Laboratory Sample Pulverizers

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It is desirable to use material which has as nearly the same metallurgical characteristics as the samples with which the standards will be included. This is usually a difficult chore. For many reasons, including the particle size at which a significant amount of the gold mineral is liberated, the sampling characteristics of even -150 mesh material may preclude the use of geologically and metallurgically similar ore as a standard. It is usually easier to get material with desirable grade characteristics with the necessary sampling properties, than to find geologically and metallurgically similar material with the required sampling characteristics. High grade standards are especially difficult to find and prepare. This is because as grade increases the size of the gold particles usually increases. Larger gold particles are liberated and tend to segregate during comminution and the homogeneity of the material cannot be maintained. For grades much above 3 g/t it is very difficult to find material which has the proper sampling properties.

Likely candidate material for assay standards is old mill tailings. Some of these have sufficiently homogeneous mineral content so that the sampling errors can be effectively controlled. Where mill tailings are either not available or are unacceptable, mineralization that has exhibited homogeneous results in re-assay of pulp material is also a good candidate for the standard. Finally, the mineralized rock being sampled may and should be used if adequate homogeneity in the -150 mesh material exists. (“Adequate” homogeneity or “Acceptable” homogeneity is defined in a later section).

Once candidate material has been chosen, a lot (lot as used here and what follows means the batch of material) must be collected and prepared for testing. A lot to be used as a standard must contain sufficient material. This means that at least 50 kg must be available for use as the standard lot. Lots used for pilot testing of the sampling characteristics of the fine material may be made up of as little as 10 kg. The lot should be collected to minimize the inclusion of foreign or undesirable material. For example, mill tails are typically layered in the tailings impoundment. Material tends to be more homogenous within a layer than across layers. Therefore, when the tailings are collected, the collection of a lot should be confined to a single layer.

Once the lot is collected, the entire mass must be reduced in particle size to -150 mesh (the 150 mesh size assumes that a sampling nomograph has been produced and the relative standard deviation for sampling error at 150 mesh is less than two percent as given by that nomograph). The fine material should be screened through a 150 mesh screen and material not passing the screen should be returned to the pulverizer for further comminution. After two iterations material retained on the screen may be discarded (remember – this is not acceptable sampling practice for determining the grade of the lot, but that is not the objective in this case).

When the lot is reduced in particle size, it must be divided for analysis. This is probably best done using the fractional shoveling technique. The lot should be divided to produce 100 sub-samples of 35-40 g per sample (This weight is based on the assumption of using a one assay-ton portion for fire assay. The weight may be adjusted according to the weight to be used for the fire assay.) The weight of the sub-sample is not absolutely critical so it may vary within a few grams, but it probably should not exceed 40 g or be much less than 35 g. Restricting the amount of material to 35-40 g precludes the testing lab from duplicating assays and reporting an “average” value. Averages have a different distribution of errors than individual samples. Including at least 35 g insures that even if a small amount of material is lost in handling, there will be sufficient pulp for a one assay-ton lot for testing.

The 100 samples should be assigned to one of five groups using a systematic random sampling procedure. When the assignment is complete, each group should have 20 samples. Each sample should then be assigned a sample number designating the group to which it belongs and its number in the group and so labeled.

Pulverizing with Ring Roller Mill

The pulverized method of firing is currently the dominant combustion method in large lignite-burning power plants. A continuing research program has been conducted to determine the pulverization characteristics of lignite in mills of different types, the variations which can be encountered between mine sources, and the influence of moisture content, and to test and study possible improvements in operating techniques. This report describes the results of tests performed on a laboratory-size ring-roller mill of the MB series manufactured by the Foster Wheeler Corp. Under a cooperative agreement between the Foster Wheeler Corp. and the Bureau of Mines, the cooperator provided the test MB mill for this investigation.

Lignite as mined is variable from one mine source to another and even within each mine. It is generally characterized by moisture content ranging from 35 to 44 percent, averaging about 37 percent; ash content ranging from 4.2 to 9 percent and averaging 6 percent; and a heating value ranging from 6,000 to 7,300 Btu/lb and averaging 6,800 Btu/lb. For pulverized firing, lignite is generally pulverized so that 55 to 65 percent passes a 200-mesh screen (0.0029-inch opening). Pulverizers are manufactured by a variety of companies and the major difference between them is the method of applying the roller-type grinding action. These units use heated air to sweep the mill and to dry, classify, and transport the pulverized product to the burners. The moisture removed from the lignite is carried into the boiler with the pulverized lignite and primary combustion air.

