Hydraulic Media Packing

Hydraulic media packing takes place in all continuous fine media mills, despite the fact that a properly designed media mill is configured to circulate the media throughout the mill and, particularly, away from the exit end of the mill against the linear flow of product from the mill inlet to the mill exit. Media mill design normally creates a minimal cross-sectional area between the mill shell and the agitator at the point where the disc or pegs are located. This area is the point of greatest linear flow of product from the inlet end of the mill to the exit end of the mill. If this product linear flow rate exceeds the rate at which the media is being pumped back against the product flow, the media will concentrate at the exit end of the mill. When the local concentration of media at this end of the mill reaches about 94%, the media interlocks into a nonflowable configuration and stops moving. Succeeding interlocked media layers will build up in the mill with further operation until this static media bed covers the mill screen or rotary gap separator. Mill operating pressures will rise with increasing speed as the separator is blocked and the increased concentration of media also acts as a brake and increases the mill power requirement. This nonflowable media configuration can also occur at the shell wall without blocking the media separator and will eliminate the flow of product past the shell cooling surface at the exit end of the mill. In this case, the symptom of hydraulic media packing is the rapid and sudden rise in the product exit temperature.

The highest product velocity in a fine media mill normally occurs at the point where the product passes through the screen or rotary gap separator. In most cases, the screen is either rotating or is swept by a rotating surface in close proximity to prevent the media from blocking the screen. However, it is possible for the product flow through the rotary screen or gap slot to be high enough for individual media to be held in the slot by the product flow, despite the centrifugal force developed by the rotating screen or gap. In this case, the mill pressure normally increases rapidly, particularly when very fine media is used.

A number of factors define the product feed rate at which hydraulic packing starts. Higher product density results in increased media flotation, which reduces the feed rate at which hydraulic media packing will occur. A similar effect results from higher product viscosity. Increasing the media diameter or density will increase the product feed rate at which hydraulic packing will occur. However, increasing the amount of media loaded to the mill will reduce the effective product flow area between the media, thus increasing its flow velocity and reducing the product flow rate at which hydraulic packing occurs.

Mill configuration also plays a significant role. Increasing the cross-sectional flow area of the mill will increase the limiting product flow rate at which media packing initiates. If the separating screen or rotary gap area is the limiting factor, increasing the screen or gap area will increase the product flow rate at which screen or gap blockage occurs.

Another parameter that defines the maximum product feed rate is the rotational speed of the mill agitation system. Increases in agitator speed result in more activity of the media bed and the rate at which the media is pumped back against the overall product flow in the mill. This increases the linear product flow rate required to concentrate media at the exit end of the mill. Agitator speed is adjustable in most mills.

Modern Fine Media Mills

However, the lack of a new generation mill doesn’t mean that a mill user can’t use the circulating process. Prior generation mills can operate the circulation milling process, albeit at a reduced grinding efficiency. Whether the user chooses to do so depends on an understanding of the process limitations, which, in turn, requires a comprehension of the effects of the residence time distribution, which takes place during the passage of the product through the mill.

Figure 1/ Single Pass Residence Time Distribution

Single-Pass Residence Time Distribution

RTIME = (LVOL) / RATE

where: RTIME = Average residence time in minutes

LVOL = Mill liquid volume after the media charge has been added (liters)

RATE = Product feed rate in liters per minute

In fine media mills, the product and media undergo considerable mixing during the passage of the product through the mill. As a result, some segments of the product feed pass through the mill faster than the linear flow velocity of the product while passage of other segments is substantially retarded, resulting in a considerable variation in the amount of residence time that the individual product segments spend in the media mill under shear.

Characterization of this residence time distribution can be achieved by instantaneously injecting a dye or radioactive tracer into the product as it enters the mill and then measuring its concentration in the mill effluent as a function of the time since its injection (see Figure 1). There is a short period of time after the injection before any of the tracer appears. The maximum tracer concentration occurs before the average residence time is attained and the tracer is still being measured at small levels at residence times that are four or five times the average residence time. Over 60% of the injected tracer appears in the effluent before the average residence time occurs, indicating that the majority of the product feed is under shear for less than the calculated average residence time.

