Abstract Different chelating agent-HF mixture systems have been used to stimulate deep hot sandstone formations. Typically, chelating agents are used as a substitute for HCl and organic acids in HF based systems to stimulate hot and exotic oil and gas sandstone formations. Apart from reducing the risk of corrosion, secondary and tertiary reactions, the system can be deployed in formations containing high clays and carbonates as well as iron and zeolite bearing sandstone formations. However, several fields’ cases indicated precipitation of aluminum-based scale following acid treatments that include these systems. The focus of this paper is to identify the type of precipitations that occur during the reaction of chelating-HF systems and determine the factors that affect these precipitations. Solutions of different ethylenediaminetetraacetic acid EDTA containing two ammonium bifluoride ABF concentrations of 0.5 and 1.0 wt% were examined in this study. Aluminum chloride or calcium chloride was added separately to each chelating-HF solution to contain various concentrations of aluminum or calcium. The filtered liquid was analyzed using inductively coupled plasma ICP, while formed solid precipitate was analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The main type of precipitate in solutions containing aluminum was identified as AlF3. The amount of precipitate was found to be dependent on solution initial aluminum and free fluoride concentrations. In other words, the main factor that controlled the aluminum-fluoride precipitation was found to be F/Al ratio and above a critical F/Al value, AlF3 precipitates.

Introduction Hydrofluoric based acids have been extensively used to stimulate sandstone formations with variable success rate. The poor results are mainly attributed to the rapid spending rate, subsequent precipitation, and decomposition of sensitive clays and matrix unconsolidation (Smith and Hendrickson, 1965; Gidley, 1985; Brady et al., 1989; Simon and Anderson, 1990; Shuchart, 1995; Gdanski and Shuchart, 1996; Thomas et al., 2001). Hydrofluoric based acids went through several optimization and development processes to overcome these potential limitations and the main drivers were developing acidizing fluids with low corrosivity, retarded reaction rate and without undesirable reactions by-products (Rae and Di-Lullo, 2003). In terms of regular mud acid, in order to avoid undesirable reactions by-product, HCl/HF ratio is recommended to be maintained high i.e. 9/1 (Gdanski and Shuchart, 1996). Nevertheless, strong acids such as HCl decompose sensitive clays i.e. illite and chlorite by extracting aluminum resulting in amorphous silica precipitation and fines migration (Simon and Anderson, 1990; Thomas et al., 2001). Apart from regular mud acid, several approaches have been tried to address high temperature sandstone acidizing. These include the use of retarded mud acids including that based on AlCl3, fluoboric acid and phosphonic acid, and organic-HF acids (Thomas and Crowe, 1978; Gdanski, 1985; Lullo, 1996; Shuchart and Gdanski, 1996; Al-Harbi et al., 2011). However, these retarded HF acids remained prone to the same mud acid problems as they provided a marginal reduction in the reaction rate. In addition, fluoboric-based retarded mud acid will form potassium-based precipitate, KBF4, when it reacts with either illite or K-feldspars. Aluminum chloride HF retarded system is very susceptible to aluminum fluoride precipitation (Al-Dahlan et al., 2001). Organic-HF acids, on the other hand, can tackle some of these challenges due to their retarded nature, low corrosion rate and compatibility with clays. However, organic-HF acids are more susceptible to aluminum fluoride precipitation because of the high percentage of free fluoride (Schucart, 1997; Al-Harbi et al., 2011; Al-Harbi et al., 2012). Chelating agents have been widely used as iron control agent in spent HCl acid (Frenier et al., 2000; Taylor et al., 1999) and as primary dissolution agent in carbonate and sandstone matrix stimulation (Fredd and Fogler, 1998; Frenier et al., 2004; Parkinson et al., 2010; Mahmoud et al., 2011). Frenier et al. (2004) showed results of laboratory tests and field treatments using HEDTA. HEDTA could extract 1,100 ppm of aluminum when it was reacted with kaolinite at 300oF for 250

IPTC 16969

Evaluation of Chelating-Hydrofluoric Systems B.G. Al-Harbi, SPE; M.H. Al-Khaldi, SPE; M.N. Al-Dahlan, SPE; Saudi Aramco

Copyright 2013, International Petroleum Technology Conference This paper was prepared for presentation at the International Petroleum Technology Conference held in Beijing, China, 26–28 March 2013. This paper was selected for presentation by an IPTC Programme Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the International Petroleum Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the International Petroleum Technology Conference, its officers, or members. Papers presented at IPTC are subject to publication review by Sponsor Society Committees of IPTC. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the International Petroleum Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, IPTC, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax +1-972-952-9435

2 IPTC 16969

min. Field tests showed that HEDTA can be used to improve the productivity significantly by removing calcite and dolomite content. Ali et al. (2002) reported that EDTA containing no HF performed better than HCl and organic acid for stimulating damaged sandstone formations containing carbonate and acid sensitive clays. Additionally, Na3HEDTA was used to stimulate Berea core and better results were obtained compared to HC and almost similar results to acetic acid followed by 9/1 mud acid. Mahmoud et al (2011) conducted extensive laboratory work to evaluate different chelating agents including GLDA, EDTA and HEDTA. GLDA caused significant improvement to the permeability of Berea cores and it performed better than HEDTA at pH 4 and 300oF, whereas at pH 11 the performance was comparable. Mahmoud et al. (2011) also reported the results of GLDA containing different concentrations of HF acid. GLDA performed better than GLDA containing HF and as HF concentration decreases the performance improves. Besides, GLDA performed better than HEDTA for stimulating Bandera and Berea sandstone cores. Parkinson et al. (2010) compared the performance of Na3HEDTA, Na3HEDTA/HF and regular mud acid for stimulating Berea sandstone and the laboratory tests showed that Na3HEDTA as standalone acidizing fluid is the most effective among the tested acid mixtures. The main objective of this study is to assess the interaction between different EDTA/HF mixtures with both calcium and aluminum, two main ions present in spent sandstone acid solutions.

