2009-anionic flotation of high-iron phosphate orescontrol of process water chemistry and depression of iron minerals by starch and guar gum

1、Int. J. Miner. Process. 92 (2009) 4957Anionic otation of high-iron phosphate oresControl of process water chemistry and depression of iron minerals by starch and guar gumBalakrishnan Nanthakumar a, Dennis Grimm b, Marek Pawlik a, a University of British Columbia, Norman B. Keevil Institute of Mining

2、 Engineering, 517-6350 Stores Road, Vancouver, BC, Canada V6T 1Z4b Agrium Incorporated, Kapuskasing Phosphate Operations, P.O. Box 92, Kapuskasing ON, Canada P5N 2Y1a r t i c l ei nf oa b s t r a c tArticle history:Received 30 May 2008Received in revised form 19 January 2009 Accepted 8 February 2009

3、Available online 14 February 2009The fatty acid otation of a difcult-to-oat igneous phosphate ore (26.2% P2O5, 15.4% Fe2O3) was investigated through batch otationtests in thepresence of soda ash. The poor otation of this otherwise high qualityorewas attributedtoveryhighconcentrationsofcalciumandsulf

4、ateions(about1000and2000mg/L,respectively)found inprocesswater,whichindicatedthat theotationpulp wassaturatedwithrespecttocalciumsulfate.Despitethe very high concentrations of calcium, theotationresults and pulp titrationtests as afunction of soda ash dosage showed that high concentrate grades (N36%

5、 P2O5) and phosphate recoveries (80%) could readily be achieved straight from such a high-calcium environment using pulp pH and conductivity as parameters for controlled addition of soda ash, especially during a continuousplant process. The main role of soda ash wasto precipitate calcium and other i

6、nterfering cations from the pulp thus allowing the tall oil collector to interact with the phosphatecomponents.Acomparativestudyof cornstarchand guar gum asirondepressantsfortwophosphate oresvaryinginmineralogyofironminerals(high-sideriteandlow-siderite,andvariousoxides)showedthatguar gumwasamoreeff

7、ectiveirondepressantthanstarchintermsofP2O5 gradesoftheconcentrates.However,longer otation times were necessary in the presence of higher doses of guar gum in order to achieve high P2O5 recoveries. The depressant performance of guar gum was superior to that of starch especially for the high-siderite

8、 ore. 2009 Elsevier B.V. All rights reserved.Keywords: Froth otation Phosphate ores Fatty acids Iron oxides Soda ash1. IntroductionMgsincephosphate minerals (andinvariably presentcarbonates, such as calcite and dolomite) are sparingly soluble in water and always release a small amount of these catio

9、ns into the pulp. Such naturally- low levels of cations do not pose major otation problems.In the case of high-iron phosphate ores, the depression of iron minerals is usually achieved using starch and the use of this polysaccharide in this role is based on the very successful application of various

10、starches in beneciating hematite-based iron ores. How- ever, otation practice at AgriumKapuskasing Phosphate Operations (Kapuskasing, Ontario) indicates that the performance of starch is not always satisfactory, and it has been difcult to clearly correlate the variable action of starch with other pa

11、rameters, such as ore grades or process water chemistry.This collaborative study was undertaken in order to identify sources of otation problems with representative samples of two ores from AgriumKapuskasing Phosphate Operations (AgriumKPO), to investigate the potential of using guar gum as an alter

12、native depressant of iron minerals, and to propose measures to improve the otation of such problematic ores.The anionic otation of phosphate ores generally relies on the use of fatty acid surfactant-type collectors. The selectivity of the process results from a high chemical afnity of the anionic su

13、rfactants towards these sparingly-soluble salt-type minerals (Peck and Wadsworth 1963, 1965). The adsorption of fatty acids on phosphate minerals (apatites) is often treated as the formation of the corresponding metal (calcium or magnesium) carboxylate on the mineral surfaces.The collecting efciency

14、 of fatty acids is strongly affected by the presence of polyvalent cations in process water, most commonly calcium and magnesium. The interfering role of calcium and magnesium is two-fold: these ions form water-insoluble precipitates with fatty acidsa process that normally results in high reagent co

