Proceedings of the 7th World Congress on Mechanical, Chemical, and Material Engineering (MCM'21)
Prague, Czech Republic Virtual Conference – August, 2021
Paper No. MMME 115
DOI: 10.11159/mmme21.115

Impact of Recirculating Mining Process Water on the Flotation
Performance of a Sedimentary Phosphate OreMorake D. Hlahane, Willie Nheta, Mamookho Elizabeth Makhatha
University of Johannesburg
Mineral Processing and Technology Research Centre
P.O. Box 17011, Doornfontein 2028, Johannesburg, South Africa
dhlahane@gmail.com; wnheta@uj.ac.za

Abstract – Water is one of the scarce resources in the world and the mining industry like many other sectors is expected to discretionallyutilise this resource sustainably. It was on the premise of adding to the body of knowledge related to water sustainability and flotation
performance that an investigation was prompted to evaluate the impact of recirculating mining process water on the flotation performanceof a sedimentary phosphate ore. A series of flotation locked cycle tests were conducted with the objective to simulate recycling processwater, which by default modifies the water chemistry and influences flotation performance. The laboratory flotation results showed thatrecycling process water in the laboratory scale did not show significantly noticeable influence on P2O5 grade and recovery performance.
A saleable product of 33.27% P2O5 grade at reasonable average recovery of 65% was achievable. Water analyses revealed that there wasan increase in total dissolved solids [452 to 846mg/L], electrical conductivity [662 µm/cm to 1025 µm/cm], CaCO3 [146 to 304mg/L],
sodium [57.7 to 76.6mg/L], chloride [100.7 to 109.7mg/L] and magnesium [10.4 to12.6mg/L] after circulating the process water for 15cycles. The results achieved however confirmed the importance of water quality and understanding process water chemistry in flotation
processes. The compounding salts and inorganic elements in the water necessitate the need for water clarifiers for process water prior tobeing recycled to the flotation process.

Keywords: Sedimentary phosphate, Water quality, Flotation, Process water, Water chemistry.

1. Introduction

Water chemistry in flotation processes continues to be a challenge although there has been previous studies and details
in literature [1, 2] . The present work focuses on the Sedimentary phosphate ore fines circuit (-212+20µm) flotation feedsamples, where bench-scale simulation of recycled process water was carried out to evaluate the influence on flotation
performance and water quality. One of the major categories of a flotation system is the chemistry component which includes
amongst others collectors, depressants, frothers and pH adjusters which are interrelated [1]. There are various parametersthat influence the quality of water and they include amongst others, total suspended solids (TSS), total dissolved solids(TDS), electrical conductivity (EC), Iron (Fe) content and typical contaminants such as amongst others Magnesium (Mg),Sulphate (SO4), Calcium (Ca), Potassium (K) and Sodium (Na). All these parameters are critical and each or combined havea direct influence on the quality of water [2].
The use of strong inorganic acids causes accumulation of ions in the process water, especially Ca2+ and PO42- whichleads to problems in apatite recovery and in the environment [3]. The influence of water quality on the flotation performanceof complex sulphide ores showed that the influence of process water on lead flotation depends on its composition andconcentrations of constituents. It was established amongst others that the addition level of recycled water (during the grinding
or at the start of flotation) has no significant effect on the flotation of the complex sulphide ores [4].
In this study, a comprehensive water analysis of recirculated process water and its impact on Apatite recovery from aphosphate sedimentary rock is presented.

MMME 115-1

2. Method

The sample used in this study was sourced from sedimentary phosphate mine in Western Cape, South Africa. A
series of laboratory flotation tests were conducted in the laboratory using Denver D12 flotation continually simulating
recycling process water. For each cycle completed, a sub-sample of recycled process water was collected and sent forchemical analysis.
The flotation tests were conducted continuously on a bench-scale and the products for each test were filtered, dried,weighed and submitted for X-Ray fluorescence (XRF) chemical analysis. A total of 15 locked cycles were conducted
and process water was simulated by decanting and recycling water streams. Fig. 1 shows the sample preparation process
flow prior to flotation tests.

