ABSTRACT

Mineral flotation is a wet concentration operation that has a differentiating effect dependent on the surface properties of the minerals. Water quality is integral to the phosphate ore flotation, since the presence of certain ions in the solution can compromise the effectiveness of the operation. The objective of this study was to compare the effectiveness of phosphate ore flotation (dispersed air) before and after effluent treatment using coagulation, flocculation and dissolved air flotation (DAF) techniques. The mineral flotation tests were performed for new water (NW) and three effluents tailing dams (TD), simulated effluent with high concentrations of calcium and magnesium ions (SE) and simulated effluent after DAF treatment (SE_DAF). The simulated effluent underwent two stages of aggregation followed by DAF for cation removal. The results showed that the proposed technique removed 91.6% of magnesium and 66.4% of calcium from the original simulated effluent (SE). The SE_DAF presented better average values of mass recovery, content, and recovery of P₂O₅, confirming the effectiveness of the removing ions suggested as deleterious for mineral flotation. The flotation performance of the DAF-treated effluent was comparable to that of new water. This study underscores the significance of water quality management in optimizing mineral flotation processes.

Keywords:
Dissolved air flotation; Ca²⁺ and Mg²⁺ ions; Mineral flotation; Phosphate ore; Water reuse.

1. INTRODUCTION

Water is a scarce resource, and its control has become increasingly important. Some mineral processing operations are not authorized to use new water due to environmental regulations, thus forcing companies to use relatively impure primary water sources with high salinity levels, including calcium, magnesium, and iron salts as potential precipitates [¹[1] MUZENDA, E., “An investigation into the effect of water quality on flotation performance”, World Academy of Science, Engineering and Technology, v. 69, pp. 237–241, 2010.]. This issue has led to developing strategies to improve water sustainability, including water reuse as one of the most widespread practices, especially in mineral flotation; however, this reuse tends to alter water quality [²[2] BAZMANDEH, M., SAM, A., RANJBAR, M., “Effect of water quality on flotation performance case study gol-E-gohar iron mine Iran”, MSAIJ, v. 14, n. 8, pp. 326–329, 2016.]. Most research has focused on understanding the issues associated with water quality in flotation, where studies focus on three main aspects: (1) understanding the reasons for water quality variation, (2) quantifying the effect of water quality variation on flotation efficiency by investigating the responsible mechanisms, and (3) testing different solutions [³[3] LIU, W., MORAN, C.J., VINK, S., “A review of the effect of water quality on flotation”, Minerals Engineering, v. 53, pp. 91–100, 2013. doi: http://doi.org/10.1016/j.mineng.2013.07.011.
https://doi.org/10.1016/j.mineng.2013.07... ].

Flotation is the most commonly used operation for phosphate ore concentration. However, factors, such as the presence of ionic particles or organic matter from residual reagents in the water solution can impact process performance [⁴[4] SANTOS, M.A., SANTANA, R.C., CAPPONI, F., et al., “Dissolved air flotation in mineral effluent treatment aiming at improving the apatite flotation process”, In: Proceedings of the 25th National Meeting of Mineral Treatment and Extractive Metallurgy, pp. 219–226, Goiânia, 2013.]. When recycled water containing high levels of dissolved salts and metallic ions is employed, apatite [Ca₅(PO₄)₃(F, Cl, OH)] recovery [⁵[5] SANTOS, M.A., SANTANA, R.C., CAPPONI, F., et al., “Effect of ionic species on the performance of apatite flotation”, Separation and Purification Technology, v. 76, n. 1, pp. 15–20, 2010. doi: http://doi.org/10.1016/j.seppur.2010.09.014.
https://doi.org/10.1016/j.seppur.2010.09... ], and consequently, flotation selectivity can be affected, which is significant because flotation selectivity is critical in the operation [⁶[6] SANTOS, M.A., SANTANA, R.C., CAPPONI, F., et al., “Influence of the water composition on the selectivity of apatite flotation”, Separation and Purification Technology, v. 47, n. 4, pp. 606–612, 2012.].

