A processor of aluminum silicate minerals (i.e. clay) was experiencing inefficiencies in its particle reduction process. The inefficiencies were so severe that the corporate office began to inquire.
At this facility raw materials are brought in and stockpiled both in indoor and outdoor areas. The processing of the material involves drying and particle size reduction milling. Some of its customers require course granular materials and others require the minerals to be reduced to a very fine powder. The finer powders are primarily used in porcelain and ceramic markets, where the largest demand is. The main issue was that the company could not achieve efficient processing of the finer powder. The final step in the process consists of passing the milled powder through a screening classifier. The product that is successfully reduced to the desired particle size passes through the screening classifier. If the material has not been reduced to the desired particle size it will not pass through the screeners. This out-of-specification material is ejected and re-directed back to into the milling process using a pneumatic (pressure) conveying line. The diameter of the conveying line (metal pipe) is approximately 8 in.
Investigation indicated that a significant amount of material was not being milled to the desired particle specification (the finer powder) on the first pass through the mill. The classification screeners were catching the out-of-spec material and re-directing it back into the mill. Of course, this re-direction took capacity, time, and energy away from the ability to process additional new material entering the mill. The mill itself was properly designed and maintained to mill to any size particle in one pass.
Additional examination revealed the moisture content of the raw stock was directly proportional to the ratio of the amount of material being re-directed back into the mill. The moister the material, the more difficult it was to produce the fine powders to specifications. It was tough to control the moisture level of the raw material since it arrived from different sources and was stored in various indoor and outdoor areas of the facility. One answer was to increase the temperature of the dryer. However that resulted in inefficiencies in the drying process since the moisture content of the feedstock varied. The dryer had to be set at a temperature that would sufficiently dry the worst-case damp feedstock. When relatively dry feedstock was processed, the hotter dryer setting was simply wasting energy.
As a solution the mineral processor implemented a QuantiMass ultra mass flow measurement system, which was installed on the pneumatic conveying line that redirected the out-of-spec material back into the mill. The QuantiMass would provide real-time measurements of the quantity of material passing back into the mill. An increase in the flow rate reported by the QuantiMass system indicated the mill efficiency was decreasing. Adjustments could then be made to increase the dryer temperature to ultimately restore efficiency.
The company felt an in-line (also referred to as on-line), continuous solids flow measurement sensor would help them achieve its goals. The real-time monitoring of the flow-rate for the actual amount of feedstock being ejected and re-directed from the screener to the mill would allow them to determine quickly that the material was too moist and the process was becoming inefficient, which would allow them to make appropriate adjustments to the dryer. They could easily install the sensor into their process. Plus, the company connected the solids flow measurement sensor to the optional controller to have immediate on-site visibility to the flow data.
Before the sensor was installed, the company and sensor manufacturer collaborated to determine the location in the pneumatic line to place the sensor. Using the optional controller, the company was able to have a visual of the output signal in an easily accessible and visible location. The QuantiMass system has been successfully working and the company is pleased with the results. It has incorporated the QuantiMass solids flow sensor in various processes in multiple facilities.
In order to achieve maximum accuracy from this type of solids flow measurement sensor, a few factors should be taken into consideration:
- Vertical pneumatic or gravity conveying lines are usually best. Horizontal installations are possible, but there is the potential for compromised accuracy. Settled materials sliding down a chute are to be avoided.
- A steel pipe or chute is needed. It is acceptable to sleeve the sensor location with steel.
- Pneumatic conveying must be dilute phase conveying only, not dense phase conveying.
- Particles must be airborne and the velocity of the conveyed material should be constant.
- It is best if particle sizes are same size, or if size differences exist, the mixture of sizes should be consistent.
- The sensor should ideally be installed in a location that is in a straight section as far away as possible from pipe transitions, bends, or elbows in the line.
- If the conveying line is to handle different material at different times, it is possible to set up the sensor, like the QuantiMass system, for separate calibration profile(s) for different materials. The QuantiMass can store up to 24 different calibration profiles. For remote switching between profiles an optional BCD switch is available.
Compact, in-line solids flow measurement sensors are commonly based off of microwave technology. The measurement process of the sensor is centered on the Doppler effect. The mass flow-rate is determined by evaluating the frequency and amplitude changes during the measurement process. Reproducible measurements can be made for most powders, dust, pellets and granules, usually up to the size of 0.75 in.
Common reasons for operating these types of sensors include obtaining in-line, continuous solids flow measurements for accurate validation of material quantities, measuring for proper mixing (blending) of additives in specific recipes, and monitoring for variable flow quantities due to disturbances like different densities.
Some of the key criteria that the company was looking at when it chose the QuantiMass system were:
- Ability to easily incorporate the system into their process and receive continuous, real-time measurements without the use of weight scales
- Compact sensor size and design for simple installation into existing processes
- The proven microwave Doppler effect technology to provide accurate and reproducible flow measurements
- Sturdy, non-intrusive sensor design that minimizes maintenance and wear & tear on the instrument (this also means no obstruction to the material flow)
- Fast measuring and adjustable sensitivity to produce quick, precise data for the specific material being processed at the time
- The possibility to do totalizing. The system has an integrator/totalizer feature that can provide a sum of the mass flow for a period of time that can be obtained via the controller or an output signal can be provided to an existing PLC to accomplish external data logging functions.
- Application versatility: the sensor is suitable for powders, dust, pellets, and granular up to 0.75 in. in pneumatic conveying and free-fall processes.
In summary, an in-line solids flow measurement sensor can be an easy way to incorporate a system that helps automate and verify the quantity of material being conveyed in a process. It can usually be added without requiring major changes to existing processes.