2023 Volume 64 Issue 6 Pages 1225-1231
Effect of pH and precipitations on copper–molybdenum ore rougher flotation has been investigated in seawater with flotation experiments followed by precipitation estimation with thermodynamic calculations, turbidity measurements and XRD analysis. Results of flotation experiments in seawater indicated that the effect of pH on copper and molybdenum flotation maximum recovery seems limited. On the other hand, pH drastically influences copper and molybdenum flotation kinetic constant, it decreased a lot with increasing pH, and results of pH 8.5 and 9.0 is quite similar. From thermodynamic calculation, precipitation effect on seawater less than pH 9 seems limited since CaCO3 and Mg(OH)2 does not exist on this pH region. To estimate precipitation on pH in seawater, turbidity measurements were carried out with controlled pH and results indicated even pH is less than 9, noticeable turbidity can be seen. To confirm precipitation in seawater, precipitation was collected from controlled pH seawater solution. XRD analysis of precipitation indicated that obtained precipitation at pH 8.7 is CaCO3, CaSO4 and Mg(OH)2. This result is not same as the result of thermodynamic calculation and it might be due to the activity coefficient and ionic strength effect. Although flotation kinetic is influenced with turbidity, turbidity influence on maximum recovery is limited. Effect of kinetic might be due to that precipitation exist as suspension and it prevent bubble and mineral attachments. Small effect of precipitation on maximum recovery might be due to exist of few precipitation on the surface of copper and molybdenum mineral since most of mineral in rougher flotation is gangue minerals and also most of precipitations might be on gangue minerals. These results indicated that seawater flotation have to take into account of precipitation effect more for flotation kinetic than maximum flotation recovery.
Flotation is important process in mineral processing to concentrate mineral ore to concentrate especially for base metal sulfide such as copper, zinc and lead etc. For normal case in copper sulfide ore, copper percentage can increase from 0.3% to 20% with flotation process. Copper ore flotation to concentrate mainly consists from rougher flotation and further cleaner flotation. Rougher flotation recovers most of sulfide minerals as float whereas remove gangue silicate minerals as sink, following cleaner flotation is for selective flotation to separate sulfide minerals such as necessary copper sulfide and unnecessary pyrite. Since most contents of copper ore is gangue silicate minerals, copper ore rougher flotation consumes vast of water and it is important to improve copper ore rougher flotation efficiency to reduce cost and environmental impact. Flotation water quality is important factor to estimate flotation recovery and it is reported that seawater and/or saline water is adversary effect of floatability since precipitations on the surface of minerals.1–5) One of this effect is explained since Ca2+ and Mg2+ ion inside sea water will react with collector and these ions makes hydrophilic precipitations on the surface of minerals at higher pH region such as more than pH 9.2,5) There are also report for seawater flotation that seawater effect is not only adversary, it makes bubble size smaller and froth stability1) and effect of seawater is complex in current status. Although seawater have adversary effect on flotation, it is used for rougher flotation process since scarcity of freshwater and stringent regulations on the quality of discharged water.6–8) Estimation of utilize seawater on rougher flotation of copper ore and optimize flotation condition is important and it is necessary of systematic estimation on the effect of pH on copper ore rougher flotation since seawater pH influence largely on flotation behavior due to precipitation production. However, precipitation influence on flotation mechanism with kinetic model on each mineral is not conducted.
In this study, considering above situation, the effect of pH on copper ore (copper–molybdenum containing ore) rougher flotation were estimated in seawater and the feasibility of flotation condition and mechanism of seawater effects were investigated on copper and molybdenum minerals from the point of view of precipitation production during flotation and precipitation effect on the flotation kinetic and recovery.
The copper–molybdenum ore sample was mined from a porphyry copper deposit in Chile. The ore sample was crushed to 6 mm or less at the mine, and then was crushed to less than 10 mesh with a roll-mill-crusher in the laboratory. The final particle size of the crushed sample was 1241 µm as P80. The ore was mixed uniformly by conical quartering method and divided into several bags (0.875 kg/bag) for the flotation tests. Each ore sample was stored in a freezer at −40°C to prevent further oxidation of the mineral surface. For chemical analysis, ore sample was mixed with sodium peroxide (Na2O2, Kanto Chemical Co., Inc.) and melted at 800°C. Then sample was dissolved in diluted 10% HCl (Kanto Chemical Co., Inc.) and analyzed by inductively coupled plasma optical emission spectrometer (ICP-OES, Agilent ICP-OES 5100, Japan).