+2mm fraction

Runof Mine(-4mm+0mm)-2mm+425µm+425µm- 425µm-425µmCoarse float feed(-425µm+212µm)-212µm

20µm

Fines float feed(-212µm+20µm)-20µm

Fig. 1: Sample preparation (Milling and screening)

From Fig. 1, sample preparation involved dry screening Run of Mine (ROM) at 2mm screen and further screening
the -2mm fraction at 425µm. The +425µm fraction was milled using laboratory ball mill (D 0,265m, L0,305m) in closed
circuit to produce 100%-425µm which was wet screened at 212µm to produce fines (-212µm) and coarse (425µm+212µm). The stream of focus for the test work was the fines circuit where the -212µm was further deslimed at20µm to remove ultra-fines that have proven to be a challenge in flotation processes. For the purpose of the bench-scale
test work, approximately 11kg bulk sample (-212+20µm) was produced and split homogenously using a rotary splitter
into 700 grams representative samples for further laboratory test work.
Laboratory flotation tests were conducted using a Denver D12 flotation machine as shown in Fig. 2.

MMME 115-2

Fig. 2 shows that the fines (-212µm+20µm) from milling and screening were used for 15x locked cycle flotation test.
All the exiting water streams were collected, combined and reused to simulate recycling process water. The process involved
scrubbing the float feed sample in the presence of Sodium Silicate at 50-60% solids for 3min, with the aim to enhanceflotation by increasing flotation rates of phosphates when compared to silicates and therefore improving selectivity andyielding faster flotation rates. The scrubbing action is expected to aid in removing impurities and with separating clay
coatings from phosphate mineral surface.
The resulting pulp was deslimed using a 20µm screen. Desliming oversize fraction was transferred into a Denver D12
for direct flotation. Anionic collector (herein referred to as Fatty Acid (FA) was added and conditioned at approximately
60% solids and floated until froth collapsed and dissipates. The Anionic collector is expected to attach to the mineral surface
and with the introduction of airflow produce bubbles loaded with the positively charged mineral surface apatite. The FA
concentrate was de-oiled with sulphuric acid (H2SO4) and the pH was shifted from acidic to alkaline medium (7.5-8.5) prior
to reverse flotation for every cycle.
Cationic collector (herein referred to as Amine) was conditioned for 30 seconds at approximately 50% solids and withintroduction of airflow, reverse flotation was effected and silica was collected over the cell-lip. For reverse flotation, Amine
collector attached to the negatively charged minerals surfaces (silicates) whilst the negatively charged mineral surface(apatite) stayed in the liquid phase due to the bubbles failing adhere to its surfaces, thus being hydrophilic and herein referred
to as ‘reverse-conc’. The Fatty acid and Amine chemicals used were from Arrmaz (USA).

Decanted H2O recycled

Recycled H2O-212µm+20µm

ScrubbingDirect flotation
Direct Tails
H2SO4

Decanted
H2O recycled-20µm
decanted H2O

AcidWash

Reverse flotation

Acid
diluted H2OReverse Conc

Reverse TailFig. 2: Laboratory flotation test work process flow

For each cycle completed, a sub-sample of recycled process water was collected and sent for chemical analysis. Theflotation tests were conducted continuously on a bench-scale and the products for each test were filtered, dried, weighed andsubmitted for major phosphate minerals analysis using XRF press pellet method.

MMME 115-3

3. Results

3.1 Bench Floatation Results
The flotation cycles were grouped to minimised inconsistencies and laboratory errors. During the flotation tests, novariables were altered, reagents dosages were kept consistent for all the flotation cycles. The major elements in the headgrade, direct and reverse flotation concentrates are summarised in Tables 1, 2 and 3.

