Laboratory Study of an Organic Binder for Palletization of a Magnetite Concentrate

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Laboratory study of an organic binder for palletization of a magnetite concentrate Minerals & Metallurgical Processing, Aug 2010 by de Moraes, S L, Kawatra, S K Abstract This study aimed to identify a way to reduce the use of bentonite in the pelletization of magnetite. With this goal, different combinations of binders were compared to bentonite by examining the quality of pellets obtained by balling drum agglomeration. Pellets were subjected to routine tests that included simple compressio
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  Laboratory study of an organic binder forpalletization of a magnetite concentrate Minerals & Metallurgical Processing,  Aug 2010 by de Moraes, S L,  Kawatra, S K   Abstract This study aimed to identify a way to reduce the use of bentonite in the pelletization of magnetite. With this goal, different combinations of binders were compared to bentonite byexamining the quality of pellets obtained by balling drum agglomeration. Pellets were subjectedto routine tests that included simple compression of the wet and dry 105 o C pellets, dropping wetpellets to determine their ability to survive handling and submitting pellets to thermal shock todetermine how they tolerate drying and preheating. The best results use sodium silicate (1.5%) asa binder and show that it is possible to pelletize iron ore without using bentonite.Key words: Pelletization, Magnetite, Iron/ iron ores, Binders, Bentonite Introduction Pelletization is a method of agglomeration used to turn fine fractions of iron ore into an adequateproduct (pellet) to be fed to blast furnaces and direct reduction reactors, where it will be reducedto pig iron or sponge iron.Binder must be added to the pelletization mixture to raise the liquid phase viscosity inside thecapillary spaces, to maintain the cohesion of green pellets and to improve the compressivestrength of the thermally treated pellets, by contributing to the formation of ceramic or iron oreoxide bridges or to scorify discrete points during thermal treatment.The classic binder for iron ore pelletization is bentonite, which is characterized by its highswelling power, great superficial area, cation exchange capacities around 60 to 170 meq/100gand property of thixotropy. In contact with water, the bentonite platelets separate, forming acolloidal gel.Bentonite enhances the strength of the iron ore pellets by the following mechanism: the presenceof colloidal material shortens the distance between particles, raising the intensity of the Van derWaals forces . The disposition of bentonite plates in the point of contact between ore particlesalso enhances the pellet's compressive strength. The typical dosage of bentonite as a binder foriron ore pelletization is 0.5 to 0 .7% of the ore mass (dry basis) . A disadvantage of bentonite isthat it adds the undesirable contaminants alumina and silica to the pellet.  Binders that would not leave a residue after thermal treatment would be extremely interesting.They would be advantageous for minimizing the variations in quality of the pellets, lowering thedosage to be used to around 0.05%. Materials and methods Sample ore and binders. Asample of magnetite concentrate from the Empire Mine (Palmer, MI)and received by Michigan Tech on 16/03/2009 was utilized. The average chemical compositionof the sample is shown in Table 1.The work was developed using the following binders: bentonite , sodium hydroxide , sodiumcarbonate , sodium metasilicate , sodium tripolyphosphate and carboxymethyl cellulose. Thetypical chemical composition of bentonite is shown in Table 2 and the specification of thebinders in Table 3.The choice of binders to be tested was based on the author's previous experience using thesebinders in the agglomeration of hematite concentrate (Moraes and Cassola, 2008).The sodium hydroxide and sodium silicate were used in the form of a 50% (w/w) solution inwater.The moisture of bentonite was determined by drying it in an oven at a temperature of 105 o Cuntil constant weight. The moisture content of bentonite was 13% (dry basis).Homogenization and reduction of sample mass. The sample of iron ore concentrate was receivedin three containers containing approximately 15 kg (wet weight) each. It was dried at atemperature of 100 0 C for 24 hours.The dry sample was disaggregated using a 28-mesh (0.71 mm) sieve, blended and split by arotatory splitter. 24 subsamples of approximately 1.7 kg each were produced. Figure 1 shows theprocess of homogenization and reduction of mass of the sample.Pelletization tests. For each test the ore , binder and moisture were added to a laboratory mixer(Fig . 2a) for two minutes . The tests were carried out using a laboratory-scale balling drum (Fig.2b) with aliquots of 1.7 kg (dry basis) ore. The balling procedure consisted of: 1) Delump mixed binder and concentrate through an eightmesh screen;2) Add a small amount of ore mixture to drum rotating at 25 rpm; moisten it with a water sprayto produce pellet seeds; 3) Enlarge seeds by adding more concentrate while spraying with water, periodically removingand screening to keep uniform size. Pellets are finished when they are between 1 .6 and 12.7 mm(0.06 and 0.5 in.) in diameter.  