15、nsumption rates, and at the high pH values required for phosphate otation calcium and magnesium hydroxy-complexes (e.g., Ca(OH)+) specically adsorb on all mineral surfaces rendering even the gangue minerals positively charged (Clark and Cooke, 1968). This surface charge reversal leads to enhanced co

16、llector adsorption onto the unwanted components of ores and the selectivity of the otation process towards phosphates is drastically reduced. In practice, however, it is impossible to completely avoid the presence of Ca and1.1. Flotation practice at Agrium Inc.Kapuskasing Phosphate Operations (KPO)T

17、he KPO phosphate ores belong to an igneous deposit and differ widely in mineralogy and phosphate grades. The ores are broadly Corresponding author. Tel.: +1 604 827 5034; fax: +1 604 822 5599.E-mail address: mppmining.ubc.ca (M. Pawlik).0301-7516/$ seefront matter 2009 Elsevier B.V. All rights reser

18、ved. doi:10.1016/j.minpro.2009.02.003Contents lists available at ScienceDirectInt. J. Miner. Process.jour nal homepage : www. elsevier. com/ locate/ijminpro50B. Nanthakumar et al. / Int. J. Miner. Process. 92 (2009) 4957divided on-site into three main classes. The easy-to-oat A-type ores do not pose

19、 any major otation problems, but the B-type and the so- called “marginal” ores are not always readily oatable using the current reagent suites and process parameters (pulp density, feed particle size, pH). A-type ores contain normally much more than 15% P2O5 and less than 8% ferric oxide equivalent

20、(Fe2O3). In fact, most A- type ores are of such a high P2O5 grade that no otation is required, and simple mechanical screening/desliming or magnetic separation are sufcient to bring the concentrate grade to well over 36% P2O5 the minimum P2O5 grade required by the Redwater fertilizer plant in Albert

21、a (also operated by Agrium Inc.). B-type and marginal ores are approximately of the same P2O5 grades as A-type ores but their iron content may exceed 30% Fe2O3. Marginal ores also contain higher amounts of other gangue minerals, such as silicates and quartz. Any material containing less than 15% P2O

22、5 is rejected as waste rock. Blends of ores of different types are also processed. The main phosphate mineral recoverable by froth otation is uorapatite (Ca5known double-oat Crago process, the otation circuit does not include any further reverse otation of silica, and no other reagents are used at t

23、he plant. The nal concentrate is thickened, ltered, and thermally- dried. The P2O5 grade of the concentrate should be at least 36% and the equivalent contents of Fe2O3 and MgO should be less than 2.5% and 0.6%, respectively.2. Experimental2.1. Ore preparation and characterization200250 kg of two dif

24、ferent ore samples were provided by AgriumKPO. One of the ores was a marginal ore while the other one wasa B-type ore. The relatively high-grade marginal oreunexpectedly did not respond at all to froth otation at the KPO plant, giving basically barren froth regardless of reagent dosages and process

25、conditions. The marginal ore had been stockpiled at the site for about 2 years. The B-type ore was sampled straight from the mine pit.Both ores were prepared in the same way. All the material was crushed to below 3.35 mm (6 mesh). Each ore was then divided into two 100-kg samples, of which one was f

26、urther rifed to produce 1-kg samples for subsequent grinding and otation tests. Each 1-kg sub- sample was stored in a zip-lock plastic bag.A number of 1-kg samples of each ore were then carefully wet- ground by controlling the grinding time in order to reach the target particle size distribution sho

27、wn in Table 1, as determined by wet screening.The above particle sizes were reproducible to within 2% among different samples, and the top size (0.3 mm) as well as the amount of slimes (0.020 mm size fraction) were in very good agreement with routine lab data from KPO. It should, however, be mention

28、ed that the actual amount of slimes in the KPO plant may reach 35% depending on the grindability of the different ore types.After desliming at 0.020 mm (wet screening), the two size fractions (0.02 mm and +0.02 mm) were collected and sent for chemical assays, while the mineralogy of these two sample

29、s was determined by X-ray diffraction (Rietveld renement). Table 2 presents the results of chemical assays.It should be noted that the otation feed constitutes about 80% of the ground product, with the remaining 20% of the material rejected as slimes (Table 1). This gives, from mass balance, overall

