Effect of calcium concentration on calcite flotation from apatite using carbonic gas

Abstract

The flotation process currently considered for apatite concentration from the Santa Quitéria phosphate deposit (Brazil), involves bulk flotation of apatite and calcite with anionic collector at pH=10 followed by calcite flotation at pH=5.5, adjusted with H₃PO₄. Although this concept is efficient from the point of view of separation between apatite and calcite, the intensive use of inorganic acids causes ion accumulation in the process water, especially Ca²⁺ and PO₄ ^2-, which leads to problems in the flotation stage as well as in the environment. CETEM has been studying a process for the separation of carbonate minerals and apatite that involves the use of carbonic gas injected into the bubble generation system of flotation machines instead of inorganic acids. The aim of this study was to evaluate the effect of Ca²⁺ ion concentration on the water during the calcite flotation stage of the Santa Quiteria phosphate ore, between 6 mg/L (standard test) and 670 mg/L (maximun concentration) on the flotation performance in terms of P₂O₅ grade and its loss and the CaO/P₂O₅ ratio (RCP). The results indicated a reduction in the selectivity on the flotation of calcite from apatite for Ca²⁺ concentrations from 6 mg/L to around 285 mg/L. Despite this, the results obtained in this study indicated that the process based on the application of CO₂ for the separation of calcite and apatite may be a technical alternative that causes less impact in flotation performance for the phosphate concentration ores with carbonated gangue.

keywords:
process water; calcium ions; calcite; apatite; carbonic gas

1. Introduction

The agribusiness sector is of significant importance for the Brazilian economy. Between 2000 and 2015, fertilizer use in the country went up by 87%. However, the national production of fertilizers is historically lower and did not grow as much as the national demand. As a result, dependence on imports has been increasing year after year and, in 2015, about 65% of the total fertilizer consumption was supplied by imports (Cruz et al., 2017). Phosphate rock is a nonrenewable resource, which is the only economically feasible source of phosphate fertilizers. Around 60% of the world’s marketable phosphate are concentrated by the flotation process (Abouzeid, 2008; Sis and Chander, 2003). In Brazil, 13 phosphate mines were responsible for the phosphate production in 2014. However, most of the national production - 82% - is located in the states of Minas Gerais and Goiás.

Brazil has some ongoing phosphate mining projects that can increase the national production of phosphate rock concentrate. One of them is the Santa Quitéria uranium-phosphate ore deposit located in the northeast of Brazil. Besides the fertilizer production, another interesting question about this project is that the uranium contained in the apatite mineral supplies the Angra 3 nuclear power plant. The phosphate deposit is hosted in precambrian metamorphic rocks composed of fluorapatite, calcite, quartz and graphite, which represent around 90% of the total composition. Several studies have been carried out aiming to concentrate the apatite from the Santa Quitéria ore, mainly focused on the aspects of calcite and apatite separation by flotation (Aquino and Furtado, 1985; Albuquerque, 2010; Louzada et al., 2010; Paiva et al. 2010, Matiolo et al., 2016; Santos et al., 2015). The flowsheet currently considered for industrial scale was designed by the Center for Development of Nuclear Technology, Brazil (CDTN). The flotation process involves bulk flotation of apatite and calcite with anionic collector (fatty acid soap) at pH=10 followed by calcite flotation at pH=5.5 using H₃PO₄ as an apatite depressant. Calcite is collected in the froth phase and apatite concentrate is the sink product (Aquino and Furtado, 1985).

The most widely developed flotation process for the treatment of phosphate ores containing carbonate as gangue minerals uses inorganic acids as apatite depressants. Studies have been performed on the flotation process and the fundamental flotation behavior of carbonaceous phosphate ores (Houot, 1982; Zhong et al., 1991; Elgillani and Abouzeid, 1993; El-Shall et al., 2003; El-Midany, 2004; Abouzeid, 2009). Abdel-Khalek (2000) performed studies on different flotation strategies to separate both calcite and silica from a sedimentary phosphate ore from Egypt. Among three different processes for carbonate flotation, the results revealed a better selectivity when phosphoric acid was used as a depressant for phosphate. Al- Fariss et al. (2013) carried out studies with the AI-Jalamid phosphate rock from Saudi Arabia by column flotation. The optimum conditions achieved were 2.0 kg/ton of Na₂SO₄ (depressant), 1.82 kg/ton of oleic acid dissolved in kerosene (1:2.5) as a collector, at pH 6.0 - 6.5 (pH adjusted using sodium hydroxide and sulfuric acid). The best experimental beneficiation results were P₂O₅ grade above 35%, recovery above 95% and a lower CaO:P₂O₅ ratio of 1.53 from a feed containing 25% P₂O₅ and CaO:P₂O₅ 2.1.