Since the residence time under shear is the primary determinant in the degree of dispersion obtained in a media mill, it follows that a variation in the residence time will result in a variation in the product quality. Thus, any sample taken from the mill contains product segments of varying quality in proportion to the length of time implied by the residence time distribution. Hence, residence time distribution is an indicator of product quality distribution.

Figure 2/ Effect of Feed Rate on Single Pass Residence Time Distribution

Multiple-Pass Residence Time Distribution

Figure 2 compares the single-pass residence time distribution for a product having a 20-minute average residence time (i.e., single-pass process) with the same product having a four-minute residence time per pass (i.e., a five-pass process). It is obvious that the single-pass residence time distribution is substantially narrower for the product with the higher feed rate.

The four-minute residence time distribution is repeated for each of the five passes required to attain the same average residence time as the single-pass process. The cumulative effect of the four additional passes, combined with the narrower residence time distribution per pass, results in a major reduction in the overall residence time distribution and product quality uniformity at the same average residence time. This reduction can be of considerable importance when a narrow particle size distribution or a high product transparency are used as the basis for product quality measurement. In particular, the reduction of the low time end of the residence time distribution curve is significant because of the elimination of the underdispersed and, thus, lower product quality fraction, which otherwise might require more processing to meet a high quality specification.

Figure 3/ Single Pass Dispersion Process

Figure 4/ Single Pass Dispersion Process

Single-Pass Dispersion Process

Single-pass product feed rates are normally pretty slow because of the need to achieve the required product quality in a single passage through the media mill. With difficult-to-disperse pigments, the residence time required to achieve the desired product quality can be so long that full media fluidization cannot be attained in vertical mills. Since the single-pass process has the broadest residence time distribution, it also has the broadest particle size distribution of any fine media mill dispersion process and thus often requires a higher average residence time distribution to achieve an acceptable product quality than would be required by two or more passes under the same milling conditions, other than the product feed rate.

One disadvantage of the single-pass process is that the average residence time required to achieve product quality is not accurately predictable and must be determined for each batch. After each feed rate adjustment, the operator must wait until a product volume equivalent to 3–5 times the mill liquid volume has passed through the mill to obtain an accurate measurement of the resultant product quality before processing the batch. Other than the low attendance time, the major advantage of this process is that it produces acceptable product immediately after being adjusted to the proper product feed rate, while the multiple-pass process produces acceptable product only on the last pass through the mill.

The basic limitation of this process is in its broad residence time distribution and the resultant broad particle size distribution, a major portion of which is underdispersed. This characteristic essentially limits this process to the dispersion of relatively coarse, easy to disperse particulates and to industries that do not require high quality dispersions having a narrow particle size distribution.

Figure 5/ Multiple Pass Dispersion Process

Multiple-Pass Dispersion Process

The multiple pass process uses a similar equipment configuration to the single-pass process, differing only in the addition of piping and valves that allow each of the agitated mixing tanks to be used alternatively as a feed tank or product receiver, as shown in Figure 5. In its initial stage of operation, the product premix is fed to the mill at a predetermined feed rate, passed through the mill and sent to the second mixing tank, which, at this stage, is acting as a product receiver. This sequence constitutes a “pass.” At the completion of the pass, the mill will sense the lack of product feed and shut itself down, unless attended or suitably automated. The process must then be restarted by opening and closing the appropriate valves to feed the product from the receiving mixer (which is now the feed mixer) to the media mill. After a short period of circulation back to the new feed tank to flush the remaining material from the prior pass from the mill, the mill output is sent to the original feed tank (which is now the receiving tank). This procedure is repeated as often as required to achieve the necessary product quality. In extreme cases, 20 or more passes may be used.