Dissolved air flotation (DAF) is an advantageous technique for floc separation in water treatment systems that receive several different types of raw water [⁷[7] REALI, M.A.P., PATRIZZI, L.J., “The influence of the contact zone configuration on the performance of a DAF pilot plant applied to water treatment”, Water Science and Technology: Water Supply, v. 16, n. 1, pp. 186–192, 2016. doi: http://doi.org/10.2166/ws.2015.127.
https://doi.org/10.2166/ws.2015.127... ]. In DAF, bubble generation is accomplished by saturating part of the effluent with air in tanks operating above atmospheric pressure, with the availability of air linked to the operating pressure of the system [⁸[8] NUNES, D.G., FRANÇA, S.C.A., COUTO, H.J.B., “Mineral industry effluent treatment using a pilot dissolved air flotation unit”, In: Proceedings of the 17th Scientific Initiation Day, pp. 1–7, Rio de Janeiro, CETEM, 2009.]. This process is followed by sudden decompression through a needle-type valve or flow constriction devices, resulting in the formation of microbubbles in the range of 10 to 100 μm.

This technique has been employed in several areas, particularly for treating some industrial effluents [⁹[9] RUBIO, J., SOUZA, M.L., SMITH, R.W., “Overview of flotation as a wastewater treatment technique”, Minerals Engineering, v. 15, n. 3, pp. 139–155, 2002. doi: http://doi.org/10.1016/S0892-6875(01)00216-3.
https://doi.org/10.1016/S0892-6875(01)00... ] and in the agricultural sector [¹⁰[10] MUÑOZ-ALEGRÍA, J.A., MUÑOZ-ESPAÑA, E., FLÓREZ-MARULANDA, J.F., “Dissolved air flotation: a review from the perspective of system parameters and uses in wastewater treatment”, TecnoLógicas, v. 24, n. 52, pp. 281–303, 2021. doi: http://doi.org/10.22430/22565337.2111.
https://doi.org/10.22430/22565337.2111... ]. Examples of industrial applications for removing suspended solids and adsorbed ions in Fe(OH)₃ precipitates occur in Chile, where DAF plants are used, enabling the reuse of treated water for irrigation and recycling in various unit operations for ore treatment [¹¹[11] RODRIGUES, R.T., RUBIO, J., “DAF–dissolved air flotation: potential applications in the mining and mineral processing industry”, International Journal of Mineral Processing, v. 82, n. 1, pp. 1–13, 2007. doi: http://doi.org/10.1016/j.minpro.2006.07.019.
https://doi.org/10.1016/j.minpro.2006.07... ]. AZEVEDO et al. [¹²[12] AZEVEDO, A.C., OLIVEIRA, H.A., RUBIO, J., “Treatment and water reuse of lead-zinc sulphide ore mill wastewaters by high rate dissolved air flotation”, Minerals Engineering, v. 127, pp. 114–121, 2018. doi: http://doi.org/10.1016/j.mineng.2018.07.011.
https://doi.org/10.1016/j.mineng.2018.07... ] conducted bench-scale and pilot studies for the removal of suspended solids and cations using the same technique, aiming to produce treated water suitable for reuse in the beneficiation of sulfide ores, achieving removals exceeding 90% for Cu²⁺ and Pb²⁺ through the precipitation of metal hydroxides and cation adsorption on the Fe(OH)₃ matrix. In iron ore beneficiation, PINHEIRO et al. [¹³[13] PINHEIRO, V.S., BALTAR, C.A.M., LEITE, J.Y.P., “Flotation with amine: the importance of water quality”, Journal School of Mines, v. 65, n. 4, pp. 549–552, 2012.] demonstrated the feasibility of process water reuse in iron ore flotation after treatment by DAF for the removal of Ca²⁺ and Mg²⁺ cations. OLIVEIRA et al. [¹⁴[14] OLIVEIRA, H.O., LIMA, N.P., AZEVEDO, A.C., “Process water treatment by coagulation-flocculation-dissolved air flotation (DAF) for reuse in iron ore processing”, In: Proceedings of the 29th National Meeting of Mineral Treatment and Extractive Metallurgy, Armação dos Búzios, 2022.] achieved high turbidity removal (approximately 98%) and removal of Mn²⁺ cations (approximately 80%) and Al³⁺ (>95%) through coagulation-flocculation-DAF of process water from tailings filters and sludge thickeners in iron ore processing. FAUSTINO et al. [¹⁵[15] FAUSTINO, L.M., BRAGA, A.S., WHITAKER, W., et al., “Dissolved air flotation applied to the treatment of water polluted by iron ore sludge”, In: Proceedings of the 27th National Meeting of Mineral Treatment and Extractive Metallurgy, v. 1, Belém, PA, 2017.] treated water polluted by iron ore sludge with ferric chloride (FeCl₃) and DAF, achieving residual turbidity of 25 NTU, with reductions of up to 99%. NUNES et al. [⁸[8] NUNES, D.G., FRANÇA, S.C.A., COUTO, H.J.B., “Mineral industry effluent treatment using a pilot dissolved air flotation unit”, In: Proceedings of the 17th Scientific Initiation Day, pp. 1–7, Rio de Janeiro, CETEM, 2009.], using a dissolved air flotation (DAF) pilot unit, treated a phosphate rock effluent with particle removal efficiencies above 90%.