Studies of lignite pulverization have been made with a hammer mill pulverizer and a ball mill. The investigation described in this report was conducted in a laboratory-size mill utilizing the rolling-compression type of grinding action utilized in all commercial-size units currently operating in North Central United States.

Previous studies established that lignite from different mine sources have diverse pulverization characteristics. Variation also occurs between different seams within the same mine and even at different levels within the same seam. Extensive comparison of the pulverization characteristics of lignites from different mine sources was therefore not included in this study. Sufficient samples were tested, however, to establish that the capacity and operating characteristics of the type of pulverizer being tested were also significantly influenced by the variable characteristics of lignite.

A major factor in pulverization of lignite is its moisture content. The effect of drying on the pulverization and grindability characteristics of lignite has been demonstrated in a previous Bureau study. In its natural state, lignite is tough, resilient, and difficult to grind. If moisture is removed either before or during pulverization, power requirements are significantly reduced, the fineness of the product is increased, and the mill capacity is increased. The effectiveness of pulverizers used in commercial practice on lignite is therefore a function of the degree of drying which can occur in that particular type of mill. In laboratory scale, the particular mill tested in this investigation utilized a sweeping gas system with internal size classification similar to commercial-scale units. The majority of testing in the MB-1 pulverizer was directed to study the drying process and develop techniques to increase the degree of drying, if possible. Data were also developed for pulverization of predried lignite, an operation which provides maximum benefits from drying, for comparative purposes.

Experimental Test System and Procedure

The test system is shown schematically in figure 1. Provided are a source of sweeping gas and methods to control the rate and temperature of the sweeping gas flow and lignite input rate. Power input, pertinent temperatures, rate of product production, and a variety of static pressures can be measured. A sample of the pulverized product can be collected at any desired time.

Sweeping Gas

Fuel oil burned with stoichiometric quantities of air produced both inert sweeping gas and the sensible heat in the gas for mill operation at elevated temperatures. The hot inert gas produced by the generator (fig. 1) was passed through a heat exchanger, through a cold water wash to remove corrosive oxides, and then compressed and stored in a bank of high pressure cylinders. When needed, the inert gas was reheated to temperatures as high as 700° F in a heat exchanger prior to entering the pulverizer. In tests in which sweeping gas at low temperature levels was desired, room air was used. For a special series of tests, auxiliary valves and an exhaust blower were provided in the system to permit operation at either positive or negative pressure in the mill.

The flow rate of the sweeping gas was controlled by a bypass valve at the gas blower and measured with an orifice-type meter. Inlet and exit temperatures of sweeping gas at the pulverizer were measured with bimetallic thermometers. Static pressures were indicated with liquid-filled manometers. A manometer was installed to measure the pressure drop across the grinding zone.


A schematic of the test mill is given in figure 2. Grinding is done by three steel rollers which rotate between a spring-loaded stationary upper race and a driven lower race or table. The lower table is driven by an electric motor through a V-belt and gear-reduction drive. The sweeping gas is introduced into a chamber under the table and passed upward through angled slots which surround the lower races. The gas entrains particles as it passes the edge of the rotating table and carries them to the classifier located at the

ring-roller mill pulverization test system

top of the mill. Figure 3 shows lower table, steel rollers, and spring-loading mechanism of the upper race. The slant of the slots imparts a swirling motion to the sweeping gas to increase both gas turbulence and residence time between the grinding zone and the classifier section.

Figure 4 shows the classifier section of the mill. The larger particles are removed from the gas stream by the classifier and returned to the center of the table for further grinding. The pulverized product entrained in the sweeping gas leaves the mill through the top outlet. Fresh feed is added through an auger feeder in the side of the mill and discharges above the center of the rotating table. Centrifugal action carries the fresh feed and the partially ground particles returned by the classifier outward to the grinding zone under the steel rollers. The test mill with supporting stand and motor is shown in figure 5.

Test size of lignite was 0.125 by 0.046 inch. This size was selected by comparing the diameter of pulverizer rollers in commercial units to the usual feed size in these units. The ratios between diameter of rollers and size of feed lignite for commercial units and for the test unit are reasonably comparable.

ring-roller mill pulverizer

ring-roller mill spring loading mechanism

ring-roller mill classifier section

ring-roller mill support stand

Rate of feed was controlled by a variable-speed drive on the auger. Calibration of the feed system in pounds of lignite delivered per unit time versus setting of speed drive provided a reasonable estimate of feed rate. However, the variable characteristics in feeding of different lignite samples prevented total reliance on this method of calibration. Lignite charged was weighed and operating time was recorded so that more accurate rates could be calculated.

