Frother dosage optimization
CHAPTER 4: LABORATORY RESULTS AND DISCUSSION
4.1 O
VERVIEW OF ANALYSIS AND REPEATABILITYThe laboratory experiments were divided into three series as stated in section 3.2.1. Series A refers to tests performed on the column cell with the E2 sparger (E2 column) and the mechanical cell. Series B refers to tests performed on the column cell with the E3 sparger (E3 column). Both series A and B utilized the same ore sample and were analysed for Cu/Ni content (detailed procedures are available in section 3.2).
4.1.1 Analysis of the rougher tailings
The data from initial tests showed a significant variation in the recalculated head grades of Cu and Ni in the feed. The sampling of the rougher tailing was identified as a significant contributor to this variation. This was due to difficulties in obtaining a representative sample for analysis from approximately 9.5 kg of rougher tails remaining in the flotation cell (Some of the water was removed after settling and the pulp was mixed using the impeller. Samples of pulp were removed, dried and split). It was decided that a more reliable method was to use a standard rougher flotation procedure and compare the cleaner efficiency. The degree of fluctuations in the recalculated head grade of the rougher concentrate was not as significant as the rougher feed.
4.1.2 Problems encountered with repeatability and its possible causes
The experiments performed in series A and B were analysed in batches. It was concluded from the analysis that the recalculated head grades in different batches still showed significant variations. This sometimes occurred within the same batch as well as for particular concentrates within individual tests.
An obvious consideration for such variation would be the sampling of the ore itself. It has been explained in the experimental procedure that the ore was screened (3mm). The coarse fraction was crushed in a jaw crusher and re-screened. The crushed ore and fine ore was individually blended, sub-sampled and riffled and then combined to form 5kg portions (detailed procedures in Section 3.2.2). It was therefore concluded that all reasonable measures to obtain a representative feed were applied.
The subsequent considerations were either that there were errors in the experimental method or in the analysis itself. General precautions were put in place to ensure that the experimental method was consistent. Significant shifts in the grade occurred between different batches sent for analysis. This led to the conclusion that the analysis techniques had to be investigated.
The analysis itself can be divided into the acid digestion of the ore sample and the ICP analysis.
These techniques are discussed in section 3.2.12. The acid digestion was performed by the author and the ICP analysis was performed by a local laboratory. A study into the possibility of errors in both techniques was pursued. The investigation entailed the following:
Multiple samples of a particular concentrate were individually digested with acid to investigate the extent of errors in the digestion technique.
Multiple sub-samples of a single concentrate were submitted for analysis to investigate the possibility of errors in the ICP analysis within a particular batch of tests.
Liquid samples from previously digested concentrates were resubmitted to investigate errors in ICP analysis between different batches of tests.
Substantial variations in Cu/Ni grade were discovered for tests that were resubmitted from previous batches. In a particular case a concentrate that was reported to have a Ni grade of 3156 ppm two months prior, was re-analysed to have a grade of 5960 ppm. It was also evident that there were discrepancies within the same batch of tests as well. A repeat sample for the above concentrate in the same batch reported a Ni grade of 5120 ppm. However, these discrepancies were infrequent and were less drastic.
The investigation into experimental errors was inconclusive to a certain degree because having ascertained that errors did exist in the ICP analysis it was not possible to determine whether the differences in the individually digested tests were from the digestion itself or the ICP. The differences in the grades of Ni and Cu for the individually digested concentrates were of a much smaller degree (200-400 ppm) that that of the ICP analysis for repeated batches (more than 800 ppm as in the example above).
Attempts were made to determine exactly what was causing these inconsistencies in the ICP analysis. Repeat analyses were conducted at adjusted calibrations and fresh standard solutions were utilized but neither option yielded much success.
In total, 80 flotation tests were performed in Series A, B and C. Series A and B consisted of 50 tests which had been analysed for Cu/Ni content locally. The plausible but expensive alternative was to analyse the pgm content of these tests. This technique also required significantly larger masses of concentrate for analysis. After multiple attempts at Cu/Ni analyses, there was not
enough sample left from the tests performed in Series A and B to be re-analysed for pgm content.
A new ore sample was required and series C contained a repeat of the base tests. It may be questioned as to why the Cu/Ni analyses were not discontinued sooner as they currently form the bulk of the experimental work. It was initially hoped that the cost of direct pgm analysis could be avoided but it was eventually decided that PGM analysis was required to draw final conclusions. Nevertheless the initial tests performed with Cu/Ni analysis provided essential experience for developing experimental procedure. Physical difficulties, such as the crack in the E2 sparger and mechanical problems whilst milling, hampered and prolonged ICP investigations.
The tests analysed for Cu/Ni content could not be discarded altogether because tests that were analysed in the same batch could provide suitable data. These tests were selected based on criteria that will be discussed below.
4.1.3 Section Criteria for tests in Series A and B
It was decided that only tests with a similar calculated head grade of rougher concentrate would be used for comparison. The average rougher grades of the selected tests were 0.37% Cu and 0.54% Ni. The head grade for each test was required to be within 12% of the average head grade stated above. About 46% of the tests did not satisfy this criterion (for the reasons discussed in Section 4.1.2) and they were discarded. In order to make the selection process more transparent to the reader, graphs depicting the head grade and total recovery for each test are shown below.
Each graph compares the mechanical and column data for selected changes in operating conditions. Tests that were discarded can be identified by hollow markers.
Figures 4.1 and 4.2 show all base tests performed in series A and B. Certain tests may appear within range, such as the Ni grade (65.1%, 0.53%) of a base test with the E2 column. Figure 4.2, however, shows that the corresponding Cu grade for that test was too low (0.29%). Similarly, the Ni grade for the mechanical cell test (78.9%, 0.55%) seems to be acceptable but the corresponding Cu recovery was uncharacteristically low (44.1%). It is not possible to visually judge from the graphs which Cu and Ni head grades correspond to the same test, therefore the both grades appear together for each test in Table 4.1. The values in bold signify those tests that were regarded as acceptable.
Figure 4.1 Head grade vs total recovery of Ni for base tests in series A and B
Figure 4.2 Head grade vs total recovery of Cu for base tests in series A and B 0.00
0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80
0 20 40 60 80 100