However, its use in treating tailing dams or industrial wastewater for reuse in mineral flotation applications is still a challenge and requires further investigation [¹⁶[16] SANTOS, M.A., CAPPONI, F., ATAÍDE, C.H., et al., “Wastewater treatment using DAF for process water reuse in apatite flotation”, Journal of Cleaner Production, v. 308, pp. 127285, 2021. doi: http://doi.org/10.1016/j.jclepro.2021.127285.
https://doi.org/10.1016/j.jclepro.2021.1... ]. To address this challenge, this study aimed to treatment of simulated effluent (SE) by coagulation, flocculation and DAF for subsequent application in the apatite flotation by conventional froth flotation with dispersed air. As a secondary objective, use of four types of water – TD, NW, SE and SE_DAF for phosphate ore flotation and evaluation of Ca²⁺ and Mg²⁺ cátions interference.

2. MATERIALS AND METHODS

The ore sample, effluents from the tailing dams (TD), and new water (NW) were obtained through a partnership with a phosphate mining company from Alto Paranaíba/Minas Gerais State – Brazil. The sample of phosphate ore was collected in the industrial flotation conditioning stage and was composed, in mass, by 85% coarse (80% less than 150 μm) and 15% fine (80% less than 74 μm) fractions. The ore was homogenized and quartered, with aliquots taken for material characterization and mineral flotation tests. The ore density was measured by gas pycnometry (ACP gas pycnometer, model Autodensity 200) at the Ore Processing Laboratory of CEFET/MG. Chemical characterization was performed by X-ray fluorescence spectroscopy (XRF) at the partner company’s Chemical Laboratory. The effluents TD and NW were collected at the plant of the partner mining company during a non-rainy period. The characterization of these types of water involved determining parameters such as turbidity, pH, and concentration of calcium and magnesium ions. The turbidity of the effluents was determined using a Hanna turbidimeter, model HI93703C. The pH of the liquids was measured using a Hanna pH meter, model Edge HI2002-02. The concentration of calcium and magnesium ions was determined through titration, following the procedure detailed by ALVES [¹⁷[17] ALVES, J.V.S., “Evaluation of magnafloc and sodium alginate reagents for treatment of mineral effluent from fertilizer companies using dissolved air flotation”, M.Sc. Thesis, Federal Center for Technological Education of Minas Gerais, Araxá, MG, 2022.] and was carried out at the Chemistry Laboratory of Campus IV of CEFET-MG.

