The design of FAM special hammer mills for comminution and drying of bulk solids

1‌‌‌ Introduction

Over the last 40 years, the demand for fine- and ultrafine-grained powders has increased exponentially in many branches of industry worldwide, especially in the construction materials industry. However, the starting materials for these comminution and drying processes generally have a moisture content of 3 to 12 %. To produce defined, fine-grained products, the comminuted product must be sized by means of air classification or screening into the corresponding grades. These classification problems can, however, only be resolved when the comminuted product has a maximum moisture content of 0.1 to 0.5 % depending on the grain size.

In recent years, the technology of drying, e.g. in a drying trommel upstream of the comminution process, has been increasingly replaced by combined grinding and drying in a comminution machine, e.g. in a hammer mill. This process integration, i.e. with more than one process taking place in one machine or a closed grinding and classifying system, offers considerable process advantages. For combined grinding and drying, diverse combinations of comminution machines, like, for example, hammer mills, drum mills, roller mills, impact mills and sizing machines as well as e.g. air classifiers and screens are used.

The use of FAM special hammer mills in grinding and drying technologies with internal or external material circulation enables a wide range of raw materials to be processed and final product finenesses to be obtained. In the design of the special hammer mills for combined grinding and drying, both comminution- and drying-related aspects have to be taken into consideration. A process compromise must be found that guarantees optimum technology. While pilot testing is preferred for the comminution-related design aspects, the thermal design, as an example shows, can be performed with the calculation program presented in the following.

2 Configuration and operation of FAM special hammer mills and FAM rod cage rotor classifiers

The FAM special hammer mills are a special type of hammer mill (Fig. 1) in which the comminuted product is discharged upwards by the air stream. The core component in the special hammer mill is the disc rotor in which the hammers are mounted to bars that are staggered to obtain a high-turbulence stream in the grinding chamber. The number of hammer rows (6, 8 or 12 rows) depends on the rotor diameter. The hammer bars are made of stainless and acid-resistant steel. The hammers are divided into the hammer arm and hammer head. This allows the hammer heads, which wear quicker, to be replaced easily without any need for the hammer arms to be removed, too.

In the design of the housing, importance has been attached to easy maintenance and repairs, with a rear wall that can be folded back hydraulically and a grinding chamber with wear-resistant grinding plates that can be inspected and maintained easily through large doors (Fig. 2 and Fig. 3). The geometry of the grinding plates is adapted to the specific feed material.

When the grinding and drying system is heated, the housing in welded steel plate expands faster than the rotor shaft. For that reason, the use of a restraint-free bearing system – a carb bearing on the floating bearing side – is expedient to compensate for differences in the thermal length change between housing with the bearing seats and the shaft.

To keep the roller bearing temperature below 80 °C, the oil bath in the housing of the oil-lubricated bearing and the rotor shaft are cooled. In addition, the temperature of the drilled-through rotor shaft is lowered with a coolant that is supplied by means of a special rotary inlet (Fig. 4).

The drive of the special hammer mill can be configured as a V-belt or direct drive. The rotor speed essentially determines the fineness of the comminuted product and can be adjusted within a wide range (e.g. between 40 and 70 m/s) by means of a frequency converter.

Another possibility to influence the grain size of the finished product is the volume stream of hot gas and therefore the flow velocity at which the comminuted product is transported out of the special hammer mill to the air classifier. Accordingly, the volume flow of hot gas, the inlet temperature and recirculation of the drying air into the special hammer mill have to be adapted to the material properties (e.g. feed moisture content, feed grain size) and the required end size. The dwell time of the feed material is therefore determined by the material properties of the feed material as well as in wide ranges by the technological parameters.

The FAM special hammer mills work in a closed circuit with a FAM rod cage classifier. The classifier (Fig. 5) is designed such that with a widening of the cross-section between the classifier interior and exterior cone (3 and 4) the flow velocity is reduced, and as a result, part of the coarser content of the comminuted product can be separated.

Actual separation in the classifier takes place at the rod cage (1). With the frequency control, the speed of the rod cage and therefore the ability of the finished product to pass between the rods, and ultimately the cut size can be adjusted over a wide range. A further possibility to influence the fineness with flow-related measures is the adjustable guide vane ring (2).

The coarser particles separated by the rod cage are fed together with the feed material by means of a rotary valve feeder to the hammer mill. For wet and cohesive feed material, rotary valve feeders with raking arms have proven effective, which, for example, have eight rounded chambers that are divided and staggered four times over the cell length. As a result, a quasi-continuous feed is achieved, which has a favourable effect on the operation of the hot gas generator and the drive of the special hammer mill.