The total copper grade and the acid soluble copper assays were 0.39% and 0.026%, respectively. The oxidation index of the copper mineral obtained by dividing the acid soluble copper assay by the total copper grade was 6.7. The total molybdenum assay was 0.061%, and the acid soluble molybdenum was below the detection limit. X-ray diffraction (XRD) pattern of the ore sample indicated that it contains quartz (SiO2), phlogopite (KMg3AlSi3O10(OH)2), microcline (KAlSi3O8), and albite (NaAlSi3O8), which are a kind of granite being the components of plutonic, were detected. The albite may change into clay minerals, such as smectite, which can cause a decrease in the actual flotation recovery. The mineral composition of ore sample was analyzed by Mineral Liberation Analyzer (MLA: FEI, MLA650F, United States of America) and the result is presented in Table 1. Since MLA decide mineral from element composition which determined from EDS spectra, mineral determined from MLA can be include from the mineral determined from XRD i.e. phlogopite can be included as mica, microcline and albite can be included as feldspar minerals. From this result, total copper and molybdenum mineral assay is 1.1%. Each copper and molybdenum mineral composition are shown in Table 2. As shown in this table, main copper mineral is chalcopyrite (CuFeS2, 89.0%) and other copper minerals are bornite (Cu5FeS4, 4.05%), chalcocite (Cu2S, 2.80%), covellite (CuS, 1.61%), atacamite (Cu2(OH)3Cl, 2.5%) and native copper (Cu, 0.02%). Meanwhile, molybdenite (MoS2, 95.8%) is the main molybdenum mineral.
To estimate precipitations in various pH in fresh water and seawater, solution turbidity measurements and produced precipitation analysis were carried out. 1 L of tap water or seawater was stirred with magnetic stirrer and solution pH was controlled with lime for 30 min and turbidity was measured by turbidity meter (Turbidity meter 2100QIS, DKK-TOA corp.). To confirm precipitation, 3 L of seawater was stirred with magnetic stirrer with controlled pH with lime for 30 min then solution was filtered by 0.2 µm glass fiber filter to collect precipitation. After dry precipitation, XRD analysis was carried out. Seawater in this research is taken from Niihama, Japan. Table 3 shows the chemical analysis of seawater.
The flotation tests were carried out using a thionocarbamate as a copper collector, diesel oil as a molybdenum collector, methyl isobutyl carbinol (MIBC) as a frother, and lime as a pH modifier. Two types of waters, with different dissolved ion concentrations were used for flotation tests. One is freshwater taken from tap, and other was seawater.
Figure 1 shows the flowchart of the rougher flotation test. 0.875 kg of ore and 0.5 L of fresh water (tap water) or seawater water were added to the mill to give a slurry concentration of 63% (w/w). Thionocarbamate (30 g/t) and diesel oil (15 g/t) were added to the slurry. The pH of the slurry was controlled by adding lime. Various pH conditions (7.7, 8.5, 9.0 and 9.5) were used in the laboratory-scale flotation. Rods and stainless balls were used as grinding media. The ore sample was then ground in a laboratory ball mill for a given grinding time that was previously investigated to produce a P80 size of 170 µm.
Flowchart of batch flotation test.
The test was conducted once under each condition. The ground products were put into a flotation cell with a capacity of 2.4 L. The slurry concentration was adjusted to 33% solid by adding fresh water or seawater. Afterwards MIBC (15 g/t) was added as a frother followed by 1 min conditioning. Denver D12 flotation machine (Metso Minerals, Inc.) was used for the flotation tests. The rotation speed of the shaft was set to 1150 rpm, and the flotation concentrates were recovered in 4 stages (3, 8, 15, and 30 min). Each product sample was dried at 110°C, weighed, and then digested with aqua-regia followed by analysis with ICP-OES (Agilent ICP-OES 5100, Japan).
Rougher flotation tests on a laboratory-scale were conducted at pH range of 7.7–9.5 using seawater. Figure 2 show the recovery of (A) copper and (B) molybdenum with time at various pH in seawater. As shown in these figures, copper and molybdenum recovery increased with time and it became stable after 30 min. These figures also indicated that recovery kinetic seems higher when pH is low but final copper and molybdenum recovery is similar.