A simulated effluent (SE) with a concentration of 100 mg/L of both Ca²⁺ and Mg²⁺ ions was produced to investigate the effects of the contaminants on flotation performance, more critical scenario. SANTOS et al. [⁵[5] SANTOS, M.A., SANTANA, R.C., CAPPONI, F., et al., “Effect of ionic species on the performance of apatite flotation”, Separation and Purification Technology, v. 76, n. 1, pp. 15–20, 2010. doi: http://doi.org/10.1016/j.seppur.2010.09.014.
https://doi.org/10.1016/j.seppur.2010.09... ] showed calcium and magnesium ions react with vegetable oil soap, impairing the collection mechanism and increasing reagent consumption, especially for concentrations greater than 100 mg/L. The following reagents were added to the freshwater: calcium nitrate (hydrated) and magnesium nitrate (hydrated). Two stage coagulation, flocculation and DAF could then be conducted on the SE to achieve levels of calcium and magnesium below the maximum provided by the operators of the mining company (60 mg/L for each one). The attempt to remove the ions in a single step proved to be inefficient from a technical standpoint. In a previous study with simulated efluente (SE), ALVES et al. [¹⁸[18] ALVES, J.V.S., PIRES, N.J., OLIVEIRA, M.S., “Study for the removal of Ca2+ and Mg2+ ions from water by Magnafloc 338 and sodium Alginate in a compressed air flotation system”, Tecnologica em Metalurgia, Materiais e Mineração, v. 20, e2744, 2023. doi: http://doi.org/10.4322/2176-1523.20222744.
https://doi.org/10.4322/2176-1523.202227... ] optimized the levels of reagent dosage, conditioning time, pH, and recycling flow rate to achieve greater ion removal using the experimental design technique. The operational parameters of the tests are detailed in Table 1, as indicated by ALVES et al. [¹⁸[18] ALVES, J.V.S., PIRES, N.J., OLIVEIRA, M.S., “Study for the removal of Ca2+ and Mg2+ ions from water by Magnafloc 338 and sodium Alginate in a compressed air flotation system”, Tecnologica em Metalurgia, Materiais e Mineração, v. 20, e2744, 2023. doi: http://doi.org/10.4322/2176-1523.20222744.
https://doi.org/10.4322/2176-1523.202227... ]. The simulated effluent after DAF was denominated SE_DAF. The experimental system for DAF comprised a 3 L saturation tank and a 2 L flotation cell. The saturation pressure was 4 atm. The reagent’s coagulant, and flocculant used in removing Mg²⁺ were aluminum sulfate (Al₂(SO₄)₃.(14–18)H₂O) and PAC (Magnafloc 338, anionic polyacrylamide) respectively, in the first stage of aggregation followed by DAF. The flocculant used for Ca²⁺ removal was sodium alginate (NaC₆ H₇ O₆) in the second stage. The reagents were conditioned within the flotation cell itself using an IKA RW 20 mechanical helix agitator. The removal efficiency of Ca²⁺ and Mg²⁺ ions was calculated by Equation 1.

(1)

Where E_r is the removal efficiency of the analyzed ion; C_i is the initial concentration of the ion in the effluent before treatment, C_t is the ion concentration in the treated effluent, and %_recycle is the percentage of water added to the flotation cell.

Mineral flotation tests (dispersed air) were performed twice with NW and with each of the three effluents (TD, SE, and SE_DAF) using a bench Denver mechanical cell and a 1250 mL vat (useful volume). The operational parameters of the flotation tests were similar to those used at the phosphate mining plant, as detailed in Table 2. The pulp was conditioned in the flotation cell tank. After the conditioning time, air was aspirated, and flotation was carried out for about 6 minutes, until the foam was exhausted.