3 Combined grinding and drying technologies with FAM hammer mills

Depending on how the feed material is fed to separation after comminution, a differentiation is made between grinding plants with external and internal material circuit.

In the case of FAM combined grinding and drying plants with internal material circuit, the comminuted product of the special hammer mill is transported by the air flow to the classifier and the coarser particles recirculated into the feed material. A dry finished product can be produced with a defined upper particle size, like, for example. 15 % >90 µm. Depending on the required product grain size, the feed size is 40 ... 60 mm and the throughput rate final depending on plant size (determined by the size of the special hammer mill) is between 4 and 100 t/h.

The FAM grinding and drying plants with internal material circuit consist, as Fig. 6 shows, of the following plant components: The feed system – a belt-type weighfeeder (3) – regulates as a function of the motor power of the special hammer mill (5) the feed rate from the feed bin (1), which is fed via the rotary valve (4) to the connection upstream of the special hammer mill. The rotary valve feeder is used for the uniform feeding and air sealing of the grinding system. The mill feed material in the mill intermediate component is picked up by the turbulent stream of hot gas, generated in the hot-gas generator (8), and comminuted and dried in the special hammer mill between hammers made of special cast steel and the grinding plates.

The fineness of the comminuted product is determined essentially by the rotor speed and can be varied by means of a frequency converter in a wide range (e.g. between 40 and 70 m/s).

In FAM combined grinding and drying plants with external material circuit, impact hammer mills with grinding track or with grate discharge work in the circuit with air classifiers and/or screens. In the comminution of, for example, a medium-hardness limestone from 60 mm to < 2 mm in the circuit, e.g. throughput rates up to 120 t/h can be realized. An excerpt from a design of a FAM grinding and drying plants with external material circuit is shown in Fig. 7.

Besides the production of limestone flour for flue gas desulphurization plants, other applications for FAM grinding and drying plants are clay preparation for heavy clay and whitewares products, the production of fertilizers in the size range 0 – 1 mm and the combined grinding and drying or calcination of natural, FGD gypsum (gypsum from flue gas desulphurization plants) and gypsum production waste.

4 Design of combined grinding and drying plants with hammer mills

In combined grinding and drying systems, the design must ensure that the main equipment for the sub-processes of comminution and drying is sufficiently dimensioned. Here the hammer mill must be selected such that in respect of motor power and machine size the required throughput is achieved with desired product fineness. The safest method is the transfer of findings, especially concerning the specific energy consumption, from existing systems with comparative equipment to the new machine. The specific energy consumption refers to the electric motor power relative to the throughput rate. The specific energy consumption can also be determined by means of tests with pilot-scale comminution machines and larger laboratory machines. The machine size can be upscaled roughly based on the proportionality m. ~ L x D², where L is the rotor width and D the rotor diameter. Independent of these “calculations”, ultimately the engineer’s/operator’s own experience should lead to definition of the machine size.

The dimensioning of the hammer mills must, however, also meet the process conditions with regard to drying. The hot gas volume flow must be defined with regard to the moisture content of the feed material and finished product, the throughput rate and other material properties. From the thermal and quantitative balance, the necessary heat requirement can be calculated. With the help of the combustion calculation, the combustion and flue gas quantity result which with the water vapour and mixed gas quantity is needed to adjust the hot gas temperature of the gas volume flow in the process chambers, the volume in the hammer mill [1].

In the next section, the drying-related design by means of a calculation program is shown with reference to two examples.

In combined grinding and drying systems, the minimum inlet temperature of the hot gas should not be lower than 300 °C in general and 550 °C for grinding and calcination. The flue gas temperature in drying systems lies mostly below 80 °C and has to have a sufficient difference to the dew point. In grinding and calcination of gypsum, depending on the gypsum properties and processing technology, the flue gas temperatures are between 160 and 165 °C. The velocity at the transition of the special hammer mill to the riser must be set so that it is between 18 und 22 m/s depending on the required product fineness.

In the combined grinding and drying of soft and moist feed material, a high hot gas volume flow is needed, i.e. the hammer mill must be bigger than would have been necessary for comminution to enable the residence time necessary for drying. On the other hand, for medium-hard feed material, e.g. limestone, with low moisture content up to 5 %, the design of the special hammer mill is based on comminution requirements. It is easy to see that a compromise must be found in the dimensioning of the system.