Flotation
CyclesCycle 1 - 3
Cycle 4 – 6Cycle 7 – 9Cycle 10 - 12
Cycle 13 - 15

Table 1: Head grade analysis of flotation feed sample (-212µm+20µm)

P2O5
7.69
8.25
8.07
8.08
8.04

Cycle 1 - 3Cycle 4 – 6Cycle 7 – 9Cycle 10 - 12Cycle 13 - 15

Mass pull
wt %
22.34
34.64
36.55
25.38
25.43

Cycle 1 - 3
Cycle 4 – 6Cycle 7 – 9Cycle 10 - 12
Cycle 13 - 15

Mass pull
wt %
10.98
20.6
21.89
14.72
12.25

SiO2
77.5
76.05
76.34
76.62
76.75

Head grade (%)
MgO
CaO
0.09
10.96
0.13
11.67
0.12
11.42
0.10
11.40
0.11
11.42

Al2O3
0.86
0.88
0.85
0.91
0.86

Fe2O3
0.57
0.61
0.62
0.61
0.58

Recovery %
P2O5
SiO2
67.61
10.51
86.50
19.93
88.05
22.22
71.90
12.40
66.14
11.57

CaO/P2O5
1.38
1.38
1.38
1.38
1.38

Table 2: Direct flotation concentrate

Grade %
P2O5
SiO2
23.27
36.47
20.60
43.76
19.45
46.41
22.90
37.43
22.68
37.89

Table 3: Reverse flotation concentrate

Grade %
P2O5
SiO2
35.20
5.93
32.15
14.01
29.23
20.84
34.32
7.93
35.47
5.29

Recovery %
P2O5
SiO2
50.27
0.84
80.30
3.8
79.26
5.97
62.50
1.52
54.09
0.84

CaO/P2O51.38
1.38
1.37
1.37
1.37

The overall average head grade was 8.02% P2O5 with 76.65% SiO2 and CaO/P2O5 ratio of 1.42. Direct flotation enriched
the phosphate grade from 8.02% to an average of 21.78% P2O5 and rejected 60% of the silica content. Reverse flotation tests
on the enriched direct flotation concentrate (21.78% P2O5) produced a final average concentrate grade of 33.27% P2O5 at areasonable average 65.28% P2O5 recovery.
The CaO/P2O5 ratio of averaged 1.37 was achieved which is reasonably accepted to industry specification of 1.6 fordownstream processing. A high CaO in phosphate concentrate may increase the consumption of sulfuric acid during theproduction of phosphoric acid [5].
The flotation results achieved during this bench-scale tests can be considered satisfactorily given that there was nooptimisation carried out. It can be said that with further optimisation of reagents and primary objective of achieving
32%P2O5 grade, P2O5% recoveries above 85% could be achieved on this specific feed sample.

MMME 115-4

1 000430

900

380800330700280600230500180400300

130

200

80CaCO3 (mg/l)

Total Dissolved Solids (TDS)

3.2 Simulated Processing Water Analysis
The impact of circulating processing water on total dissolved solids and water hardness was investigated and the results
are shown in Fig. 3. Fig. 3 shows that total dissolved solids (TDS) compounded as the flotation cycles increased from 452mg/l
to 864mg/l after 15 flotation cycles. A similar observation was evident with recycled water hardness (CaCO3) increasingfrom 146mg/l to 304mg/l. The persistent increase of TDS and the total hardness of process water could lead to operationalchallenges related to ineffectiveness of mineral and particle interactions, surface wettability and other factors that influences
flotation response. The reagents effectiveness could be significantly compromised which will lead to poor mineral surface
absorption, selectivity and ultimately loss of mineral recovery.

Testwork cycle
TDS (mg/l)

Tot Hardness as CaCO3 (mg/l)

Fig. 3: Total dissolved solids (TDS) and water hardness per cycle.

The effect of circulating processing water on pH and electrical conductivity (EC) was investigated and the results areillustrated in Fig 4. Fig. 4 shows that the pH gradually decreased from 7.45 to 6.37 as the process water was recycled,indicating that the process water shifted from alkaline to acidic medium. [6] found that it should be easy to separate quartz
from minerals, using reverse flotation in acidic circuit, or by direct flotation in neutral or alkaline circuits. Electricalconductivity (EC) consistently increased from 662 µm/cm to 1025 µm/cm as the process water was continually recycled. A
positive linear correlation of EC and TDS can be observed from the data herein analysed.

MMME 115-5

7,60

1 100

7,401 0007,20900

7,00800

6,80

700

6,60

600

6,40500

6,20400

6,00

300

5,80

pH

EC (µS/cm)

1 200

Testwork cycle
Electrical conductivity

pHFig. 4: Electrical conductivity (EC) and pH measurement of simulated process water per cycle.

