1.6 Modified Proctor/Fagerberg test procedure for bauxite

1.6.1 Scope

• .1 The test procedure specified in this section (this test) should only be used for determining transportable moisture limit (TML) of bauxite cargoes containing both:

  • .1 more than 30% of fine particles less than 1 mm (D₃₀ < 1 mm); and

  • .2 more than 40% of particles less than 2.5 mm (D₄₀ < 2.5 mm).

• .2 The TML of a cargo is taken as equal to the critical moisture content at 80% degree of saturation according to the modified Proctor/Fagerberg test procedure for bauxite where the optimum moisture content (OMC) of the bauxite tested occurs at saturation levels greater than or equal to 90%.

• .3 The TML of a cargo is taken as equal to the critical moisture content at 70% degree of saturation according to the modified Proctor/Fagerberg test procedure for bauxite where the OMC of the bauxite occurs at saturation levels less than 90%.

• .4 Where moisture freely drains from the sample such that the test sample compaction curve cannot extend to or beyond 70% saturation, the test is taken to indicate a cargo where water passes easily through the spaces between particles. Therefore, the cargo is not liable to liquefy.

1.6.2 Test equipment

• .1 The Proctor apparatus (see figure 1.6.2) consists of a cylindrical iron mould (the CBR mould) with a removable extension piece that is screwed onto the top and a compaction tool guided by a pipe open at its lower end (the compaction hammer).

• .2 A weighing balance (see 3.2) capable of weighing the sample and the container, as received, with an accuracy of better than ± 1 g, and suitable sample containers.

• .3 A drying oven with a controlled temperature interval from 100°C to maximum 105°C.

• .4 A container for hand mixing. Care should be taken to ensure that the mixing process does not reduce the particle size by breakage or increase the particle size by agglomeration.

• .5 A gas or water pycnometry equipment to determine the density of the solid material as per a recognized standard (e.g. ASTM D5550, AS 1289, BS 1377 Part 2).

Figure 1.6.2 – Equipment and definitions

1.6.3 Temperature and humidity (see 1.1.3 of this appendix)

1.6.4 Procedure

• .1 Establishment of a complete compaction curve

  • A representative sample according to a relevant standard (see 4.7 of this Code) of the test material is utilized.

• .2 Sample homogenization and division

  • Divide the as-received sample into individual subsamples using a sample dividing apparatus as specified in ISO 6140:1991 "Aluminium ores – Preparation of samples". Place these subsamples into heavy-duty plastic buckets that can be sealed to preserve water content.

• .3 Reconstituted sample preparation procedure

  • Reconstitution is required for bauxites if the amount of material in the representative sample that is coarser than 25 mm is greater than 10% of the total mass (by dry weight). In this case the reconstitution process below should be applied (see also figure 1.6.4.3). In this process, particles above 25 mm are removed from the sample and replaced by an equivalent mass of particles in the range 6.3 mm to 25 mm. Note that 6.3 mm is a standard sieve size. If it is not available, a 6 mm sieve may be used instead. Through reconstitution, a final reconstituted sample of sufficient mass for TML testing is generated which contains a maximum particle size of 25 mm, but which better reproduces the compaction characteristics of the full as-received particle size distribution with particles greater than 25 mm.

  

  Figure 1.6.4.3 – Schematic diagram of reconstitution sample procedure

  Step 1:	The first step in sample preparation is to sieve the sample at 25 mm to determine the amount of dry weight that is larger than 25 mm. All material retained on the 25 mm sieve is to be discarded, while material passing 25 mm is to be tested (see figure 1.6.4.3). Screen about 40 kg of bauxite for one Proctor/Fagerberg compaction curve if reconstitution is required. If the amount (by dry weight) that is larger than 25 mm is 10% or less, then no reconstitution is required. A total of about 50 kg of 25 mm screened bauxite is needed for one Proctor/Fagerberg compaction curve. Obtain this and proceed to Step 4 if no reconstitution is required.
  Step 2:	If reconstitution is required, then from a separate representative subsample of the same ore screen at 25 mm and 6.3 mm and retain the portion between these two sieve sizes. Produce about 15 kg of screened bauxite between these two sizes for one Proctor/Fagerberg compaction curve (see figure 1.6.4.3).
  Step 3:	Add the 6.3 to 25 mm fraction from Step 2 to the <25 mm fraction from Step 1. The amount of 6.3 mm to 25 mm fraction added must be equal to the mass of > 25 mm bauxite removed and discarded in Step 1.
  Step 4:	Fully mix the reconstituted or scalped sample.

• .4 Compaction

  • Compaction tests are executed for 5 to 10 (but typically 7) different moisture contents in separate tests. The number of tests should fully define the compaction curve, including accurately determining the optimum moisture content and conditions near to or at full saturation. A total of about 40 kg to 100 kg of unscreened bauxite is typically needed for one series of tests, depending on whether reconstitution is required or not.

    Divide the screened bauxite into 5 to 10 subsamples and place each in a sealed plastic bucket. The water content of the subsamples is adjusted by adding a suitable amount of water into each bucket in order to obtain water contents ranging from the as-received water content to almost saturated subsamples.