Product Collection

The mixture of pulverized product and sweeping gas passed to a cyclone where the pulverized product and gas were separated (fig. 1), Product from the cyclone was collected in two containers in series. Each container rested on an indicating scale. The first container collected the major portion of product and the second, a drum-type vacuum cleaner, removed the dust and vented the product collection system. Incremental production rates were determined by recording the accumulated weight in the containers at intervals while a test was in progress. Samples of the product stream were collected by switching the total product flow to a sample container instead of to the collection drums.

Sweeping gas leaving the product cyclone was exhausted through a large cloth filter bag. Originally the bag was suspended on the same scale as the secondary product container. The weight of moisture condensed in the filter, however, far exceeded the weight of dust collected. During most tests the small quantity of dust collected by the filter bag was not measured.

Power Consumption

The power requirements of the pulverizer drive motor were measured using a standard three-phase-indicating wattmeter. Net power consumption could not be calculated or reasonably estimated because the overall efficiency of the drive system (belt and gear reduction) was low. No attempt is made, therefore, to compare power consumed in pulverization in the present test mill with that of other pulverizers. Power consumption reported is gross input and includes the no-load power requirement. The power required to provide the sweeping gas flow was also neither measured nor estimated. Owing to the complexity of the system, which included a heat exchanger and compressed inert gas storage, no reliable data in respect to power consumption for gas pumping was determined.

Evaluation of Test Variables From Pulverizer Operation

The number of variables and the range of test conditions studied was limited. Useful data could not be obtained during partly loaded operating conditions because internal changes which occurred could not be controlled or accurately measured. The major problem was that the amount of recirculating material was variable and compensated for imbalance between feed and product rate. Any change in feed rate resulted in a change in the amount of recirculating material in the pulverizer; however, pulverizer power input and fineness of product remained relatively constant. When the feed rate was such that the capacity of the mill for recirculating material was exceeded, the mill became overloaded and plugged. It was found, however, that the maximum product production rate could be determined for each specific lignite tested and for each operating condition; this provided a relative measure of the pulverization characteristics of each particular feed and the effect of operational changes on mill capabilities. The higher the value of this production rate, the easier the feed was to pulverize at identical operating conditions and product characteristics; put another way, the higher the production value, the more efficient the mill operates for given changes in operating conditions on a specific feed in producing product with the same degree of fineness. Maximum capacity as reported in this investigation does not, therefore, necessarily correspond to the conditions of most efficient operation of the mill at which the maximum amount of material would be pulverized at lowest input energy. This value was not determined.

The procedure for determining the maximum capacity consisted of increasing the feed rate in successive tests until the mill became overloaded. By weighing the product containers at 5-minute intervals, a progressive change in production rate was established. For any feed rate the product production rate was found to gradually reach a maximum after startup, after which it decreased if the feed rate was such that the mill became overloaded. The maximum productivity rate was found to be reproducible within 2 to 3 pounds per hour.

It was found preferable to feed the mill at a rate only several pounds per hour greater than the determined production rate, since this gave the least change in production rate with time. At a capacity of 50 pounds per hour, the reproducibility of maximum production rate was on the order of ±3 percent. Samples and data were collected at the maximum rate of production.

Terms Used in Evaluation and Representation of Data

The terms and relationships used to specify test condition and to describe pulverizer performance are as follows:

  1. Moisture content of feed and product, percent, are given for each test because of the importance of moisture content on pulverizer operation.
  2. Maximum product production, pounds per hour, is the maximum capability of the pulverizer at test conditions expressed as weight of feed pulverized per hour.
  3. Power consumption, kwhr per ton, is based on gross power input to the motor which drives the mill. The gross power input provides a relative measure of power input, although net power consumption for pulverization would have been preferable.
  4. Product fineness, percent minus 200 mesh, is the percent of product which passes a 200-mesh sieve. It provides a realistic criteria of product size. Most performance specifications for a pulverizer refer to product size in terms of percent passing a 200-mesh sieve and a minimum percent is specified as an acceptable measure of performance.
  5. Productivity is the weight in pounds of moisture-free (mf), minus 200-mesh lignite produced per hour. Incorporated in this term is the weight of product, size, and moisture content. The mf weight provides a direct measure of the fuel value of the pulverized product, regardless of the actual moisture. For any particular source of lignite, ash content is relatively constant and at a relatively low level. For this reason, the minor variations in ash content of samples have no significance on test results, and moisture-free weight is used rather than moisture and ash-free weight.
  6. Comparison factor, percent minus 200 mesh per kwhr per ton, relates the actual weight of pulverized material to power consumption. The smaller the numerical value of the comparison factor, the higher is the power requirement for producing a unit weight of minus 200-mesh material. Comparison factors determined in the present tests cannot be directly compared to those from previous studies because of differences in the measurement of power requirements.