In order to understand the complex flotation behavior of the carbonate-phosphate system, Elgillani and Abouzeid (1993) and Abouzeid et al., (2009) concluded that in acidic condition, the depression of phosphate occurs due to the adsorption (or formation) of aqueous CaHPO₄ on its surface. Another important issue raised in this study is that free Ca²⁺ in solution may affect the formation of aqueous CaHPO₄. Based on thermodynamic considerations, it may be predicted that selective carbonate flotation from phosphates in acid media can be enhanced by minimizing Ca²⁺ in the solution and increasing HPO₄ ^2- in the system. Liu at el. (2017) also developed studies to demonstrate the selective depressive effect of H₃PO₄.

Although this concept is efficient from the point of view of separation between apatite and carbonates, the intensive use of inorganic acids (>15 kg/t) causes accumulation of ions in the process water, especially Ca²⁺ and PO₄ ^2-, which leads to problems in the flotation stage and in the environment. Tavares (Personal Comunication. Rio de Janeiro, 2017) reported to CETEM that experimental tests with the Santa Quiteria ore have been developed at pilot scale using the flowsheet proposed by CDTN and achieved the expected process results. However, when the process water is recycled containing high levels of calcium and phosphates ions, it seriously affects apatite recovery.

The composition of the flotation water depends on the ore being processed, the reagent suite, the water source and the way the water system site is managed. For example, salts in solution that cause water hardness are particularly liable to react with fatty acid reagents and form insoluble complexes. Therefore, in an apatite flotation process (using H₃PO₄ as a depressant) containing high levels of dissolved salts, it is recirculated and the apatite recovery process can be seriously affected. The effects of dissolved species on flotation performance of some minerals have been studied by some authors (Aquino, 1985; Guimarães and Peres, 1999; Santos, 2010). Studies published decades ago pointed out as a common experience, low recoveries were achieved when performing flotation with recirculated process water. This would suggest the desirability of applying water treatment methods to improve the quality of the recycled water or search for an alternative flotation process towards a lesser impact on flotation performance.

Mehrotra and Sivaramakrishnan (1986) highlighted the problems associated with the use of inorganic acids as phosphate depressants, including the high costs and contamination of the water supply, thus preventing water recirculation in flotation processes. As an alternative, they patented a process to beneficiate high carbonate phosphate rocks by conditioning the pulp with CO₂ and an anionic collector. Carbonate minerals are concentrated in the froth and CO₂ acts as an apatite depressant. Along the same lines, Brazil has developed studies on the flotation process for the siliceous carbonate phosphate ore from the Araxá carbonitite (the Barreiro Mine, Brazil) based on calcite/dolomite flotation by using CO₂ as a pH modifier and bubble formation (Takata and Shimabukuro 2006). Rezende et al. (2011) patented a flotation process for the Catalão Mine that involved the rougher stage of a bulk concentrate containing apatite and carbonate minerals followed by a cleaner stage, where CO₂ was added to the pulp through a bubble generation system. As a result, the apatite concentrate was obtained in the froth phase and the carbonate minerals remained in the sink fraction.

As an alternative for the Santa Quiteria ore, CETEM has been studying a process for the separation of calcite and apatite instead of inorganic acids (Matiolo et al., 2016). The flotation process is based on calcite flotation using carbonic gas in association with coconut soap as a calcite collector, followed by apatite flotation with sulphosuccinate as a collector and corn starch as a depressant in bench scale. The results achieved in fine fraction (D₅₀ = 28µm) were similar to those obtained in the coarse fraction (D₅₀ = 100µm), reaching 33% P₂O₅ and 3.7% SiO₂ with 66% mass and 77% P₂O₅ recoveries.