100,0115,090,0110,0105,080,0100,070,095,060,090,050,085,040,080,0

Testwork cycle
Na (mg/l)

Cl (mg/l)

Fig. 5: Sodium and Chlorine content of simulated process water per cycles

MMME 115-6

Chlorine (mg/l)

Sodium (mg/l)

Process water was analysed for Sodium and Chloride content after circulation and the results are shown in Fig 5. Fig. 5
shows a similar build-up trend of Sodium (57.7 – 76.6 mg/L) and Chloride (100.7 – 109.7mg/L) ions as the process water
was recycled from cycle 1 to 15. A positive linear correlation of Sodium and/or Chloride ions with TDS is quite evident fromthe data. TDS is a significant measure of water quality, the elevated Sodium and Chloride ions in the processing water would
be expected to negatively impact the interactions of chemicals and mineral surface properties.

135.0

14.5
14.0
13.5
13.0
12.5
12.011.5
11.0
10.5
10.0

115.0
95.0
75.0
55.0
35.0

Calcium (mg/l)

Magnesium (mg/l)

Fig.6 shows that Magnesium and Calcium increased from 10.4-12,6mg/L and 40.8- 100.6mg/L respectively. Ca2+ andMg2+ are sometimes troublesome for flotation and they are controlled by addition of soda ash to the pulp prior to anionicflotation [6].

15.0

Testwork cycle
Mg (mg/l)

Ca (mg/l)

Fig. 6: Magnesium and calcium content of simulated process water per cycles

4. Conclusion

The laboratory flotation tests conducted on the sedimentary phosphate ore showed that recycling process water on abench-scale could produce satisfactorily results with caution. The flotation results herein achieved should however beconsidered along with the complexities associated with flotation processes. The accumulation of the inorganic salts when theprocess water is recirculated cannot be overlooked because they have been proven to be detrimental to flotation performance.
The bench-scale herein conducted showed that circulating the process water compromises the water quality by altering
the process water chemistry. The incremental increase of Ca, Mg, Na, and Cl ions together with the intensified build-up of
total dissolved solids, unquantified reagent residuals and higher electrical conductivity in the process water were evident. Ina pilot scale and operational scale this effect would necessitate an intervention such as water clarifiers to remove and manage
process water salinity and inorganics build-up prior to recirculating process water to flotation processes.
The tests were not optimised and it is recommended that further tests be considered for optimisation purposes of criticalvariables such as amongst others reagents dosages, flotation stages and reagents conditioning.

References[1] S. K. Kawatra and J. T. Carlson, Beneficiation of Phosphate Ore, Englewood, Colorado, USA: Society for Mining,
Metallurgy & Exploration (SME), 2013.
[2] M. D. Hlahane, W. Nheta and E. M. Makhatha, “The impact of using Mining pit water on flotation performance of asedimentary phosphate ore,” EARET, 2019.
[3] A. Soares, S. F. de Mello and R. T. Rodrigues, “Effect of calcium concentration on calcium flotation from apatite using
carbonic gas,” Beneficiation of Phosphates VIII, 2018.

MMME 115-7

[4] K. Boujounoui, A. Abidi, A. Bacaoui, K. El Amari and A. Yaacoubi, “The influence of water quality on the flotation
performance of complex sulphde ores: Case study at Hajar Mine, Morrocco.,” Journal of the Southern African Instituteof Mining and Metallurgy, vol. 115, no. 12, pp. 1243-1251, Dec 2015.
[5] H. Boujlel, G. Daldoul, H. Tlil, R. Souissi, N. Chebbi, N. Fattah and F. Souissi, “The beneficiation Processes of LowGrade Sedimentary Phosphates of Tozeur-Nefta Deposit (Gafsa-Metlaoui Basin: South of Tunisia),” Minerals, p. 2,
2019.
[6] A. A. Yousef, A. M. Arafa, S. S. Ibrahim and A. M. Abdel Khalek, “Seawater usage in flotation for minerals
beneficiation in Arid regions (Arab Countries) (Simulation and application),” In Proceedings: XXII International
Mineral Processing Congress., p. 1029, 2003.

MMME 115-8