Experimental Results

Variation in Pulverization Characteristics of Lignites From Different Mine Sources at Three Levels of Moisture Content

Pulverizer operation was compared on lignites at different levels of moisture content. Samples of lignites from three different mine sources were pulverized at their natural moisture content; lignite samples from these mines and three additional mines were dried and pulverized at the middle moisture range of 16.4 to 20.9 percent; and three lignite samples were dried and tested at low moisture levels, 6.4 to 12.0 percent. Kincaid and Indianhead lignites were tested at all moisture levels. Feed size for test samples was 0.125 by 0.046 inch. All tests used room air for sweeping the pulverizer to minimize moisture change during pulverization. Moisture content of product was usually slightly lower than that of feed, but in some tests on low moisture samples the product moistures were slightly higher than those of the feed because of moisture regained from the air, which had a high relative humidity.

Results are summarized in table 1. Maximum product production rate (pulverizer capacity) is markedly different for as-mined lignite from the different mines. For as-mined lignite from the Indianhead mine, the fineness of product partially compensated for low productivity when compared with Kincaid lignite, so that both the comparison factor and productivity were virtually identical. As-mined lignite from the Glenharold mine, however, had a lower maximum capacity rate, product fineness, productivity, and comparison factor, indicating that this lignite is more difficult to pulverize than the others. Productivity is plotted versus moisture content in figure 6 for tests in this phase of the investigation. Test data collected in other phases of the investigation but which are applicable to illustrate the influence of moisture content have been plotted as “supplemental test data.”

ring-roller mill effect of lignite source

In powerplants lignites are partly dried during pulverization so the moisture content of the product is more nearly in the middle moisture content range. The maximum capacity of the mill increased for all lignites when pulverized at the middle moisture content range, and maximum capacity of the mill for the three other lignites are within the range established by Kincaid and Glenharold lignites. In the case of Glenharold lignite, the productivity and comparison factor at middle moisture range are almost identical to that of natural lignite, which again indicates the unusual pulverization characteristics of this lignite.

For the three lignites dried to low levels of moisture ranging from 6.2 to 12.0 percent, the increase in maximum capacity and fineness of product at reduced power consumption are substantial. Both productivity and comparison factor reflect the significant improvement in pulverization characteristics of dried lignite compared to natural lignite.

ring-roller mill pulverizer productivity

Pulverization of Lignite Compared to Pulverization of Bituminous Coal

Pittsburgh seam coal obtained at the Arkwright mine, Monongalia County, W. Va., and a sample obtained from the Sunnyside mine, Carbon County, Utah, were tested. Both coals were high-volatile A bituminous. Hardgrove grindability index was 56 for the Arkwright and 48 for the Sunnyside coal.

Averaged test results obtained on the different lignites at each moisture level presented in the preceding section are representative of lignite pulverization at each moisture level and provide a basis for comparison of lignite pulverization to pulverization of bituminous coals. In table 2, average results at each moisture content level from table 1 are tabulated with the data obtained on the two samples of bituminous coal.

ring-roller mill comparison of pulverization

Pulverization of lignite dried to a low level of moisture content is comparable to pulverization of bituminous coals of 48 and 56 Hardgrove grindability index. At higher levels of moisture pulverizer capacity is reduced, product fineness is decreased, and power requirements are higher than for bituminous coals.

Comparison of lignite pulverization with that of bituminous coal is difficult because of the influence of moisture content on the characteristics of lignite. Moisture content of pulverized lignite should be specified if comparison is to be made with bituminous coal. Similarly, the Hardgrove grindability index varies with the moisture content of the lignite sample tested.

This variation has been studied and efforts to correlate Hardgrove index to pulverizer operation have been made. The Hardgrove indices for lignites tested in this study are not reported because the relationship between pulverizer operation and measured Hardgrove index is subject to many uncertainties. The determination of Hardgrove grindabilities of lignites is in itself subject to procedural variations within the specifications of the ASTM standard test. Because specific procedures for low-rank coals have not been established, Hardgrove grindability test results on lignites tested are not presented.

Pulverization of Lignite With in-the-Mill Drying

Pulverization of lignite in powerplants without drying is subject to many operating difficulties which reduce reliability. The cost for pulverizer units is also higher because more mills are required to obtain desired capacity. In-the-mill drying is a satisfactory compromise between no drying at all and a separate drying system integrated into plant operation to dry the pulverizer feed. A separate drying system in powerplants would benefit pulverizer operation; however, there are offsetting system considerations. These include the increase in risk due to possible fires and explosions associated with handling dried coal in the plant, the capital investment requirements for a separate drying system, and the added problems of integrating a drying operation in the power generating system of the plant The evaluation of these considerations requires knowledge of the magnitude of the benefits to be obtained by predrying. Pulverization of lignite with in-the-mill drying is therefore compared to pulverization with predrying to aid in these evaluations.