Due to the lack of information on the influence of Ca²⁺ ions on the calcite flotation, this study seeks to evaluate the effect of calcium ions on calcite flotation from apatite using carbonic gas in a sample from the Santa Quiteria deposit. Primary experiments were carried out in a batch flotation and the flotation performance was assessed in terms of grade/recovery and loss of P₂O₅, the ratio CaO/P₂O₅ (RCP) and the mineralogical characterization.

2. Experiment

2.1 Ore sample

A uranium-phosphate sample from the Santa Quiteria deposit was utilized in this study. The sample preparation involved crushing, grinding, sieve classifying in coarse fraction (> 106 µm) and fine fraction (< 106 µm > 15 µm), desliming and flotation. The experiments were carried out using the sample from the coarse fraction of the circuit. The chemical analysis of the coarse stream was 20.3 % P₂O₅ grade, 47.7% CaO, 7.1% SiO₂, 0.5% MgO and RCP 2.3. The particle size was D₈₀=106 µm and D₄₄=38 µm. The main mineral phases in the sample were apatite - around 50%, calcite - 38% grade and quartz - 6%. These three minerals represent around 95% of the total sample composition. More details on the flowsheet were developed and the ore sample characterization can be found in Matiolo et al. (2016).

2.2 Calcite flotation

Calcite flotation tests were carried out in a Denver D12 model, bench scale device (Fig.1). In order to assess the calcium concentration, two different strategies were used:

Figure 1
Ilustrating calcite flotation carried out in a Denver model D12 device (bench scale).

• Effect of the addition of calcium [Ca²⁺] between 6 mg/L (standard test) and 670 mg/L (maximun concentration) on the flotation performance in terms of P₂O₅ grade and loss and the CaO/P₂O₅ ratio (RCP).

• Effect of recirculating process water in calcite flotation in terms of P₂O₅ grade and loss and the CaO/P₂O₅ ratio (RCP) as well as evaluate the calcium ions concentration in the process water according to recirculating water in the operating cycles.

Chemical analyses were carried out by the X-Ray Fluorescence technique and the mineralogical characterization was performed through X-Ray Diffraction associated with Rietveld refinement to identify and quantify the mineral species. The calcium ions concentration was analyzed through atomic absorption spectrometry (AA).

2.3 Effect of Ca ²⁺ concentration in calcite flotation

A set of experiments was proposed to evaluate the effect of different calcium ion concentrations. The tests were carried out using a cell with 1.8L. For the first test, tap water from CETEM`s supply network was used (analysed 6mg/L of Ca²⁺ ions concentration). For subsequent tests, the addition of Ca²⁺ ions in the process water for each desired concentration was performed through the solubilization of Ca(OH)₂. Therefore, the concentrations studied were 95, 246, 419 and 670 mg/L of Ca²⁺ ions. The calcite flotation tests were carried out with 500g of sample each. The pulp was conditioned with optimum conditions achieved by Matiolo et al. (2016). Thus, the coconut fatty acid soap collector was fixed at 500 g/t and the conditioning time was 5 minutes (natural pH,8) at a rotation of 800 rpm with 50% solids percentage, already adjusted with the water at the pre-established Ca²⁺ ion concentration for each test. After conditioning, the pulp was fed to the flotation cell (35% solids) and flotation started with carbonic gas injection at a 2-3 L/min rate through the bubble, generating a system for the flotation machine and as a pH regulator (5.8) and for bubble generation. Rougher flotation was carried out until froth exhaustion and the froth obtained in the rougher fed a cleaner stage that was carried out with carbonic gas rate at 2 L/min and flotation was accomplished until froth exhaustion. In both stages the pulp volume was kept constant with the water at the pre-established Ca²⁺ ion concentration. The cleaner froth corresponded to the final calcite concentrate (float) and the rougher and cleaner sink fractions were considered tailings (Figure 2). The yielded products (sink and float) were filtered, dried at 100°C and weighed for chemical and mineralogical characterization (XRF and XRD). The calcium ions concentration in the residual water from each calcite flotation test was determited by AA.