    Typically one test is carried out at the as-received water content. Four to eight tests are carried out at higher water contents. Depending on the as-received water content, one or two tests are carried out at lower water contents than the as-received water content. These lower water content tests are obtained by partially drying the bauxite sub-sample at room temperature. Note that full drying of the bauxite samples is not to be carried out. Note that some bauxites are not sensitive to pre-drying, but unless this is comprehensively demonstrated for the bauxite being tested, pre-drying is not permitted.

    The subsamples are mixed at the target water contents before being allowed to rest and equilibrate overnight in the sealed plastic buckets. The plastic bucket contents are remixed immediately before compaction to ensure that a homogeneous sample is tested.

    The required quantity of prepared bauxite per compaction test is about 5 kg, so this is the target amount for each subsample. The subsamples will be compacted at the prepared water contents to define a full compaction curve.

    Approximately one fifth of the subsample in a plastic bucket is filled into the mould with the extension piece attached (figure 1.6.2) and levelled to form the first layer increment. Bauxites that display plasticity may tend to stick together into clumps at the wetter water contents. It is important to gently disaggregate the clumps by hand into small particles when loading the mould.

    The bauxite is then tamped uniformly over the surface of the layer. Tamping is executed as per the Proctor/Fagerberg method D, by dropping a 150 g hammer 58 times through the guide pipe, 0.15 m each time. The guide tube is held by hand and positioned so that it is just touching with the surface of the ore, but is not resting on it. This allows the hammer to fall the full specified height and compact the bauxite, while avoiding additional (and excessive) compaction arising from the weight of the guide tube, itself, which should not be compressing the bauxite.

    The performance is repeated for all five layers so that the last layer only just overfills the mould-extension piece junction. The aim is to fill the mould by compacting in five nearly equal layers in order to produce a uniformly compacted sample in which the specified energy has been put into bauxite that occupies the mould volume.

    Bauxite should not extend more than 1 cm in height above the mould into the extension piece. If a height of more than 1 cm of bauxite is found, the sample has been under compacted and the test should be redone.

    When the last layer has been tamped, the extension piece is removed and the sample is levelled off along the brim of the mould with care, ensuring to remove any large particles that may hinder levelling of the sample, replacing them with material contained in the extension piece and re-levelling.

    The levelling process should be a horizontal cutting action, not a pushing action as that would compress more material into the mould volume than had been compacted by the specified energy input.

    After the weight of the cylinder with the tamped sample has been determined, the cylinder is emptied, the full sample is dried at 105°C as per ISO moisture determination standard for bauxite (ISO 9033:1989 "Aluminium ores – Determination of the moisture content of bulk material") and the weight is determined. The test is then repeated for the other samples with different moisture contents.

• .5 Definitions and data for calculations (see figure 1.6.2)

  • - empty cylinder, mass in grams: A

  • - cylinder with tamped sample, mass in grams: B

  • - wet sample, mass in grams: C

    • C = B − A

  • - dry sample, mass in grams: D

  • - water, mass in grams (equivalent to volume in cm³): E

    • E = C − D

      Volume of cylinder: 2,304.5 cm³

• .6 Calculation of main characteristics

  • - density of solid material, g/cm³ (t/m³): d

  • - dry bulk density, g/cm³ (t/m³):

    • 

  • - net water content, volume %: e_ν

    • 

  • - void ratio: e (volume of voids divided by volume of solids)

    • 

  • - degree of saturation, percentage by volume: S

    • 

  • - gross water content, percentage by mass: W¹

    • 

  • - net water content, percentage by mass: W

    • 

• .7 Presentation of the compaction tests

  • For each compaction test the calculated void ratio (e) value is plotted as the ordinate in a diagram with net water content (e_v) and degree of saturation (S) as the respective abscissa parameters.

    

    Figure 1.6.4.7.1

  • Form of bauxite compaction curve when the optimum moisture occurs at a saturation greater than 90%. TML is determined as the critical water content at 80% saturation.

    

    Figure 1.6.4.7.2

  • Form of bauxite compaction curve when the optimum moisture occurs at a saturation less than 90%. TML is determined as the critical water content at 70% saturation.

• .8 Compaction curve

  • The test sequence results in a specific compaction curve (see figures 1.6.4.7.1 and 1.6.4.7.2).

    Should the optimum moisture content (OMC) of bauxite occur at or above 90% saturation, the critical moisture content is indicated by the intersection of the compaction curve and the line S = 80% degree of saturation (see figure 1.6.4.7.1). The TML is the critical moisture content.

    Should the optimum moisture content (OMC) of bauxite occur below 90% saturation, the critical moisture content is indicated by the intersection of the compaction curve and the line S = 70% degree of saturation (see figure 1.6.4.7.2). The TML is the critical moisture content.

    Where the OMC is not clearly defined by carrying out a sufficient number of tests at water contents close to and on either side of the OMC, then a TML based on S = 80% cannot be adopted. The TML shall be based on S = 70%. This is to guard against the erroneous adoption of a high OMC due to insufficient definition of the compaction curve.

    Where moisture very freely drains from the sample at moisture content such that the test sample compaction curve does not extend to or beyond 70% saturation, the test is taken to indicate a cargo where water passes freely through the spaces between particles.