5 Design program for FAM special hammer mills

In the first design example, a hard limestone with a feed moisture content of 5 % is to be dried to a product moisture content of 1 %. The throughput rate is 50 t/h. If, for example, a hot gas temperature of 400 °C is defined at the mill inlet, then from a drying perspective results a special hammer mill with a rotor diameter of 1.2 m and a rotor width of 1 m. This mill size is obviously too small for a throughput rate of 50 t/h. From a comminution perspective, the size of the special hammer mill should have a rotor diameter of 1.4 m and a rotor width of 1.8 m. For this machine size, in the calculation, the hot gas temperature at the mill inlet can now be lowered to 300 °C. The resulting gas outlet velocity out of the special hammer mill is too low at 11 m/s so that an additional fresh air volume flow of 20 000 m³/h must be added. With this, the exit velocity out of the special hammer mill SHM1418 is increased to 18 m/s.

If the task of the grinding and drying system is the processing of a soft but wet limestone with 10 % moisture content, a special hammer mill must be selected to be large enough to handle the necessary hot gas volume flow. If the same SHM 1418 special hammer mill is to be used, the hot gas inlet temperature into the special hammer mill must be increased to 350 °C and the additional fresh air reduced to around 10 000 m³/h. For these operating conditions, the outlet velocity increases to 22 m/s.

An important result of the calculations is verification that the flue gas temperature does not fall below the dew point.With the input of the circulating load, the loading of the exit gas volume flow can be checked. Another advantage of this program is the possibility to calculate the grinding and drying of coal and lignite in inert gas operation, as shown in the next section.

6 Combined grinding and drying of coal

In the combustion of coal for the purpose of energy generation, direct firing mills are often used. The decreasing acceptance of coal combustion is pushing the material utilization of coal more and more into focus. This is where the use of special hammer mills comes in.

In the drying of coal in a gas mix, the explosion behaviour must be taken into consideration. An ignition and combustion/explosion of coal can only take place if sufficiently fine-grained coal and oxygen and an effective ignition source are present. Ignition sources in a hammer mill can be sparking metal parts, e.g. foreign particles in the special hammer mill, but also sparks from the hot gas generator. Whirled-up coal dust with sufficient fineness and a concentration within the explosion limits is also present. Of the above-mentioned three conditions for ignition, in order to avoid explosion in the hammer mill, the only remaining option is the reduction of the oxygen content.

As the flue gases from the hot gas generator have a 3- to 4-times-higher temperature than the temperature necessary for drying in the hammer mill, to adjust the drying temperature, relatively large quantities of fresh air are added, which increase the oxygen content above a permissible limit which is below the limiting oxygen content (LOC). The water vapour content added in the drying process does have an inertizing effect, but generally is not sufficient to safely remain below the LOC necessary for explosion protection.

For this reason, so much drying gas saturated with water vapour (vapours) is recirculated until the LOC is achieved or undershot. This recirculation of vapours can, however, only be increased so far as that the dewpoint is not undershot. The content of vapours circulated has an advantage in terms of heat economy as less fresh air has to be added downstream of the hot gas generator.

The design program for drying presented in Section 5 enables the adjustment of the oxygen content to the LOC of the drying gases based on the step-by-step increase in gas recirculation with simultaneous observation of the development of the dew point. The dew point temperature is determined from the absolute moisture content of the circulating gases and compared with the gas temperature. To reliably avoid the temperature falling below the dew point, and accordingly condensation of the water vapour, the gas temperature should be at least 20 °K higher than the dew point temperature. The absolute humidity of the gases results from the amount of water evaporated during drying of the coal, the water content from combustion in the hot gas generator and the moisture content of the returned gas.

For the user of the design program, in the combined grinding and drying of coal, 9 % is displayed as the limiting oxygen content and 8 % is shown as the LOC for lignite. These values should only be regarded as guide values, which provide sufficient certainty for many coals. For especially highly flammable lignites or high-grade coals, lower or higher limiting values may be necessary/permissible.

In this respect, IBExU Institut für Sicherheitstechnik GmbH Freiberg (an institute affiliated to Freiberg University of Mining and Technology) recommends that the required safety characteristics of the relevant coals (especially the limiting oxygen concentration) be determined and evaluated or estimated for the specific temperature and gas conditions in the hammer mill. If nitrogen is to be used as inert gas in the hammer mill (N₂ has a lower inertization effect than water vapour), safety values also have to be determined to allow for this.