Figure 2
Flowsheet illustrating the test on the effect of the addition of calcium ion concentration in calcite flotation.

2.4 Effect of recirculating process water in calcite flotation

These tests sought to evaluate the effect of recirculating process water on calcite flotation performance using carbonic gas by bench scale simulation of the process of water recirculation in industrial plants. The tests were carried out using a different cell size (10L-5L-3L-8L) for each test. The experimental procedure maintained the process conditions described in the previous topic, such as rougher/cleaner stages, solid percentages, conditioning time and collector dosage. After the first cycle ended, with tap water supplied by CETEM, the float and sink products from the calcite flotation test were filtered and the residual water was used to run the second cycle with a new ore sample, and the procedure was repeated until completing the cycles as shown in Figure 3. The yielded products (sink and float) were filtered, dried at 100°C and weighed for chemical and mineralogical characterization (XRF and XRD). The calcium ion concentration in the residual water from each test was determined by AA.

Figure 3
Flowsheet illustrating the test to evaluate the effect of recirculating process water in calcite flotation.

3. Results and discussion

3.1 Effect of Ca ²⁺ concentration in calcite flotation

Table 1 shows the effect of the Ca²⁺ ions concentration on the grade/recovery and P₂O₅ losses from the float and sink products of the calcite flotation. The concentration of Ca²⁺ studied ranged from 6 mg/L to a maximum value of 670 mg/L. The P₂O₅ losses of calcite concentrate (float) ranged from 9% to values up to 12%, which may be considered a slight variation. On the other hand, it was observed that the P₂O₅ grade in the sink fraction decreased, going from 25% for the standard test with 6 mg/L (Ca²⁺) to 22% for the tests run with 670 mg/L calcium concentration. In addition, an increase of the RCP in the sink fraction, from 1.9 up to 2.2 was observed, showing that calcite recovery was affected by the increase of the Ca²⁺ ion concentration in the process water. Moreover, in the float fraction, an increase in the P₂O₅ grade was observed as a function of a calcium concentration increase and RCP decrease to 5.1 considering the Ca²⁺ concentration (670 mg/L) from values of 7.6 for the lowest ions Ca²⁺ concentration. These results indicated a reduction in the selectivity of calcite flotation for Ca²⁺ concentrations studied, as a tendency in the increase of apatite flotation was evidenced by the increase of the P₂O₅ grade in the float fraction and the decrease of RCP values.

Table 1
Effect of the Ca²⁺ ion concentration on the yielded products (sink and float) of the calcite flotation. Collector dosage = 500 g/t; pH 5.8.

Concentration (mg/L)	Mass Recovery (%)	Losses (%)	Float (%)			Sink (%)
Ca2+	Float	P2O5	P2O5	CaO	RCP	P2O5	CaO	RCP
6	27.0	9.2	6.9	52.2	7.6	25.0	46.8	1.9
95	27.3	8.8	6.6	52.2	7.9	25.8	45.5	1.8
246	27.3	12.7	9.3	52.1	5.6	24.1	46.2	1.9
419	21.5	10.9	9.9	52.2	5.3	22.3	47.1	2.1
670	23.3	12.3	10.2	52.5	5.1	22.0	47.3	2.2

The mineralogical compositions of calcite flotation (sink and float fractions) were identified by the Rietveld method (Fig. 4). The results for the apatite grade in the sink fraction varied from 63% - for the test with 6 mg/L Ca²⁺ concentration - to 57% and, in contrast, an increase in the calcite grade (22.1% - 28.5%) in the sink fraction was observed as it increased the calcium ion concentration in the process water. However, in the float fraction, the apatite grade increased from 14% to 23%, corresponding, respectively, to 6 and 670 mg/L of calcium concentration. In addition, the calcite grade decreased from 80.5% to 71.0% as the calcium ion concentration increased. As for the quartz grade, no variation was observed along the entire range of the calcium ion concentration studies, evidencing that such mineral was not affected by the variation of the ions concentration present in the process water.

Figure 4
Effect of Ca²⁺ concentration in calcite flotation on mineralogical characterization in the sink (a) and float (b) fractions. Collector dosage = 500 g/t; pH 5.8.