The process of pulverization is more complex when drying is combined with the size-reduction process. In lignites, moisture is present not only on the surface of particles, but is an inherent portion of each particle. To remove inherent moisture, the entire particle must be heated-a process which requires time. The more the particles are dried, the higher is the temperature to which particles must be heated and the longer it takes to accomplish this.

The sweeping gas provides the heat for drying, entrains particles at discharge from the grinding zone, transports them to the classifier, and finally, transports the pulverized product and vaporized moisture to the burners. The smaller the particles, the more rapidly and easy they are to dry. The portion of the drying capacity of the sweeping gas which dries particles as they are carried from the classifier to the burners is not beneficial to pulverization. The portion of the drying which is beneficial in pulverization is that drying which is accomplished on the larger particles which are recirculated to the grinding zone.

In the grinding zone, particles are a mixture of different moisture contents ranging from as-mined moisture content to near-product moisture content The moisture content of this mixture as it enters the grinding zone is always lower than as-mined lignite, but is never as low as the product. In-the-mill drying cannot achieve the benefits obtained by predrying lignite to identical product moisture contents because a part of the mixture entering the grinding zone is always undried.

The major efforts in testing were directed toward modification of procedures and overall system techniques, with the objective of improving lignite pulverization in general. Tests are not reported with in-the-mill drying in which the optimum settings of the mechanical adjustments were established. In any installation, these settings would be established during actual plant operation on the particular coal supplied. Such operational variables apply only to a particular type of mill and are not applicable to other pulverizers. Consequently, detailed data in respect to the optimum table rotation rate, grinding race pressure, and classifier setting are not given.

The extent of in-the-mill drying was changed in experimental efforts by varying the quantity and temperature of sweeping gas. These tests had but limited success in establishing any relationships. Although physical dimensions of the test mill were in proportion to those of a commercial-size mill, the capacity of the test mill is about 1/50 the size of the smallest commercial mill of this type. The residence time of particles in the sweeping gas during transport from the grinding zone to the classifier was therefore much less in the test mill than in the larger commerical-size mills because the distance traveled by the particles was less. Considering the great influence of the duration of drying time, in-the-mill drying in the test unit could not achieve the drying that would be expected in commerical-scale operation. Other factors which contribute to these differences are the ratio of area of internal surface/volume, the degree of turbulence and mechanical mixing, and the ratio of coal in the mill zones to throughputs. These relationships are undoubtedly not comparable between the test mill and larger-size mills.

Variations in temperature and quantity of sweeping gas were found to change classifier operating characteristics as well as particle residence time, so that one change tended to influence the other. For example, increasing the temperature of sweeping gas increased product production but decreased fineness. The complexity of in-the-mill drying in lignite pulverization and the limitations imposed on applicability of data obtained on the small test pulverizer negated a comprehensive study of this variable.

In-the-Mill Drying Compared to Predrying of Lignite for Pulverization

An extensive review of all test data collected on Kincaid and Indianhead lignites was made to develop, if possible, a comparison between the performance of the pulverizer using in the -mill drying and operation with predried lignites. Product moisture content was designated the principal parameter for comparison. Accordingly, all test results obtained with in-the-mill drying were grouped in ranges of 2.5 percent product moisture. Within each specific group by moisture content, a variety of test conditions of temperature and flow rates were studied. These groups were further subdivided in 1.0-unit ranges of the ratio of weight of sweeping gas to weight of lignite feed rate as a means of restricting the range of test conditions being considered. Test results in each of these subgroups was averaged. Operations with in-the-mill drying, within a limited range of sweeping gas flow rates but at similar product moisture contents, are reflected in these values. Those values obtained with in-the-mill drying were compared to results obtained during operation on predried lignites at the most similar conditions of moisture content and test conditions.

Test results for the two lignites which produced product at three levels of moisture content with in-the-mill drying and for operation on predried lignite with comparable moisture content are given in table 3. All mechanical pulverizer adjustments such as table speed, classifier setting, and spring tension were identical.

For Kincaid lignite, test data at similar product moisture content and at reasonably similar air-to-coal ratios was available. For Indianhead lignite, air-to-coal ratios for in-the-mill drying were low compared to operation on predried lignites, but more comparable data was not available.

ring-roller mill drying

The advantage of predrying lignite over in-the-mill drying is demonstrated for each set of compared tests. The benefits of increasing the amount of drying is also demonstrated for both types of operation. In each case, the greater the degree of drying, the greater the productivity and the higher the comparison factor.