The multiple-pass process requires either more operator attention or more automation than either the single-pass or circulation processes for fine media mill processing. However, multiple-pass processing always provides a slightly narrower residence time distribution and particle size distribution than the circulation process when run under identical process conditions. This is because, as will be discussed later, the circulation process introduces an additional unique residence time distribution that results in an overall broadening of the residence time distribution for the process.

This process is used wherever very high particle size dispersions are required. High-transparency pigment dispersions required for automotive metallic finishes can be produced by way of this process because of the narrowing of the residence time distribution. Narrower residence time distributions sharply reduce the fraction of underdispersed particles in the product. It is these particles that scatter light, providing haze and reducing the “glamour” of automotive finishes. The emphasis on formulating products of increasing glamour has led to a continual increase in the transparency requirement of the pigment dispersions and, therefore, to the number of passes necessary to achieve the appropriate pigment dispersions. Other areas where multiple-pass dispersion is used are in the manufacture of magnetic tape coatings and ceramic molding dispersions, where uniform particle size dispersions are required for effective end use.

The multiple-pass process is the most complex of all of the fine media mill processes. While it requires the least residence time to achieve a given dispersion quality under identical processing conditions, the process requires more operator attention or a higher level of automation than the other processes for effective operation. It also suffers from the disadvantage that it produces acceptable product only during the last pass.

Figure 6/ Initial Circulation Grinding Process

Circulation Milling Dispersion Process

The recent development of a new class of media mills has led to increasing use and publicity for this process. Its adoption is being driven by the simplicity of its operational use. Experiments indicate that the maximum product quality achievable by this process is, at best, equivalent to that attained by the multiple-pass process using the same operating conditions.

The process equipment requirements in the initial process are shown in Figure 6. The installation uses an agitated mixer to prepare a comparatively uniform mixture of particles and liquid. This mixture is then pumped through the media mill at a high feed rate, processed and returned to the mixing tank, where it is blended with the mixer contents for further circulation through the mill. The process continues until the mixer contents have been dispersed to the desired quality level. Very high circulation and product feed rates are used with the new media mills. These rates normally range from 100–400 times the media mill’s liquid volume, measured after the media has been loaded to the mill. These high circulation rates can be used throughout the processing cycle when dispersing relatively coarse, easy-to-disperse particles that have comparatively low associative forces binding the individual particles together. However, when dispersing fine particle size, dense and hard-to-disperse particles, the initial circulation feed rate may have to be much lower than the full circulation feed rate — typically in the range of 25–33% of the full circulation feed rate — to avoid a special form of hydraulic packing. These latter particles are normally strongly associated and tend to form hard-to-disperse clumps during the premixing cycle. If this type of product is fed to the media mill at the normal high circulation rate, these clumps will enter the mill at a faster rate than they can be dispersed and, in effect, become more media, driving the media concentration at the feed input end of the mill above the onset of hydraulic packing.

Figure 7/ Circulation Grinding Process Unprocessed Product vs. Equivalent Passes

Circulation Process Induced Residence Time

The author has developed a mathematical model of the circulation process based on the stepwise simplification outlined above. The model calculates the process-induced residence time distribution, ignoring the mill-induced residence time distribution. While there are no discrete passes in the circulation process, the time required to circulate one batch volume through the media mill is normally considered as a “pass.” A circulation grinding pass can also be considered in terms of the number of mill liquid volumes or “turnovers,” which are equal to the batch volume. For example, if a 792.6-gal batch is processed at a feed rate of 792.6 GPH in a media mill, having a mill liquid volume of 7.9 gal, then a pass would be generated every 60 minutes and would consist of 100 turnovers.

The model results, shown in Figure 7, indicate that the circulation process procedure of mixing returned processed material with the feed mixer contents results in a comparatively high level of unprocessed material after the first pass. The 792.6-gal batch will have 36.6% unprocessed material in its composition at that time. However, as Figure 7 indicates, the level of unprocessed material diminishes with each pass, reducing below 1% after five passes, and seven passes are required to reduce the unprocessed material level below the 0.1% level normally considered as the minimum for architectural finishes. Very high quality dispersions, such as those required for automotive topcoat end use, will require a minimum of 12 passes. In no case should the circulation process be operated using fewer passes than the multiple pass process it replaces. These results are largely independent of changes in the batch size and the product feed or circulation rate.