3.2 Effect of recirculating process water in calcite flotation

The effect of recirculating process water in calcite flotation can be observed in Table 2. The concentration values for the Ca²⁺ ionic species are presented as well as the results of P₂O₅ grade/recovery and losses in the sink and float fraction. The Ca²⁺ ion concentration went up from 8 mg/L to 573 mg/L. The P₂O₅ losses on calcite concentrate (float) ranged from 18% to values up to 24%, in addition to the 11.1% increase of P₂O₅ grade in the second cycle compared to the first cycle, which was 8.5%. RCP decreased from 6.1 to 4.7. In the first test, the P₂O₅ grade in the sink fraction was 31% and the values remained constant in the order of 28% in the subsequent tests. However, the RCP varied significantly from 1.4 to 1.7. The results indicated that when the water is recycled in the first cycle, the Ca²⁺ concentration went up from 8 mg/L to 285 mg/L. The considerable decrease in calcite flotation performace in the presence of high calcium concentrations was due to the increase of P₂O₅ grade and the decrease of RCP values. Nevertheless, after the second cycle, the increase of the ion concentration did not affect the calcite flotation performance, since the P₂O₅ grade in the sink fraction remained constant at around 29% and showed P₂O₅ losses of approximately 21%.

Table 2
Effect of recirculating process water in calcite flotation on the yielded products (sink and float) of the calcite flotation. Collector dosage = 500 g/t; pH 5.8.

Concentration (mg/L)	Mass Recovery (%)	Losses (%)	Float (%)			Sink (%)
Ca2+	Float	P2O5	P2O5	CaO	RCP	P2O5	CaO	RCP
8	45.5	18.4	8.5	51.8	6.1	31.5	42.6	1.4
285	45.4	24.5	11.1	51.7	4.7	28.5	44.2	1.5
418	42.1	20.1	9.9	52.4	5.3	28.6	43.6	1.5
573	41.9	22.2	10.7	52.6	4.9	26.9	44.5	1.7

The mineralogical compositions of the yielded products (sink and float) of the calcite flotation were identified by the Rietveld method (Fig. 5). The calcite grade in the float fraction was 78.3% for the lowest calcium ion concentration (8 mg/L) to 71.1% for the maximum calcium ions concentration (573 mg/L), whereas the apatite grade results ranged from 17.2% to 25.0%. In contrast, the apatite grade in the sink fraction was 78.4% for the lowest calcium ion concentration (8 mg/L) and up to 67.6% for the maximum calcium ion concentration (573 mg/L). An increase in the calcite grade is observed in the sink fraction according to the accumulation of calcium ions due to recirculating process water (5.3% to 15.7%). As noted in the previous test, no variation along the entire range of the calcium ion concentration studies for the quartz grade was observed, evidencing that this mineral is not affected by the variation of the concentration of ions present in the process water.

Figure 5
Effect of recirculating process water in calcite flotation on mineralogical characterization in the sink (a) and float (b) fractions. Collector dosage = 500 g/t; pH 5.8.

The study on the effect of recirculating process water showed that the accumulation of Ca²⁺ ions increased 30% the P₂O₅ losses on calcite flotation performance when using carbon dioxide in the range of Ca²⁺ concentrations from 8 mg/L to 285 mg/L. Although the P₂O₅ grade in the sink fraction and P₂O₅ losses were practically constant for the Ca²⁺ ions concentrations above 285 mg/L, the RCP went up from 1.5 to 1.7. Furthermore, the mineralogical composition results showed an increase of calcite grade in the sink fraction 5.3% for the standard test with 6 mg/L to 13.0% for the tests run with 285 mg/L calcium concentration and a decrease in the float fraction from 78.5% to 68.2% for the same range.

The same tendency was observed in the results for the tests carried out adding Ca²⁺ concentration in calcite flotation. For Ca²⁺ concentrations ranging from 6mg/L to 246 mg/L, a reduction in the selectivity of calcite flotation was observed, since a tendency in the increase of apatite flotation was evidenced by an increase of the P₂O₅ grade in the float fraction and a decrease of RCP values (6.1 to 4.7). The mineralogical composition results presented the increase of calcite grade in the sink fraction 22.1% to 24.7% and a decrease in the float fraction from 80.5% to 74.2 % for the same range. Nevertheless, for Ca²⁺ concentrations above 246 mg/L, the values of P₂O₅ in the sink fraction and P₂O₅ losses were practically constant. The mineralogical composition results also proved this tendency.