The test data used in comparing pulverization of predried lignite with in-the-mill drying does not necessarily represent the best conditions at which to pulverize lignite with in-the-mill drying. Numerous tests were performed with in-the-mill drying at operating conditions which provided more effective utilization of the pulverizer capabilities. In figure 7 the maximum production rate, pounds of minus 200-mesh mf lignite per hour, is plotted for tests given in table 3. Data obtained in numerous other tests performed with in-the-mill drying but not used in the tabulated comparison have been plotted as “supplemental data” points. The corresponding curve for pulverization of predried lignite (fig. 6) is also given. The data points for Kincaid lignite with in-the-mill drying given in table 3 are particularly poor in representation of optimum operating conditions. With in-the-mill drying, the pulverizer operates as if the lignite in the grinding zone were at least 5 to 10 percent higher in moisture than the pulverized product.

ring-roller mill productivity

Size Consist and Moisture Content

A series of tests was performed on test samples of two size fractions, each at two moisture levels. The samples were tested individually and as mixtures. The objective of these tests was to determine changes in pulverizer performance as a function of controlled conditions in the grinding zone in respect to size consist and moisture content. The size consist and moisture content of each feed sample and results for each test are given in table 4. Data is grouped to simplify comparison between tests, and in some cases individual test data is repeated.

Group I compares the effect of size on lignite having a natural moisture content. At high moisture levels, size of feed has some influence on pulverizer operation, but less than could be expected.

ring-roller mill moisture content

Group II compares the effect of size on lignite of low moisture content ranging from 11 to 15 percent. At this moisture content, dried lignite of large size was significantly more difficult to pulverize than either the small size or a mixture of the two sizes.

Pulverization characteristics of the mixtures in groups I and II were similar to those of the small size, but in some respects a mixture of two sizes pulverized somewhat better than did the small size. The maximum production rate was higher for the small size, but the product fineness was higher for the mixture of two sizes, giving a somewhat higher productivity. Power requirements, however, were lowest for the test on small-size feed and, correspondingly, the comparison factor is also highest. For both groups I and II, moisture content of the small-size lignite was somewhat higher than that of the large size or of the mixture of the two, and this may well have contributed to what appears to be an inconsistency.

In groups III and IV the effect cf drying is shown on lignites of each size, and the improvements produced by drying are apparent.

In group III, the test conducted on the mixture of high- and low-moisture- content, large-size particles produced results very comparable to the test on the feed with highest moisture content, with the exception that product moisture content was lower. Both comparison factor and as-received productivity are practically identical to the test on high-moisture, large-size feed despite the fact that the overall moisture content of the mixture was appreciably lower. The proportion of high-moisture particles appears to be great enough to control pulverizer operation.

In group IV, the pulverization of high-moisture-content, small-size particles is very similar to pulverization of the high-moisture-content, large-size particles. Pulverization of low-moisture-content, small-size particles was significantly better than pulverization of low-moisture-content, large-size particles. The improvement in pulverization characteristics with moisture reduction is much more significant for the small-size test mixtures than for the large-size test mixtures.

In group V a variety of mixtures of the four initial conditions are compared. Pulverizer performance is not appreciably affected by moisture content or size until the proportion of dried, smaller-size fraction is somewhat greater than 35 percent. Test results in which the mixture contains 35 percent dried fines are very similar to those obtained in the tests in which all feed is at natural moisture content or when moisture content of the larger size fraction was low (table 4, group V). When the proportion of dried small- size fraction is at 44 percent (table 4, group V) an improvement is noted. Finally, drying both size fractions produced highest pulverizer production rates.

These test results demonstrate the important influence of moisture content in pulverization of lignite. However, test results indicate that both size consist and moisture content are controlling factors. Also, if only the fine sizes are dried, at least 50 percent of the total feed must be dried fines to appreciably benefit pulverizer operation, because the high-moisture, larger particles appear to control pulverizer operation.

Techniques To Increase Degree of Drying

A variety of techniques were tested in attempts to increase the effectiveness and degree of drying. Such techniques, if successful, could possibly be applied to existing commercial installations or incorporated in design of future units.

Preheating Feed to Pulverizer

In at least some of the commercial installations, a small portion of the heated sweeping gas is introduced into the feeder mechanism to reduce or prevent accumulations of surface-wetted lignite. The amount of sweeping gas used in this manner is negligible in comparison with that introduced into the pulverizer. In order to increase the drying accomplished by this technique in the test system, a 25-ft, hairpin-shaped length of insulated 1-½-inch steel pipe was installed between the feed hopper and the test pulverizer. Modifications were then made in the sweeping gas system to provide sufficient sweeping gas to entrain and initiate drying during transport from the feeder to the pulverizer.