The level of unprocessed material inherent in the circulation process is not normally a problem when processing difficult-to-disperse pigments using the modern high circulation rate mills. This is because the very high circulation rates result in the batch being processed for a large number of low residence time passes. However, easy-to-disperse particle systems can pose a problem in that, even at very high feed/circulation rates, the product may be dispersed at fewer than the minimum number of passes required to reduce the undispersed fraction below acceptable limits.

A similar situation can exist when the circulation process is applied to the prior generation of fine media mills that were not designed for the high feed rates inherent in circulation processing. These mills, by virtue of their configuration, are usually limited to more moderate flow rates before hydraulic media packing occurs. This results in fewer passes of longer residence time for these mills relative to those achieved by the specifically designed high flow circulation process media mills.

A second limitation of circulation processing becomes apparent when the process is compared with the multiple-pass process, operated under the same conditions at the same product feed rate. Because of the added process-induced residence time distribution in the circulation process, multiple-pass processing under the same conditions will always require less average residence time to achieve the standard quality than will circulation milling in the same mill. Figure 8 illustrates this point by comparing the ratio of the residence time of the circulation process to that of the multiple-pass process when operated in the same media mill using the same operating conditions. These model results indicate that circulation milling, using circulation rates that are less than 50 times the mill liquid volume per hour, is much less effective than multiple-pass milling under the same conditions in the same mill. This difference in milling efficiency lessens as the product feed/circulation rate increases, but never reaches the point where the two processes are equal. However, increasing the circulation rate from 200 to 400 mill liquid volumes per hour only improves the process efficiency by 0.65%. It should be noted that product feed/circulation rates in this paper are given in mill liquid volumes per hour to be applicable to any mill, regardless of its size or configuration.

While these model studies indicate that multiple-pass processing will always have a residence time advantage over circulation milling, when operated under the same conditions in the same media mill, multiple-pass operations are increasingly difficult to operate at product feed rates that exceed 40 or 50 times the mill liquid volume per hour. As product feed rates increase, the elapsed processing time for each pass diminishes and, unless the batch being processed is quite large, almost constant operator attention is required. This factor results in most multiple-pass batches being run at product feed rates that range from 10–30 times the mill liquid volume per hour. Conversely, the circulation process is usually run at much higher product feed rates in the new generation of fine media mills, ranging from 100–400 times the mill liquid volume per hour, subject to the limitations of hydraulic media packing. However, in the prior generation of fine media mills, configurational limitations usually result in the onset of hydraulic media packing in circulation milling at product feed rates in the range of 50–100 times the mill liquid volume per hour.

Figure 9/ Circulation vs. Multiple Pass Process

Effect of Higher Product Feed/ Circulation Rates

Figure 9 compares the overall productivity resulting from running the circulation process at various product throughput rates vs. multiple pass processing, in the same mill, under the same conditions except for the product feed rate. The curves reflect this circulation milling impact vs. multiple pass processing at 10 and 30 mill liquid volumes per hour. In the case of a multiple-pass process run at a feed rate of 30 mill liquid volumes per hour, operation of the same mill at a circulation rate below 53 times the mill liquid volume per hour will result in a lower operating productivity than operation of the same mill in the multiple pass mode at a feed rate of 30 mill liquid volumes per hour. Operation of the mill at a circulation rate of 53 mill liquid volumes per hour will provide the same productivity as multiple-pass operation at 30 liquid volumes per hour, operation at a circulation rate of 100 mill liquid volumes per hour will increase the mill productivity by 10% and operation at a circulation rate of 200 mill liquid volumes per hour will increase the mill productivity by 21%. Operation at a circulation rate of 400 liquid volumes per hour, if possible without hydraulic media packing, will increase the mill productivity by 30% over multiple-pass operation at 30 mill liquid volumes per hour. Note that doubling the circulation rate from 200 to 400 times the mill liquid volume per hour will only increase the process productivity by about 7%.