The results in the both studies for the tests performed with Ca²⁺ concentrations above 250 mg/L Ca²⁺ were practically constant. However, for the tests performed with Ca²⁺ concentrations from 6 mg/L to around 250 mg/L indicated a reduction in the calcite flotation. As a result, the quality of the phosphate concentrate was compromised. One of the alternatives to improve the concentrate quality (sink fraction) would be adding one more flotation stage of the sink fraction. Another point that can also be considered to understand the loss of the concentrate quality is the relationship between the collector and the free Ca²⁺ ions. According to Hanna and Somasundaran (1976), the collector reacts with the calcium present in the apatite and thus the Ca²⁺ addition consumes the available collector for the mineral, resulting in a significant reduction in the apatite recovery. Considering that the collector reacts with the calcium present in the calcite as well as with the free Ca²⁺ ions, the calcite flotation must be conducted with a higher collector dosage in order to reach the same flotation performance of contamination-free phosphate rock.

There are several studies in literature focused on the evaluation of the effects of the ions concentration on apatite/carbonates flotation, however, full understanding of the mechanism is still a challenge. Guimarães and Peres (1999) developed a batch flotation column to evaluate the effect of ions (added in the conditioning stage) on barite followed by apatite flotation of oxidized phosphate ore from Araxá - Brazil. Recovery decreased significantly as the ion concentration increased in both floation stages. In apatite flotation, the Ca²⁺ and Mg²⁺ ions reacted with the apatite collector (rice bran oil soap), forming insoluble soaps of rice oils, reducing the collector level in the system. Based on these results, limited ion concentrations - 20 mg/L Ca²⁺ and 30 mg/L Mg²⁺ - were proposed to ensure the quality of recirculating water without impairing the flotation process. For the same oxidise phosphate ore, Santos et al. (2010) presented the effects of the Ca²⁺ ions concentration on apatite recovery. Their results showed that high concentrations of calcium (300 mg/L) were harmful to the process, causing a sharp decrease in apatite recovery to values below 10%. Aquino (1987b) carried out studies with the Santa Quitéria ore evaluating the effects of the calcium and magnesium ions concentration on the bulk concentration of calcite and apatite. The results indicated that recovery decreased from 94.2% to 53.3% for P₂O₅ and 89.6% to 37.5% for CaCO₃ for concentrations of 49 mg/L Ca²⁺ and 30 mg/L Mg²⁺.

Even though the accumulation of calcium ions affects the quality of phosphate concentrate using carbonic gas, according to Mehrotra and Sivaramakrishnan (1986) the process can be a technical alternative that causes less impact in process performance when comparing with the other studies abovementioned.

4. Conclusions

The results showed that Ca²⁺ ions accumulation reduces the selectivity of the calcite flotation process using carbonic gas, as a pH regulator and bubble generation and fatty acid soap as a collector. The reduction on the flotation performance is evidenced in the tests carried out with Ca²⁺ concentrations increased from 6 mg/L to around 250 mg/L. However, for the tests performed with Ca²⁺ concentrations above 250 mg/L Ca²⁺, the process results were practically constant. Even though the accumulation of calcium ions affects the quality of phosphate concentrate (sink fraction) when using carbonic gas, the process can be a technical alternative that causes less impact on flotation performance between carbonate minerals and apatite.

Acknowledgements

The authors would like to thank all the Brazilians institutes supporting research, namely CNPq by GRANT Number 426023/2016-1, CETEM/MCTIC and UFRGS. Special thanks is also extended to Industrias Nucleares do Brasil (INB) and Yara-Galvani for permission to publish this work.

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Publication Dates

• Publication in this collection
  17 Apr 2020
• Date of issue
  Apr-Jun 2020

History

• Received
  01 May 2019
• Accepted
  31 Dec 2019