Design of combined drying and grinding mills

The specific feature of combined drying and grinding is that two processes, i.  e. fine comminution and drying, take place in one process area. In connection with the pneumatic handling, the continuous enlargement of the specific surface and the aeration of the mill feed, this process has advantages compared to conventional drying, e.g. in rotary dryers, and is distinguished by the energetically favourable utilization of the waste heat of other technological processes.

The tube mill of combined drying and grinding plants is designed according to comminution principles since the minimization of the comminution work to be carried out has priority. The consequence for the calculation program to be presented is that the plant and process engineering data of comminution, i.e. also the size of the mill, are determined in a first part. In a second part, the process engineering data of combined drying and grinding are determined. The mathematical procedures, on which both parts are based, have not been detailed since it is assumed that they are known. Furthermore, it would be beyond the limitation of this article. The specific feature of the calculation program to be presented is that the practical experience gained has been included, taking into account all process engineering alternatives which occur, and it also allows the operator’s own design and experience values to be included as far as possible (Figs. 1 and 2). Thus, the calculation program allows an optimization and adaptation to the boundary conditions of the operator’s plant or of the manufacturer of combined drying and grinding mills.

Obligatory input values for the design of a tube mill are the mill throughput, the particle size of the feed, the particle size of the finished product, the density or bulk density of the material being ground, and the grindability according to Bond, Zeisel or Hardgrove.

In this way, the user has various possibilities for the input of the particle sizes. Optionally, the particle size of the feed can be read in at 80  % undersize or as maximum particle size of the feed. Optionally, the particle size of the finished product can be read in at 80  % undersize or as a percentage of the fineness of the finished product with an optional particle size. The internal conversion is carried out by means of the RRSB distribution. If necessary, the directional coefficient can be changed. Furthermore, a distinction must be made between a wet and dry grinding mill and an open-circuit and closed-circuit mill. Depending on the mill type, i. e. open-circuit mill, drum mill, long or short closed-circuit mill, the mill diameter, the mill length, the power requirement and the mill speed can be determined by varying the speed ratio, the length-diameter ratio and the grinding media filling ratio. Due to the possibility of modifying individual parameters, the mill size can be determined according to the wishes of the operator. The calculation sheet shows a mill designed in this way. The blue input fields contain the obligatory parameters; the green ones contain the optional parameters.

A method developed by the author [1] and proved successful in a practice test lasting several years is used to determine the grinding media grading. The starting point is the experimental result that the particle size decreases over the effective length according to an exponential function. Since the ball diameter has to be chosen according to the particle size, the grinding media distribution function also follows an exponential function:

d_K = d_{K f ikt} · e ^–  gl

d_K = ball diameter in cm

d_{K f ikt} = fictitious maximum ball diameter in cm

l = effective length in m

g = grading exponent in m^-1

Since it is not actually possible to continuously assign the ball size to the effective length, the assignation is carried out in steps according to the existing ball sizes, as shown in Figure 3. The length of one step corresponds to the grinding media portion of the corresponding ball size. The maximum ball diameter is calculated according to Bond. The minimum ball diameter is calculated as a function of the fineness of the finished product. The grinding media weights per ball diameter have been tabulated. It is possible to vary the available ball diameters by putting hooks between the table of the grinding media weights and the grinding media position. As regards the grinding media position, it has been assumed that a classifying lining is used. These grinding media position data make it possible to determine the length of the grinding compartment of compound tube mills. The grinding media filling height and the grinding media weight are calculated for each grinding compartment as a function of the chosen grinding media filling ratio and the length of the grinding compartment. Since the calculation program may also be used without drying, i. e. only for the comminution engineering design, in this case the values are given for mill dedusting.

Obligatory input values are the feed moisture, the final moisture, the specific heat of the material being ground and the calorific value of the fuel. The drying gases may be handled in open-circuit but also in closed-circuit, i. e. like in an air-swept mill. In the following, first the mathematical procedure will be described when handling the drying gases in open-circuit. Due to the above-mentioned priority of the comminution design, the feed moisture is limited, which is shown as a function of drying gas handling.

The flue gases of a separate firing system are one source of the drying gases. Another source is the utilization of other technological processes, such as waste gases from kilns, preheaters and coolers, brought to a sufficient specified temperature. Here these waste gases, which are frequently available in larger amounts than can be used , are called hot incoming air volume flow. This designation will prevent possible confusion with the term waste gas leaving the combined grinding and drying system. The temperature of the hot incoming air has to be indicated. Since the specific heat of waste gases from other technological processes, which have already been included in the calculation program, may differ from that of the flue gas of a separate firing system, it is possible to input it as specific heat or, as an alternative, as CO₂ content.

It is necessary to determine a mill inlet gas temperature for setting up the heat balance and the derived gas volume flows. The minimum mill inlet gas temperature is the gas temperature at which the desired drying is just achieved. The maximum mill inlet gas temperature is the temperature, which the components of the tube admit without being damaged. If the drying gases are used in open-circuit, mill inlet gas temperatures are proposed as a function of the feed moisture, which may be modified, if required, or based on other experience gained.

The fuels that can be used in a separate firing system are gaseous fuel (e. g. natural gas), liquid fuel (e. g. petroleum) and solid fuel (e. g. coal). The specific combustion and flue gas volume flows are determined by means of the statistical combustion calculation based on the calorific value and the excess air. The gas volume flows are calculated and distributed by means of iteration of the heat balance to be set up taking into account the following two conditions:

– the enthalpy of the gas volume flows must meet the heat requirement

– the mill inlet gas temperature must be between the minimum and the maximum value.