These process productivity gains may be partially offset by the fact that the circulation milling process is usually operated with media bed volumes in the range of 80–85% of the mill empty volume. Multiple-pass processing of the same product in the same mill will normally use media bed loads that are up to 5% higher thus improving the rate of multiple pass dispersion and narrowing the productivity advantage of the modern high feed rate circulation process mills.

Figure 10/ Modern High Flow Circulation Milling Process

Modern High Flow Circulation Milling Process

One potential problem with the circulation process is its potential for uncontrollable temperature buildup during the batch. Because of energy input in the media mill, processed material normally leaves the mill at a higher temperature than it entered and is immediately blended with the remainder of the batch, raising its temperature. Jacketed mixers help to control the temperature rise but become less effective as the product’s low shear viscosity increases during processing. For this reason, it is recommended that a suitable heat exchanger is installed between the feed pump and the mill inlet to provide the cooling capacity required to control the product inlet temperature to a constant value during processing. Installing the heat exchanger at the mill exit is not recommended, since such an installation will increase the operating pressure on the mill seals.

Circulation Process Limitations

A third limiting factor is the media load used for the process. Typically, the circulation milling process uses a media load that may be 5% below that of a comparable multiple pass process to maximize the product feed/circulation rate. If the two processes are operated at otherwise identical conditions, the multiple pass process, with the higher media load, is theoretically capable of producing a higher degree of dispersion than the multiple pass process with the same product.

To Circulate or Not to Circulate?

Operation of the new generation of mills at the high feed/circulation rate inherent in the circulation process will result in a reduced processing time relative to multiple pass processing. This savings in processing time can range from 15–30% of the grinding cycle, depending on the existing product feed rate for the multiple pass process. However, with the earlier mills, the feed/circulation rate used in circulation processing may not be very different from that currently used in the multiple pass process. In this case, little improvement would be noted in the overall processing time.

Multiple pass processing, because of the higher media load normally used, has the potential of producing a slightly higher product quality than can be achieved by way of circulation milling. From a practical viewpoint, this has little significance except for those products that require maximum levels of transparency. However, since this type of dispersion is of extreme importance in the manufacture of automotive metallic finishes, conversion from multiple-pass processing to circulation milling should be considered on a product by product basis.

The processing of very easy to disperse particulates by way of circulation milling may pose a problem in that, even at high circulation rates, it is difficult to achieve enough passes of the product to reduce the unprocessed material level to an acceptable degree without overdispersing the remainder of the batch. This is not a problem with multiple pass processing.

Based on the previous discussion, the following factors should be considered when selecting between the multiple-pass and circulation milling fine media mill dispersion processes. Analysis should be done on a product-by-product basis.

• Using the new generation of fine media mills, circulation milling offers advantages in direct labor costs and shorter processing cycles when compared with multiple-pass processing.

• With earlier mill designs, hydraulic packing limitations in feed rate minimize the circulation milling cost and capacity advantages vs. multiple-pass processing. However, the operational simplicity inherent in circulation milling remains.

• Multiple pass processing provides a slightly higher potential product quality, inherent with increased media loads, than the circulation milling process.

• High viscosity products severely restrict the maximum product feed rate for circulation milling, which is basically a low viscosity product process, regardless of the type of mill used. High viscosity is much less significant in multiple-pass processing.

• Easy-to-disperse particulate systems, requiring 4 or less passes in the multiple-pass process, can pose a problem in the circulation milling process, because of the number of passes required to reduce the undispersed particulate level to an acceptable degree.

It is important to realize that no one fine media mill process is the best for all dispersions. Each has its advantages and limitations that must be considered for each product to be dispersed. Both the multiple pass and the circulation milling processes are effective and proven. The choice is up to the user. c

For more information on equipment, contact Premier Mill, One Birchmont Drive, Reading, PA 19606-3298; phone 610/779.9500; fax 610/779.9666; e-mail Sales@PremierMill.com; visit www.premiermill.com.

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