Solutions of gelatinized corn starch, saponified vegetable oil and NaOH were used as the depressant, collector and pH regulator, in concentrations of 3% (m/m), 2.5% (m/m), and 10% (m/m), respectively. The cornmeal was sourced from a commercial brand, while the hydrogenated soybean oil, supplied by RJMG Óleo Química, was of the Agem variety. A comparative analysis of the effluents SE and SE_DAF was conducted to verify the influence of the DAF on the response variable values for mass recovery (yield), P₂O₅ content, and metallurgical recovery. For this analysis, the assumption of normality of the data was verified by the Anderson–Darling test. F and t hypothesis tests were then completed to compare the variance and difference between the effluents SE and SE_DAF. Then, a comparison between the four effluents used in the study was performed through an analysis of variance (ANOVA). The main objective of this test was to determine any differences in the averages of the analyzed response variables (mass recovery, P₂O₅ content and metallurgical recovery) for all effluents. The Tukey test was used to further validate the ANOVA test results, verifying significant differences between the response variable averages of effluents. The statistical tests were completed using the software Minitab (version 19) and a significance level of 5%. The tests were conducted in duplicate. The metallurgical recovery of each flotation test was calculated by Equation 2.

Where R_metal is the metallurgical recovery, A is the mass (g) fed into the flotation cell, C is the mass (g) of the floated fraction, a is the P₂O₅ content in the flotation feed sample, and c is the P₂O₅ content in the concentrate.

3. RESULTS AND DISCUSSION

The specific mass of the ore was observed to be 2.86 ± 0.04 g/cm³, not varying significantly from the values found by UMAR et al. [¹⁹[19] UMAR, A.H., YARO, S.A., ABDULWAHAB, M., et al., “Characterization of Sokoto phosphate rock for beneficiation”, Journal of Raw Materials Research, v. 13, n. 2, pp. 74–93, 2016.] (2.90 g/cm³). Table 3 quantifies the predominant oxides found in the phosphate ore sample by percentage. Notably, the sample used in this study contained high concentrations of CaO, SiO₂, and P₂O₅, contributing to 60.39% of the sample composition.

Figure 1a presents the turbidity results of the TD efluente and NW, with the TD effluent 50% more turbid than NW. Moreover, the turbidity of the tailing dam effluent (TD) was 88% lower than the value found by SANTOS et al. [¹⁶[16] SANTOS, M.A., CAPPONI, F., ATAÍDE, C.H., et al., “Wastewater treatment using DAF for process water reuse in apatite flotation”, Journal of Cleaner Production, v. 308, pp. 127285, 2021. doi: http://doi.org/10.1016/j.jclepro.2021.127285.