The composition of each mixture is given in percentage by mass concentration based on drycomponents . Moisture content and binder dosages are shown in Table 4. Characterisation of the pellet properties . Two standard tests are used to measure the strength of pellets, whether the pellets are green pellets or fired pellets. These tests are the drop test andthe compression test.The drop test requires dropping a random sample of pellets from a height of 46 cm (18 in.) onto asteel plate. Pellets are dropped repeatedly until the pellets crack. The number of drops needed tocrack each pellet is recorded and averaged. Compression strength is measured by compressing or applying pressure to a random sampling of pellets until the pellets crumble. The pounds of force required to crush the pellets is recorded andaveraged.These two tests are used to measure the strength of both wet and fired pellets . The drop andcompressive test measurements are important because pellets, proceeding through the ballingdrum and subsequent conveyor belts , experience frequent drops as well as compressive forcesfrom the weight of other pellets traveling on top of them. Thermal shock resistance is a factor which must be taken into consideration in any process foragglomerating mineral ore concentrate . Increases in a pellet's thermal shock resistance improvethe pellet's ability to resist internal pressures created by the sudden evaporation of water whenthe pellet is heated in a kiln. If the pellet has numerous pores through which the water vapor canescape, thermal shock resistance is improved. If the surface of the pellet is smooth andcontinuous, without pores, the pellet has an increased tendency to shatter upon rapid heating.This causes a concurrent increase in the amount of fines or coarse particles in the pelletizedmineral ore. A binder which increases the pores formed in a pellet improves the pellet's ability toresist thermal shock.The pellets obtained were subjected to the tests for characterization described as follows.  Determination of moisture . After the pelletizing, about 300 g pellets were randomly sampled anddried in an oven at temperature of 105 0 C until constant weight.  Drop number green pellet  . From each batch of green pellets obtained, 20 pellets were randomlyselected and dropped freely from a height of 46 cm (18 in.) onto a platform with a steel shank.Each pellet was dropped repeatedly until the first fractures appeared. The number of dropsincurred by each pellet was recorded. The result is the average of values given by number of fallsper pellet. Wet compression strength . From each batch of pellets obtained, 20 pellets were randomlysubjected to automatic compression until breaking. At this time the load was recorded and thearithmetic mean of the values represents the test result in kgf orN/pellet.   Dry compression strength . This test is similar to that done with green pellets, differentiated anadditional step: the 20 pellets were dried in an oven at 105 o C for 24 hours . After cooling for 15minutes, the pellets were subjected to a compression test and the result was expressed in kgf orN/pellet. Thermal shock resistance test  . From the batch of green pellets, groups of 15 pellets were selectedat random. Then each group was placed in a muffle furnace at temperatures of 300 o  C, 500 o  C,700 o  C and 900 o  C for 10 minutes at each temperature. After this time, the groups were left tocool in the air and subjected to a test of compressive strength. This step also evaluates thepercentage of pellets that crack and/or explode. Results and discussion Table 4 shows the results obtained in the pelletization tests with iron ore concentrate anddifferent binders. For each value reported, the mean and standard deviation were determined for20 pellets. The error bars shown on the graphs represent the 95% confidence intervals calculatedusing the t-distribuition.The variation of the moisture content in each test was from 8.5% to 9.8%. The biggest moisturevalue was observed in the test CMC NaOH and the smallest in the test with sodium carbonate.The wet drop results are shown in Fig. 3.The highest value observed was that of the tests bonded with bentonite, which survived 3.2drops. This value is not the minimum industrially acceptable, but in this case will be the value of reference. The worst result was observed for pellets bonded with sodium silicate, which onlylasted 1.9 drops. Pellets bonded with sodium carbonate, bentonite NaOH and CMC (sodiumcarbomethyl cellulose) TPP (sodium tripolyphosphate) show the same result of 2.9 drops each.  Effect of bentonite. Figure 4 shows the compressive strength of pellets bonded with bentonite .Two tests were carried out with bentonite at the dose of 0.66%. They showed wet and drycompressive strength results of 19 N/pellet and 38.3 N/pellet, respectively, higher than theminimum industrially acceptable (9 N/pellet, wet and 22 N/pellet, dry). Pellets bonded withbentonite at 0.4% and NaOH at 0.02% showed less wet and dry compressive strength comparedto plain bentonite at O.66%. However, the result of the wet compression (18.1 N/pellet) is stillgreater than the minimum industrially acceptable standard. The dry compressive strength result(20.7 N/pellet) is almost the minimum industrially acceptable that is 22 N/pellet. These resultssuggest that the dose of bentonite and NaOH can be adjusted to reach the target strength.The behavior of pellets bonded with bentonite (0.66%) and bentonite (0.4%) NaOH (0.02%)during thermal shock can be seen in Fig. 4. As shown in this figure there are no significantdifferences between the results at 30O0C and 50O0C. At 70O0C the performance of pellets withbentonite (0.4%) NaOH (0.02%) falls and at 90O0C the compressive strength improves again.These results show that is possible to reduce the amount of bentonite without reducing thequality of the pellets.
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