30、 P2O5 contents of 24.0% and 29.5% for the marginal and B-type ore, respectively.The percent contents of the main minerals detected in the slimes and otation feed of the marginal ore are given in Table 3. The mineralogy of the B-type ore is presented in Table 4. In addition, the marginal ore also con

31、tained small quantities of biotite, siderite, aerinite, dawsonite, and anatase.The main iron minerals in the otation feed of the marginal ore appear to be magnetite and goethite, while the entire amount of hematiteandthemajorityofgoethiteareeasytoreject bydesliming. Other noteworthy features of the

32、data are the low quartz content in the feed, nearly complete rejection of kaolinite, and total loss of crandallite (CaAl3(PO4)2(OH)5H2O) to slimes.The slimes fraction of the B-type ore also contained small amounts hematite, ilmenite, zircon and crandallite. The main iron minerals in(PO4)3(F).The mai

33、n iron minerals are represented by hematite,goethite, magnetite, and siderite. KPO ores are also known to contain very small amounts of pyrite and pyrrhotite.The KPO open-pit mine is presently the only Canadian phosphate mine, in operation since 1999, located about 35 km south-west of the townof Kap

34、uskasingin Ontario. Fig. 1 schematically presents the main processing stages.The run-of-mine ore is rst crushed and ground to below 0.7 mm top particle size. The ground ore is mechanically classied to remove the 0.7 +0.3 mm size fraction, which is then reground. The 0.3 mm size fraction is deslimed

35、at approximately 0.0250.020 mm using cyclones, and the cyclone underow (0.3 + 0.025 mm size fraction) feeds the otation circuit. The otation feed is conditioned with reagents at 55% solids. Starch is added rst as a depressant of iron minerals followed by tall oil fatty acid as a phosphate collector.

36、 Typical dosages are on the order of 800 g/t of tall oil and 1200 g/t of starch. The pH of the pulp is controlled with sodium hydroxide (NaOH) and is maintained around 10.511.0. The otation circuit consists of a single rougher bank of eight 17-m3 cells, although an expansion project is planned to in

37、clude a cleaning circuit. Because the KPO igneous ores typically respond well to froth otation, the so-called high-intensity conditioning with reagentsas widely practiced in Florida phosphate plantshas never been adopted at KPO, and the main technological problem is the complete separation of iron m

38、inerals from the phosphate components of the ores. The otation concentrate is further upgraded through wet two-stage magnetic separation to remove any residual magnetic parts (mostly magnetite). In contrast to the well-the B-type otation feed are siderite(FeCO368.9% Fe2O3)andTable 1Particle size dis

39、tribution of the ground ores before desliming.Fig. 1. A schematic diagram of the phosphate processing plant at AgriumKPO.Particle sizePassing (%)0.300 mm970.150 mm600.038 mm250.020 mm20B. Nanthakumar et al. / Int. J. Miner. Process. 92 (2009) 495751Table 2Head grades (wt.%) of otation feed and slime

40、s of the tested ores.Table 4Mineralogical composition (%) of the B-type ore by X-ray l using a minimum amount of wash water and deslimed on a 625- mesh screen. The “undersize” slurry containing the slimes was then pressure-ltered to recover the process water. In order to make sure tha

41、t the otation feed is thoroughly deslimed, the recovered process water was put aside while the solids left on the 625-mesh screen were additionally washed with lab tap water to completely remove any remaining slimes. The oversize material ( 0.3 mm + 0.02 mm size fraction) was mixed with the recovere

42、d process water in a Denver 2 L otation machine to achieve a solids content of 50% (wt) and the volume of the pulp was brought up to about 23 cm below the cell lip using lab tap water. The slurry was conditioned at 1500 rpm for 5 min without any reagents. Then, a volume of a 5% (wt) soda ash solutio

43、n was added to the cell and the slurry was conditioned for a further 5 min. When the effect of soda ash dosage was investigated, the pH of the pulp was left unadjusted after soda ash addition and varied between 8.3 and 10.5 (see Figs. 5 and 7). For tests with starch and guar gum, the pH was adjusted