The result will be the flue gas volume flow of the separate firing system, the portion of the required hot incoming air volume flow taken from the existing one, the possibly required fresh air volume flow and the waste gas volume flow. The heat balance includes as output values the water evaporation, the material heating, the residual water heating, the waste gas heating and the wall heat losses.

The grinding heat, as an additional input item, is subtracted from the sum of the heat balance portions to determine the heat requirement. The grinding heat represents the portion of the grinding energy that is transformed into heat. The input items for the determination of the heat requirement comprise the quantity of heat of the fuel, the amount of heat of the incoming air as well as the grinding heat. The quantity of heat of the fuel, the amount of fuel and the specific fuel requirement related to the evaporated amount of water are determined by means of the calorific value. Finally, the design algorithm offers the possibility to check the process engineering, and to make changes, if necessary, until the final target is achieved.

The dew-point check is to ensure that no condensation phenomena will occur in the mill and in the range of waste gas. For this purpose the dew-point temperature of the evaporated amount of water and of the waste gas volume flow is determined and compared with the waste gas temperature. The mill gas velocity check is a comparison of the gas velocity inside the mill and the admissible gas velocity. The mill gas velocity is related to the free cross-section, i. e. taking into account the grinding media filling ratio. A warning is signalled in both checks if the limit values are exceeded.

If the drying gases are flowing in closed circuit, it is advisable to confirm or input the hot gas temperature and not the mill inlet gas temperature. The hot gas temperature is the temperature of the gas volume flow before mixing with the recycled gas volume flow. The specific circulating gas volume flow is introduced as further term to be confirmed or input, respectively, since a degree of freedom is added when calculating the gas circulation. The specific circulating gas volume flow is the ratio of the circulating gas volume flow in operating state to the mill throughput. Furthermore, the gas circulation, which represents the ratio of the circulating gas volume flow to the waste gas volume flow, is calculated.

Due to the high power requirement, combined grinding and drying mills with drying gases flowing in closed circuit, i. e. ­
air-swept mills, are used only for special cases. Such a case would be combined grinding and drying of coal, where an inert ­atmosphere is required for safety reasons. This is taken into account in the design program by calculating the oxygen content in the mill gas. The oxygen content is determined ­
taking into account the inertizing effect of the flue gas and of the water vapour.

Three examples of use are to demonstrate the most varied possibilities of an optimum design of combined grinding and drying mills:

– Taking into account the grinding media position, it is possible to subdivide the mill into two or three grinding compartments. Assuming that self-classifying liner plates are used, as explained under item 2, the corresponding places of residence, i. e. mill lengths, are assigned to the individual ball sizes. The length data of the grinding media position are taken to select the length of the grinding compartment on the assumption that certain ball sizes should be contained in each grinding compartment. In the calculation sheet of the tube mill design it was assumed that balls from 80 to 60 mm should be contained in the first grinding compartment, i. e. the first grinding compartment has a length of 2 m and the second one a length of 5 m. Due to the large feed screw for the first grinding compartment, the grinding media filling height may only amount to 70 cm, i. e. the grinding media filling ratio must be reduced. The latter has to be increased in the second grinding compartment in order to achieve the desired throughput. The grinding media weight is calculated as a function of the desired grinding media filling ratio and the length of the grinding compartment.    

– If hot incoming air is available for drying, the optimization criterion is the utilization of an incoming air volume flow as large as possible together with a minimum additional requirement of fuel. In this case, the limits of the mill inlet temperature have to be taken into account. Since the high-temperature waste gases of other technological processes are predominantly used for other thermal purposes of higher quality, such as steam or power generation, the upper limit for drying will not exceed 500 °C. The lower limit of utilization of waste heat is governed by economic considerations. Thus, it will be hardly be worthwhile to install separate fans and pipelines for waste gases < 150 –200 °C.

Taking into account the available incoming air volume flow and its temperature, the calculation program has considered the following 4 cases:

– Large incoming air volume flow with a temperature, which is between the minimum and the maximum mill inlet gas temperature;

The required hot incoming air volume flow meets the demand.

– Large incoming air volume flow with a temperature above the maximum mill inlet gas temperature;

The required hot incoming air volume flow meets the demand, however, fresh air must be supplied to reduce the mill inlet gas temperature to the maximum temperature admissible.

– Large incoming air volume flow with a temperature below the minimum mill inlet gas temperature;

Even if the incoming air volume flow meets the heat requirement, additional heating is necessary to achieve the minimum mill inlet gas temperature.

– Small incoming air volume flow;

Additional heating is necessary to meet the heat requirement and to achieve the minimum mill inlet gas temperature.

The above-mentioned example, which is continued in the enclosed calculation sheet for combined grinding and drying, shows that the incoming air volume flow is large enough as regards the enthalpy. However, the mill inlet gas temperature of 350 °C is not high enough, i.e. additional heating by means of a separate firing system is required.

When increasing the gas circulation of an air-swept grinding plant, the mill gas velocity is increased with simultaneously decreasing mill gas inlet temperature. Figure 4 shows the operating range for the gas circulation, which is characterized by the specific gas circulation volume flow. On the assumption that the mill gas velocity is not higher than 4.5 m/s and that the mill gas inlet temperature should not be lower than 250 °C, there is an operating range for the specific gas circulation volume flow of 1.3 to 1.7 m³/kg.