https://doi.org/10.1016/j.jclepro.2021.1... ] (288 NTU). These authors investigated the impact of tailings dam effluent on the flotation of apatite from a phosphate concentration plant situated in a region neighboring the location of the present study, within the state of Minas Gerais. The turbidity results for the TD effluent and NW were below the maximum levels allowed by the Brazilian federal legislation for effluent return to the environment (100 NTU) [²⁰[20] BRASIL, “Resolution nº 357/05. Establishes the classification of fresh, brackish and saline waters of the National Territory, Diário Oficial da União, 2005.]. Figure 1b presents the pH values of the NW and TD effluent. The pH of the effluent TD (8.10) was comparable to the pH of water from the surroundings of a phosphate mine, as stated by JAILOS et al. [²¹[21] JAILOS, P., CHIMTALI, P.J., VUNAIN, E., “Assessment of groundwater quality in areas surrounding Thundulu phosphate mine, Phalombe district, Malawi”, Tanzania Journal of Science, v. 47, n. 3, pp. 1310–1321, 2021. doi: http://doi.org/10.4314/tjs.v47i3.38.
https://doi.org/10.4314/tjs.v47i3.38... ] (8.30), considering both analyses were performed during the non-rainy period. The pH of the NW (7.70) was similar to the average values for effluents generated by processing in phosphate ore mines in Al-Abyad (7.55 ± 0.03 and 7.58 ± 0.13) and Al-Hisa (7.65 ± 0.13 and 7.66 ± 0.03), as stated by JIRIES et al. [²²[22] JIRIES, A., EL-HASAN, T., AL-HWEITI, M., et al., “Evaluation of the effluent water quality produced at phosphate mines in Central Jordan”, Mine Water and the Environment, v. 23, n. 3, pp. 133–137, 2004. doi: http://doi.org/10.1007/s10230-004-0053-z.
https://doi.org/10.1007/s10230-004-0053-... ]. Figure 1c displays the concentration of calcium and magnesium ions for the NW and TD effluent, determined through titration, indicating that Ca²⁺ and Mg²⁺ concentrations were comparable between effluents, particularly for magnesium. VINAUD [²³[23] VINAUD, L.U.P., “Removal of calcium and magnesium ions from phosphate rock mining tailings, employing banana peels, potato and coconut shell fiber as biosorbent materials”, M.Sc. Thesis, Federal University of Triângulo Mineiro, Uberaba, MG, 2018.] found lower concentrations of both Mg²⁺ ions (13.16 mg/L) and Ca²⁺ ions (1.53 mg/L) in a TD effluent, even though the effluent originates at a point near the recirculated water intake of a phosphate rock dam that receives effluents from all processes of the beneficiation plant. Figure 1d presents the removal efficiency of Ca²⁺ and Mg²⁺ ions obtained, suggesting that it is possible to remove 91.6% of magnesium (7 mg/L of Mg²⁺) and 66.4% of calcium (28 mg/L of Ca²⁺) from the SE effluent with treatment via coagulation, flocculation, and DAF in two stages (1st condition and 2nd condition).