Despite operation at various proportions of flow through the preheater, no discernible benefits in pulverization could be detected. The lack of pulverizer operational improvement was attributed to the fact that particle residence time was insufficient for appreciable drying of the larger particles.

The amount of moisture reduction which was accomplished on the smaller particles was also not enough to influence pulverizer operation. Because the larger particles control pulverizer operation, little benefit could be achieved without significant drying of these particles.

Predrier-Crusher System

Systems in which a partial drying is combined with size reduction are utilized in industry, particularly in size reduction of a surface-wetted material. One application of these systems is presently being used to reduce moisture content of lignite feed to a recently completed cyclone-burner-fired boiler.

Moisture at particle surfaces can be easily removed from lignite to reduce moisture content of the particle by several percent. To remove greater quantities of moisture, particles must be heated. The heating process requires a residence time—the factor which contributed to limited benefits of the feed preheater described in the preceding section. If size reduction is combined with the drying process new and fresh surface is formed, and drying can be accomplished more rapidly.

Tests utilizing a combination predrier and crusher were performed to determine the potential of such a system in pulverization. A test unit was constructed in which hot combustion gases and lignite could be introduced into an enclosed chute which served as the feed hopper for a swing-hammer crusher. The crusher discharged the partly dried and crushed lignite onto a conveyor. The exhaust gases discharged with the lignite and vented to the atmosphere.

A sample of 1- by ½-inch lignite of 32 percent moisture content was passed through the crusher but not exposed to drying gases. The size of the crushed lignite was 13.7 percent retained on 1/8-inch screen and 50 percent minus 1/16 inch. Similarly, a second sample of by ½-inch lignite was crushed and simultaneously exposed to the hot gas for drying. The size of the crushed and dried lignite was 7.5 percent retained on 1/8-inch screen and 65 percent minus 1/16 inch. The moisture content of the dried lignite was 22.5 percent.

Both samples were immediately used as feed to the pulverizer after pre-treatment. A third sample of 1- by ½-inch lignite was tested in the pulverizer as a reference. There was no significant difference in pulverizing the reference sample and the precrushed lignite. Although size of the precrushed lignite was smaller compared to the reference sample, the difference in feed size was not significant in comparison to the size of the pulverized material. Pulverization of the precrushed and predried lignite immediately after processing in the crusher-drier unit, however, was significantly easier than pulverization of crushed, but not dried, samples. The capacity of the pulverizer was increased 45 percent and power requirements were reduced 30 percent compared with pulverization of both the reference sample and the precrushed lignite. The moisture reduction was the major factor in improving operation. A combination drier-crusher unit installed in an existing system would provide increased pulverizer capacity and significant reduction of power requirements without modification of existing pulverizers.

Pulverizer Modifications

Within the pulverizer, particles entrained by sweeping gas are dried during transport to the classifier. A swirling motion is imparted to the sweeping gas by the air-inlet ports of the pulverizer. It appears reasonable to expect that a portion of the larger particles would be diverted out of the gas path by collision with other particles and directly back into the grinding zone without passing through the classifier. For that portion of the recirculating load, residence time in the heated gas would be reduced. To prevent particles from bypassing the classifier, a shield was constructed which enclosed the internal center portion of the pulverizer. It was expected that improvements in overall drying and thus pulverization efficiency would be achieved. Comparative tests, however, showed no detectable improvement in pulverizer operation. It was concluded that either retention time was not significantly increased or only a small portion of the particles bypassed the classifier.

Negative Pressure Operation

Pulverization of lignite should be improved by operation with negative static pressure in the mill rather than the usual positive pressure because the degree of in-the-mill drying would be increased. Modifications to the mill and sweeping gas system were made to test this type of operation.

Comparative tests were performed at three levels of static pressure ranging from plus 7.4 to minus 4.4 inches of water. As static pressure in the mill was reduced, weight of product obtained per unit time increased, but the fineness decreased. Differences in operation as a function of mill static pressure could not be related to moisture removal, but appear to be related to changes in sweeping gas velocity and mass flow relationships. Results from pilot plant tests showed no significant benefits during operation with negative rather than positive mill pressure.

Pulverization of Frozen Lignite

During winter, lignite may freeze either in storage or in the mine if the stripped lignite seam is exposed to low temperatures for extended periods before mining. Frozen lignite is usually detected in plant handling and storage facilities when hangup problems develop in conveyor and bunker systems. If the lignite is frozen, important adverse effects on pulverizer operation also occur.

