vibratory) that oscillates from side to side across the bed. Bias-cut vibratory feeders comprise a vibratory conveyor that becomes narrower as the product is conveyed across the bed. The product drops off the narrowing edge of the feeder onto the conveyor, effectively feeding the product across the bed.
Whichever type of feeder or spreader is used, its importance cannot be overemphasized. The characteristics of the product must be taken into account when selecting the feeder type. If the feeding or spreading is not right, the dryer will not be able to dry the product uniformly. The end result will be poor product quality, wasted energy and reduced capacity.
Sizing an Industrial Conveyor Dryer for Fines Agglomeration
When sizing most process equipment — and dryers are no exception — data must be generated on a pilot scale in order to develop the size required for a full-scale production plant. During drying, the characteristics of the product and the shape of the agglomeration can greatly influence the size of the equipment.
For extruded ore fines, the diameter and length of the extrusions will determine the surface-to-volume ration. Because evaporation can only happen at the surface of the material, this area should be maximized. During drying, a critical moisture level will be reached where the internal diffusion of the water will determine the drying rate. For extrusion, this means determining a practical minimum diameter.
The initial pilot testing involved fines extrusions with a diameter of 1.969 inches (50 mm). As shown in figure 1, the extrusions were allowed to break off from the die face based on the strength of the material. This resulted in a length of approximately 3.15 to 3.937 inches (80 to 100 mm).
Drying-rate tests were conducted for extrusions of 1.969 and 0.984 inches (50 and 25 mm) diameter. Under the same drying conditions, the overall drying time was in direct relation to the diameter of the extrusions.
Pilot-scale testing of extruded and pelleted fines was conducted to determine the optimal drying conditions based on the rate of evaporation and the final mechanical strength of the agglomerated fines.
The drying tests covered a range of temperatures from 347 to 401°F (175 to 205°C) and a range of air velocities (figure 2). Tests 5 and 6 used the same process conditions, but the effective diameter of the pellets was reduced from 1.969 inches (50 mm) to 0.984 inches (25 mm).
As shown, the drying time can be reduced by 50 percent with a decrease in the diameter of the extrusions. The mechanical performance results of the smaller diameter extrusions also were superior. This seemed to be due to the center core of the pellets being dryer relative to the center core of the large-diameter pellets (table 1).
Additional tests were performed on the 0.984 inch (25 mm) diameter extrusions to determine the sensitivity of the mechanical strength to moisture content and drying-air temperature. Samples of extrusions were dried to 5 percent moisture content using air temperatures ranging from 392 to 752°F (200 to 400°C). As shown in figure 3, the fines level (less than 0.010 inches [0.25 mm]) after the mechanical strength testing remained constant. During drying, the pellets showed increased cracking and fissuring at the higher drying air temperatures. This caused the decrease in the percentage of particles above the 0.374 inches (9.5 mm) size.
Samples of extrusions were also dried to a range of 1 percent to 9.25 percent moisture using a 392°F air temperature. The mechanical strength test results are shown in figure 4. The mechanical performance of the pellets improved as the dried moisture content decreased. The fines level below 0.010 inches for extrusions dried to 1 percent moisture was 50 percent of the fines level for extrusions dried to 9.25 percent (figures 3 and 4).
When Agglomerating Fines, Drying Tests for Spheres
Another method to agglomerate the fines is to form balls and add a proprietary binder to improve the mechanical strength. Again, the use of conveyor dryers will allow for drying the spheres in a static bed. Using a dryer to develop a temperature profile during drying will provide the best mechanical characteristics (table 2).
As shown in figure 5, the best set of mechanical results was for test number 5. Test 5 provided an intermediate temperature for drying that allowed the binder to cure and provide the desired mechanical strength to the spheres. Drying the spheres at a faster rate does not provide the necessary time-temperature profile to allow optimal curing. Even though the physical size of the dryer can be reduced, the spheres will not have the strength to survive the kiln operation. This will result in an increase in fines recirculation.
Agglomeration's Energy Impact on Kiln Operations
The agglomeration of fines will require a large amount of water that then must be evaporated as the fines are reprocessed. For the extrusions example, the dryer was designed for a recycle fines flow-rate of ~115,742 tons per hour (105,000 dry tonnes per hour). This will require the addition of ~72,753 pounds per hour (33,000 kg per hour) of water in the agglomeration process. If the extrusions were sent directly to the kiln, there would be an additional heat load on the kiln of 74x106 kJ per hour to evaporate the water. As discussed, the conveyor dryers can use exhaust gases from the kiln and dry to 5 percent moisture content. This will reduce the evaporation load on the kilns to 11,023 pounds per hour (5,000 kg per hour), or 11x106 kJ per hour. This is a reduction of 85 percent.
In conclusion, the traditional route of sending wet agglomerated fines to the kiln has several undesirable effects on the plant operations. Because the kiln is not designed for evaporation, the energy efficiency is poor, and the temperature profile in the kiln is also affected. The wet agglomerations have a low survival rate, which means the dust-recirculating load is high. This affects the overall production level.
Drying the agglomerated fines in a conveyor dryer will improve the strength. Testing showed that forming spheres with controlled drying also improved the performance of the binder. This will lead to a reduced recycle rate of fines to the kiln and, therefore, lead to an increase in the plant production rate.
The use of the kiln off-gas as the source of thermal energy for the dryer will reduce the overall energy demand of the plant and results in a reduction of the greenhouse gases.