The DAF (Dissolved Air Flotation) technique reduces Ca²⁺ and Mg²⁺ content by introducing fine air bubbles into the water, which attach to the suspended particles, including ions, forming buoyant aggregates or flocs. These flocs then rise to the surface of the water, forming a froth layer that can be easily off. As a result, the concentration of ions in the treated water decreases [¹¹[11] RODRIGUES, R.T., RUBIO, J., “DAF–dissolved air flotation: potential applications in the mining and mineral processing industry”, International Journal of Mineral Processing, v. 82, n. 1, pp. 1–13, 2007. doi: http://doi.org/10.1016/j.minpro.2006.07.019.
https://doi.org/10.1016/j.minpro.2006.07... ].

Table 4 displays the average results for mass recovery, P₂O₅ content, and metallurgical recovery for each type of water, from the set of ore flotation tests. The SE effluent presented the least satisfactory results for all the evaluated variables. The results of a comparison between SE and SE_DAF are displayed in Tables 5 and 6. The effluents were analyzed under the assumption of normality of the data (p > 0.05). The results of the F-test proved statistical equality between effluents for the variables of mass recovery (Table 5a), P₂O₅ content (Table 5b), and metallurgical recovery (Table 5c) (p > 0.05). The F-test indicates that p-values above 5% exhibit equal variances However, the t-test analysis of the difference between means showed that the values of mass recovery (Table 6a), P₂O₅ content (Table 6b), and metallurgical recovery (Table 6c) were statistically different (p < 0.05), with SE exhibiting lower response values than SE_DAF for all variables analyzed. A hypothesis to explain this phenomenon suggests that the presence of calcium and magnesium ions is detrimental to the flotation process for several types of ores, such as phosphate (GUIMARÃES and PERES [²⁴[24] GUIMARÃES, R.C., PERES, A.E.C., “Interfering ions in the flotation of a phosphate ore in a batch column”, Minerals Engineering, v. 12, n. 7, pp. 757–768, 1999. doi: http://doi.org/10.1016/S0892-6875(99)00062-X.
https://doi.org/10.1016/S0892-6875(99)00... ]), iron (CRUZ et al. [²⁵[25] CRUZ, D.G., GONÇALVES, P.S.M., LELIS, D.F., et al., “Effect of Ca2+ and Mg2+ ions on the reverse cationic flotation of itabiritic iron ore”, Holos, v. 3, pp. 1–11, 2021.]; OLIVEIRA et al. [¹⁴[14] OLIVEIRA, H.O., LIMA, N.P., AZEVEDO, A.C., “Process water treatment by coagulation-flocculation-dissolved air flotation (DAF) for reuse in iron ore processing”, In: Proceedings of the 29th National Meeting of Mineral Treatment and Extractive Metallurgy, Armação dos Búzios, 2022.]), zinc (ARAÚJO and LIMA [²⁶[26] ARAÚJO, A.C.A., LIMA, R.M.F., “Influence of cations Ca2+, Mg2+ and Zn2+ on the flotation and surface charge of smithsonite and dolomite with sodium oleate and sodium silicate”, International Journal of Mineral Processing, v. 167, pp. 35–41, 2017. doi: http://doi.org/10.1016/j.minpro.2017.08.001.
https://doi.org/10.1016/j.minpro.2017.08... ]), lithium (LIU et al. [²⁷[27] LIU, W., ZHANG, S., WANG, W., et al., “The effects of Ca (II) and Mg (II) ions on the flotation of spodumene using NaOL”, Minerals Engineering, v. 79, pp. 40–46, 2015. doi: http://doi.org/10.1016/j.mineng.2015.05.008.
https://doi.org/10.1016/j.mineng.2015.05... ]), and copper (JELDRES et al. [²⁸[28] JELDRES, R.I., ARANCIBIA-BRAVO, M.P., REYES, A., et al., “The impact of seawater with calcium and magnesium removal for the flotation of copper-molybdenum sulphide ores”, Minerals Engineering, v. 109, pp. 10–13, 2017. doi: http://doi.org/10.1016/j.mineng.2017.02.003.
https://doi.org/10.1016/j.mineng.2017.02... ]).

Table 7 presents the results of a comparison between the four types of water evaluated for variables of mass recovery, % P₂O₅ content and metallurgical recovery. For all evaluated responses, at least one effluent presented a statistically different average value (p < 0.05).

Figure 2 shows the results of the Tukey test for analyzing the effluent variables of (a) mass recovery, (b) P₂O₅ content, and (c) metallurgical recovery. The Tukey test indicated that SE was the only effluent that presented statistically significant difference in average value of mass recovery (Figure 2a) and metallurgical recovery (Figure 2c). SE presented the highest concentrations of Ca²⁺ and Mg²⁺, leading to the hypothesis that such ions impede the mineral flotation process, proven in the case of phosphate ore. For the P₂O₅ content, the Tukey test showed that in addition to the significant mean difference of SE, the TD and NW effluents also presented an unexpected difference among themselves. As the P₂O₅ content was a value obtained from quantitative chemical analysis, the difference between the TD and NW may be due to measurement error. Furthermore, the p-value is close to the level of significance used (p = 0.048). The statistical analysis revealed no significant difference in apatite flotation performance between TD and SE_DAF effluents. As depicted in Figure 2, the error bars for the three parameters assessed – mass recovery, grade, and metallurgical recovery – intersect zero, indicating their similarity.