44、 to 10.3, and the required dosages of soda ash were 608 g/t and 3.5 kg/t for the B-type and marginal ore, respectively. Next, a volume of a 3% (wt) caustic corn starch solution (Casco Inc. industrial starch 030050) or a 1% guar gum solution (Rantec KP4000) was added as an iron mineral depressant and

45、 conditioned with the pulp for 5 min. It should be mentioned that the addition of caustic starch typically raised the pH of the pulp to about 10.811.0, while the addition of guar gum which was added as a neutral solution slightly lowered the pH to 1010.1. The dosage of starch was kept constant at 12

46、00 g/t while the dosage of guar gum varied between 300 g/t and 1200 g/t. Finally, a suitable amount of 10% (wt) saponied tall oil (a resin acids content of less than 4%) was introduced into the cell to achieve a collector concentration of 800 g/t and the pulp was conditioned for a further 5 min. Flo

47、tation concentrates were manually collected after total otation times of 30 s, 1.5 min, 2.5 min, 3.5 min and 5 min, which in most cases was sufcient to obtain barren froth. Any material remaining in the cell after 5 min of otation was treated as tailings. Make up water was added continuously to main

48、tain a constant volume of the pulp. Each concentrate and tailings were dried in an oven, and assayed for P2O5 and Fe2O3 by X-ray uorescence (whole rock analysis).LOILoss on ignition.goethite. In contrast, the marginal ore contains only trace amounts of siderite. The absence of detectable amounts of

49、magnetite in the B-type ore is also noteworthy.Assuming that an idealized formula of uorapatite is Ca5(PO4)3(F), the equivalent P2O5 contents in the otation feed agree very well with the direct assays in Table 2. The iron oxide data from both tables are also in good agreement.2.2. Analysis of proces

50、s waterAfter wet grinding, the process water was recovered by pressure ltration and ltered further on a 0.45-m polycarbonate lter to remove any suspended colloidal solids. The clear ltrate was assayed for main ions. The main ion concentrations found in equilibrium with the tested ores are given in T

51、able 5. The data clearly show the presence of high concentrations of calcium and magnesium cations in process water produced by the marginal ore, and the reasonable working assumption is that these ions were most likely responsible for the observed poor otation of this ore type in the plant.The high

52、 concentration of sulfate ions suggests that a small amount of a calcium sulfate mineral, most likely gypsum (CaSO4H2O), may be present in the marginal ore, and is the primary source of the high amount of Ca in the process water.It should be noted that the mineralogy given in Table 3 was obtained af

53、ter wet-desliming, so any residual calcium sulfate present in the dry ore was immediately dissolved during grinding, and thus could not be detected by X-ray diffraction. It is noteworthy at this point that the molar concentrations of Ca2+ and SO42 ions are nearly stoichiometric (1:1) consistent with

54、 the composition of any calcium sulfate mineral. It should also be observed that the solubility of gypsum is about 3.15 g/L (Appelo and Postma, 2005) which agrees well with the combined concentrations of Ca2+ and SO24. In other words, the process water in equilibrium with the marginal ore seems to b

55、e saturated with respect to calcium sulfate.Finally, the pH of process water for the marginal ore was 7.5 while the conductivity was 3.9 mS/cm. For the B-type ore, process water had a conductivity of 0.44 mS/cm and a pH value of 7.8. For comparison, the conductivity of a saturated calcium sulfate so

56、lution was found to be 2.8 mS/cm (pH 6.9), as measured in a separate test.2.4. Titration of pulp with soda ashIn order to more clearly determine relationships between soda ash dosage and the pH/conductivity of the pulp, a number of titration tests were performed using a deslimed otation feed slurry

57、(50% solids) placed in the otation cell. The slurry was titrated with a 5% (wt) soda ash solution while continuously mixing. After each dose of soda ash, the slurry was mixed for 1 min after which the equilibrium pH and conductivity values were simultaneously recorded. Additional titration tests wer

58、e also performed on model calcium sulfate solutions2.3. Batch otation procedureFor each otation test, 1 kg of the 6 mesh material was wet- ground as described in Section 2.1. The slurry was removed from theTable 3Mineralogical composition (%) of the marginal ore by X-ray diffraction.QuartzKaoliniteDolomiteCalciteGoethiteMagnetiteHematiteFluor-apatiteCrandalliteFeed2.27