Comparative tests were performed to evaluate and demonstrate the effect of freezing on pulverization characteristics of lignite. In these tests the only variable was the difference in temperature of the test lignite. A sample of each of two sizes (0.125 by 0.046 inch and 0.046 by 0 inch) of lignite of as-received moisture content were exposed to ambient winter temperatures until the particle temperature was well below freezing. The samples were then immediately pulverized. A similar sample of each size but at normal room temperature was also pulverized. Room air was used to sweep the pulverizers to eliminate in-the-mill drying as a variable.

The pulverizer capacity was reduced by 25 percent and power consumption increased when feed was frozen. Maximum product rate pulverizing 0.125- by 0.046-inch frozen lignite was 42 pounds per hour compared to maximum product rate of 56 pounds per hour on an unfrozen sample. For 0.046- by 0-inch lignite, the corresponding values were 38.0 and 58.0 pounds per hour. The 0.125- by 0.046-inch frozen sample required 27.6 kwhr per ton compared to 23.6 kwhr per ton for the unfrozen lignite, and the 0.046- by 0-inch frozen sample required 25.2 kwhr per ton compared to 20.7 kwhr per ton for unfrozen sample. Frozen lignite severely reduced the capabilities of the pulverizer. When the reduction in drying capabilities in the mill are also considered because of increased heat requirements per unit weight of feed, an additional decrease in capability occurs.

Summary and Conclusions

Tests conducted in this investigation conclude a series of studies of lignite pulverization in different types of size-reduction equipment. These studies have shown and demonstrated specific characteristics of lignite pulverization common to all types of mills as follows:

  1. Pulverization characteristics of lignite vary significantly from one mine source to another. Similarly, lignite from one seam may have significantly different pulverization characteristics compared to lignite from another seam at the same mine. In this investigation, capacity of the test pulverizer ranged from 41 pounds per hour with high power consumption to 53 pounds per hour and reduced power consumption for the lignites tested.
  2. Moisture content is a major factor in pulverization of lignite. Removal of moisture either before or during pulverization produces progressive improvements in pulverizer operation. In this investigation, when lignite was predried to middle moisture content levels (15 to 20 percent), capacity increased from 41 to 53 pounds per hour to 70 pounds per hour and power requirement decreased from 29 kwhr per ton to about 18 kwhr per ton. At a low moisture content, 10 percent, capacity increased to 92 pounds per hour and power consumption decreased to 14 kwhr per ton. Predrying is more effective than in-the-mill drying because with in-the-mill drying, a portion of the material in the grinding zone is always at the natural moisture content. For the same product moisture content, pulverizer operation with in-the-mill drying is comparable to operation on predried lignite of 5 to 10 percent higher moisture.
  3. Compared to pulverization of several bituminous coals having a Hardgrove grindability index in the range of 48 to 56, mill capacity during pulverization of lignite at natural moisture content without drying is less than 50 percent, and power requirements on a per ton basis are virtually double. Capacity of the mill is increased to 70 percent of that of bituminous coals when lignite is pulverized at middle moisture content levels; when dried to low moisture content levels, mill capacity and power requirements are com¬parable to the bituminous coals tested.
  4. Techniques which increase the degree of in-the-mill drying will increase pulverizer capacity and reduce power requirements during pulverization of lignites. Techniques tested included both modifications of the test mill and alterations of the test system. A combination predrier-crusher system achieved significant benefits in pulverization. Pulverizer capacity was increased 45 percent and power requirements were reduced 30 percent. Present lignite predrier-crusher plants could provide additional pulverizer capacity without modification of existing units by installation of such pre-treatment systems.
    Entrainment of lignite feed in preheated sweeping gas was unsuccessful because of limited drying accomplished on larger particles. To be an effective technique to improve drying, sufficient residence time must be provided for drying larger particles in order to influence pulverizer operation.
    Operation of test pulverizer with a negative mill static pressure rather than a positive pressure did not improve mill capabilities. Rate of vaporization of moisture from particles should be increased at lower pressure but other conditions appear to have much greater influence on pulverizer performance. Benefits, if any, could not be detected using the test unit.
  5. When pulverizing frozen lignite, pulverizer capacity is reduced 25 percent and power requirements are increased compared with operation using unfrozen lignite.
  6. Test results obtained in this investigation were found to substantiate and enlarge conclusions reached in previous investigations. Attempts to utilize the test mill as a replication of commercial-scale pulverization were limited because a major consideration in lignite pulverization-degree of drying in-the-mill-was not comparable. The measure of performance for a particular type of pulverizer on a lignite will best be established on commercial-size mills operating on tonnage samples. Pilot plant tests serve to establish and define relative differences which can be encountered and the importance of operating variables.


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