Flotation recovery of (A) copper and (B) molybdenum with time at various pH seawater.
To estimate kinetic and recovery of flotation in detail, flotation recovery time transition results were fitted with flotation kinetic model, that is proposed by Agar9) as following equation.
| \begin{equation} R = R_{\textit{max}}(1 - e^{-kt}) \end{equation} | (1) |
This equation consists from flotation time (t), recovery (R), maximum recovery (Rmax) and kinetic constant (k). Example is shown in Fig. 3. With adjusting Rmax and k, flotation results of seawater flotation at pH 8.5 can be fitted well with this model.
Flotation result of copper recovery at pH 8.5 in seawater and fitting result with eq. (1).
Table 4 indicates various flotation results of the effect of pH in seawater, parameters are obtained from actual flotation result of Fig. 3 with fitting eq. (1). Table 4 also indicated final froth grade of copper and molybdenum as well. Result of Table 4 is summarized in Fig. 4 as the effect of pH on kinetic constant (A) and maximum flotation recovery (B). As shown in this figure, kinetic constant decreased with increasing pH, but kinetic constant at pH 8.5 and 9.0 is almost similar. On the other hand, maximum flotation recovery only slightly decreased with pH.
The effect of pH on flotation kinetic constant, k (A) and maximum flotation recovery, Rmax (B) of copper and molybdenum flotation. k and Rmax values are obtained from Table 4.
The effect of precipitation in seawater is influenced with pH and its thermodynamic calculations has been done by Suyantara et al.5) as shown in Fig. 5. As shown in this figure, the calcium ion and the magnesium ion present in the seawater can produce magnesium hydroxide (Mg(OH)2) and calcium carbonate (CaCO3) colloidal precipitates at pH higher than 9.2 and 10, respectively as also reported by Hirajima et al.2) They reported that the floatability of molybdenite and chalcopyrite was significantly reduced at high pH conditions owing to the adsorption of these colloidal precipitates on the surface. Therefore, it is more beneficial to perform the flotation at low pH range especially at the natural pH of seawater (i.e., pH 8.0–8.3) to weak alkali condition less than 9.2 which precipitate Mg(OH)2. Also this figure indicated that dolomite (CaMg(CO3)2) and calcium sulfate (CaSO4) might produce at pH region which we have carried out flotation including natural seawater pH (pH 8.3). However, since seawater with natural pH does not produce any precipitation, high ion intensity effect in seawater might influence ion activity and did not produce these precipitations on natural pH in seawater.
Species diagram of seawater (quoted from Suyantara et al.5)).
To confirm precipitation in actual system, solution pH was controlled in fresh water, seawater and precipitation production were estimated with measuring solution turbidity and precipitation analysis. Figure 6 indicated results of measurements of pH value with time by lime addition to control pH in (A) fresh water and (B) seawater and this figure shows pH can be controlled well. During this pH measurements, solution turbidity measured and results are shown in Fig. 7. As shown in this figure, turbidity in fresh water is very low (around 1 NTU) at all pH. Turbidity in seawater is higher than that of fresh water and turbidity increased with increasing pH. Since turbidity in seawater more than pH 8.5, it indicates possibility that precipitation can be produced more than pH 8.5 in seawater.
pH value with time by lime addition to control pH in (A) fresh water, (B) seawater.
Solution turbidity with time at various pH controlled by lime addition in (A) fresh water, (B) seawater.
To confirm the precipitation, 3 L of seawater pH was controlled for 30 min then precipitations were obtained and XRD analysis were carried out. Table 5 indicated the precipitation amount and lime addition to control at various pH in 3 L of seawater. As shown in this table, precipitation amount increased with increasing pH and amount on pH 8.71 and pH 9.31 is similar. On the other hand, precipitation can not be seen in freshwater. Figure 8 indicates XRD pattern of precipitation in seawater at controlled pH 8.71 and pH 9.31 by lime addition. With both pH case, peak of CaCO3, CaSO4 and Mg(OH)2 can be obtained. According to thermodynamic calculation in Fig. 5, precipitation in seawater such as CaCO3 and Mg(OH)2 can form above pH 9, and only CaMg(CO3)3 and CaSO4 can be produced at pH 8.5. However, results of turbidity measurements and precipitation analysis indicated even lower than pH 9, CaCO3, CaSO4 and Mg(OH)2 can be produced whereas CaMg(CO3)3 can not be produced. This difference between thermodynamic calculation and actual measurements might be due to the activity coefficient and ionic strength effect, and it might be influence flotation behavior.