PINHEIRO [²⁹[29] PINHEIRO, V.S., “Application of dissolved air flotation for the treatment of hard water in the Seridó/RN region”, M.Sc. Thesis, Federal University of Pernambuco, Recife, PE, 2011.] states that the effectiveness of cationic collectors in mineral systems can be affected by the presence of specific cations in the water. The cations can create complex molecules with the collector, forming a precipitate and decreasing the availability of reagent molecules in the solution that are needed to promote the hydrophobization of minerals. GUIMARÃES and PERES [²⁴[24] GUIMARÃES, R.C., PERES, A.E.C., “Interfering ions in the flotation of a phosphate ore in a batch column”, Minerals Engineering, v. 12, n. 7, pp. 757–768, 1999. doi: http://doi.org/10.1016/S0892-6875(99)00062-X.
https://doi.org/10.1016/S0892-6875(99)00... ] corroborated the findings of PINHEIRO [²⁹[29] PINHEIRO, V.S., “Application of dissolved air flotation for the treatment of hard water in the Seridó/RN region”, M.Sc. Thesis, Federal University of Pernambuco, Recife, PE, 2011.], explaining that calcium and magnesium ions reacted with the anionic collector, forming insoluble soaps and decreasing the amount of reagent available for the collection of apatite, with this effect significant for ion concentration values above 30 mg/L. PAIVA et al. [³⁰[30] PAIVA, P.R.P., GASPAR, J.C., UGARTE, J.F.O., et al., “Deleterious effect of calcium and magnesium ions on the floatability of semi-soluble minerals”, In: Proceedings of the 2nd National Meeting of Mineral Treatment and Extractive Metallurgy, pp. 273–279, Ouro Preto, 2007.] verified that in flogopithite ore, the floatability of the apatite, calcite and dolomite minerals decreased abruptly when the solution reached concentrations of 20 mg/L of calcium and magnesium ions. For concentrations above this value, the floatability of carbonates increased, and the detrimental effect of calcium and magnesium ion concentration on apatite floatability was further proven. LANGSCH et al. [³¹[31] LANGSCH, J.E., PAIVA, P.R.P., MONTE, M.B.M., et al., “Effect of residence time and pulp solids percentage on the dissolution of calcium and magnesium present in carbonated ores”, In: Proceedings of the 22nd National Meeting of Mineral Treatment and Extractive Metallurgy, pp. 281–287, Ouro Preto, 2007.] noted that one of the relevant factors in monitoring the pH value and the concentration of calcium and magnesium ions in the pulp was the precipitation of the ions in solution and adsorption on the mineral surfaces, an element that interferes with their floatability and in the selectivity of the apatite/carbonate separation. NANTHAKUMAR et al. [³²[32] NANTHAKUMAR, B., GRIMM, D., PAWLIK, M., “Anionic flotation of high-iron phosphate ores-control of process water chemistry and depression of iron minerals by starch and guar gum”, International Journal of Mineral Processing, v. 92, n. 1–2, pp. 49–57, 2009. doi: http://doi.org/10.1016/j.minpro.2009.02.003.
https://doi.org/10.1016/j.minpro.2009.02... ] believed that Ca²⁺ and Mg²⁺ ions could not be avoided in industrial practice. In addition, phosphate minerals (and invariably present carbonates, such as calcite and dolomite) have poor solubility in water (salt-type minerals), with there always being a possibility of releasing small amounts of these cations in the pulp. However, such naturally low levels of cations do not pose major flotation problems.

4. CONCLUSIONS

Mining faces significant challenges due to the increasing need for primary sources of impure water with high ion levels and the utilization of recycled water, exemplified by tailings dam waters and those from solid-liquid separation operations. In the context of phosphate ore, the physical properties and solubility of apatite, the primary mineral, are crucial. Apatite releases ions into the beneficiation process water, potentially hindering its direct use in flotation as recycled water. Utilizing dissolved air flotation, along with coagulation and flocculation, may offer a viable solution for water treatment and reuse as mineral effluent.

The dissolved air flotation technique proved to be an effective alternative for treating simulated efluent, obtaining a liquid with acceptable concentrations of calcium and magnesium ions, with removal efficiencies of 91.6% and 66.4% respectively. The flotation results of phosphate ore in this study showed that higher concentrations of calcium and magnesium ions in the simulated efluent resulted in lower values for mass recovery, P₂O₅ content, and metallurgical recovery, proving the effect of water quality on the performance of the operation. Mineral flotation tests of simulated efluent treated by coagulation, flocculation, and dissolved air flotation produced average values of mass recovery, content, and metallurgical recovery that were similar to the results for NW. These results highlight the need for a water treatment stage in mineral processing plants and prove that the process presented in this study is a viable option to improve effluent quality.

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