XRD pattern of precipitation in seawater at various pH controlled by lime addition.
Since turbidity of suspension is influenced with pH in seawater, results of the effect of pH on flotation results (Fig. 4) is re-arranged with the effect of turbidity on flotation results. Figure 9 indicates the effect of turbidity on flotation kinetic constant, k of copper and molybdenum flotation. As shown in this figure, it seems that flotation kinetic constant decreases with increasing turbidity. Figure 10 indicates the effect of turbidity on maximum flotation recovery, Rmax of copper and molybdenum flotation. As shown in this figure, it can indicated that turbidity only slightly influenced on maximum recovery of copper and molybdenum.
The effect of turbidity of suspension on flotation kinetic constant, k of copper and molybdenum flotation.
The effect of turbidity of suspension on maximum flotation recovery, Rmax of copper and molybdenum flotation.
Based on the results of flotation experiments in this report, it can be indicated that seawater flotation influenced with pH since higher pH produce precipitations and it adversary influenced on flotation kinetics whereas maximum recovery is not influenced a lot. Seawater flotation is widely investigated so far and the effect of seawater on flotation is controversy, it indicates adversary floatability or selectivity compared with fresh water flotation.1–5) On the other hand, it also indicated that seawater is not only adversary effect.1) Suyantara et al.5) indicated this difference might be due to pulp density of flotation experiments. Since precipitation amount is similar with same pH, when pulp density is lower, mineral surface can cover more precipitation compared with higher pulp density flotation. It means lower pulp density indicate adversary effect on seawater flotation recovery. Since flotation pulp density on this report is high, effect of seawater on flotation recovery is small. Also since this report is carried out flotation with copper–molybdenum ore, most of sample is gangue minerals. Precipitation might produce but most of attached precipitations can exist on gangue minerals and precipitations on the surface of copper and molybdenum minerals are smaller compared with concentrate flotation. On the other hand, seawater influenced flotation kinetics a lot. It might be due to that precipitations produced in seawater in pulp disturb bubble and mineral attachment. In current flotation process, seawater flotation is more applied on rougher flotation compared with further cleaner flotation and selective flotation. Reason is not mentioned but since rougher flotation is high pulp density and high gangue percentage, effect of precipitation on maximum recovery can be reduced. Also since rougher flotation is normally large scale and retention time is longer, lower flotation kinetics with precipitations can be reduced as well. From this point of view, seawater flotation is suitable for rougher flotation since drawback with precipitation can be reduced. Rougher flotation estimation need to take into account these point of view of investigation.
Copper–molybdenum rougher flotation estimation has been carried out with the effect of pH and precipitations in seawater with flotation experiments followed by precipitation estimation with thermodynamic calculations, turbidity measurements and XRD analysis. Results of flotation experiments in seawater indicated that the effect of pH on flotation maximum recovery is small whereas pH drastically influences flotation kinetic constant, it decreased with increasing pH. To confirm this effect, precipitation estimation was carried out from thermodynamic calculation. It indicates that precipitation effect on seawater less than pH 9 seems limited since CaCO3 and Mg(OH)2 does not exist on this pH region. Also turbidity measurements were carried out with controlled pH and results indicated even pH is less than 9, noticeable turbidity can be seen. XRD analysis of this precipitation indicated that obtained precipitation at pH 8.7 is CaCO3, CaSO4 and Mg(OH)2. This result is not same as the result of thermodynamic calculation and it might be due to the activity coefficient and ionic strength effect. With comparison of these results it can be indicated that flotation kinetic is influenced with precipitations whereas precipitation influence on maximum recovery is limited. It might be due to that precipitation exist as suspension and it prevent bubble and mineral attachments, then flotation kinetic became lower. Small effect of precipitation on maximum recovery might be due to exist of few precipitation on the surface of copper and molybdenum mineral since most of mineral in rougher flotation is gangue minerals and also most of precipitations might be on gangue minerals. It might be due to seawater appliance on rougher flotation and the higher effect of flotation kinetics than maximum flotation recovery need to take into account for rougher flotation circuit construction.
This research was supported by Kyushu University, Graduate School of Engineering, Department of Earth Resources Engineering. We appreciate the support of this research by Sumitomo Metal Mining Co., Ltd., for providing samples and useful advice.