Theory EDTA is a weak acid and it dissociates stepwise as follows, Equations 1-4:

⇾ (1) ⇾ (2) ⇾ (3) ⇾ (4)

The distribution of the acid species is dependent on both the pH and the dissociation constants as shown in Figure 1. The dissociation constants of EDTA are: Ka1= 1.02 E-2, Ka2 = 2.14 E-3, Ka3 = 6.92 E-7, Ka4 = 5.5 E-11 (Serjant et al., 1979; Perrin et al., 1987). The first dissociation constant of EDTA, Ka1, is expressed as in Equation 5:

∗ (5)

Mixtures of EDTA/HF can be prepared using EDTA and ammonium biflouride salts, Equation 6. The solution pH value will determine the the main H+ specie donator. For example, at low pH of less than 2, H4EDTa will be the H+ donating specie.

⇾ (1)

⇾ (6)

The produced HF acid will dissociate to produce H+ and fluoride ions, Figure 2. Additionally, it will react with different sandstone minerals, quartz and clays and calcite, Equations 7-9:

HF H+ + F-, pKa = 3.170 (Perrin et al., 1987) (7)

SiO2 + 4HF SiF4 + 2H2O (Yokel, 2002) (8)

Al2Si2O5(OH)4 + 18HF 2H2SiF6 + 2Al3+ + 6F- + 9H2O (Yokel, 2002) (9)

The interaction of dissolved calcium and aluminum ions with fluoride ions can result in the precipitation of calcium and aluminum fluoride (Crowe, 1986), Equation 10 and 11:

⇾ (10)

3F- + Al3+ AlF3 (11)

Al-Harbi et al. (2011) and Al-Mohammad et al. (2011) have reported that the precipitation of both aluminum and calcium fluoride is mainly dependent on solution pH value. In EDTA/HF solutions, the presence of EDTA will prevent the precipitation of both calcium and aluminum fluoride. EDTA is an excellent chelating agent for calcium and aluminum ions. Equations 12 and 13 show the reaction of tetrasodium EDTA with calcium and aluminum ions:

⇾ (12)

⇾ (13)

The EDTA-ion complex is stable because of chelation bonds, Figure 3. Similarly, EDTA is an excellent chelating agent for both magnesium and iron Equations 14 and 15:

⇾ (14)

⇾ (15)

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The chelating agent affinity for an ion is defined by the formation constant, KF, Equations 16 and 17:

(16)

(17)

The larger the formation constant, the stronger is the chelating agent-ion complex. Table 1 lists the formation constants of EDTA with calcium, magnesium and iron (III) cations. Experimental Studies Materials Sodium EDTA solution and ammonium bi-fluoride salt were obtained from local service company and it was used as is. Different EDTA/HF mixtures were prepared using ammonium bi-fluoride, sodium EDTA solution and distilled water with a resistivity greater than 18Ω.cm at room temperature. The pH of fresh EDTA/HF mixtures ranged between 4 and 5. Table 2 gives the chemical compositions of all EDTA/HF mixtures used in this study. Calcium chloride hexa-hydrate and aluminum chloride hexa-hydrate salts were obtained from Fisher Scientific Company with purity of more than 99%. Experimental procedure The interaction between EDTA/HF acid and calcium ion was investigated at 25ºC and atmospheric pressure. The experiments were conducted using different EDTA/ammonium biflouride salt concentrations. The EDTA content ranged from 30 to 50 wt% while ammonium bifluouride salt was used at 0.5 and 1 wt%, Table 2. Each EDTA/HF mixture was placed in five different plastic test tubes, each containing 10 ml of live acid mixture. The first test tube contained 0.1 g of calcium chloride salt. Then calcium chloride salt was added in increment of 0.1 g to the following tubes until the last tube contained 0.5 g. In some tubes, a precipitate was noted in the test tubes. This precipitate was filtered and the supernatant was analyzed. Following the same procedure, the interaction between EDTA/HF mixture and aluminum ion was investigated at 25ºC and atmospheric pressure. The calcium and aluminum concentrations in collected reactive fluid samples were measured using inductivity coupled plasma (ICP). To measure the pH value of the collected samples, an Orion model 250A meter and Cole Parmer Ag/AgCl single junction pH electrode were used. Collected solid samples were analyzed using X-Ray Powder Diffraction (XRD) and ESEM. Results and Discussion The interaction between different EDTA/Ammonium biflouride (ABF) mixtures and calcium and aluminum ions was investigated at 25C and atmospheric pressure. The effects of both EDTA/ABF ratio and dissolved amount of both calcium and aluminum ions on this interaction were also investigated. Figure 4 shows the amount of dissolved calcium in different EDTA/1.0 wt% ABF mixtures as a function of added amount of calcium chloride salt at 25C and atmospheric pressure. In these solutions, the average pH value ranged from 4.5 to 4.7. Additionally, all different mixtures showed comparable performance. For example, 30 wt% EDTA-1.0 wt% ABF was able to hold up to 10,000 ppm of dissolved calcium ions without precipitating calcium fluoride, as was noticed with regular 9 wt%HCl/1 wt% HF acid (AlKhaldi et al. 2011). Similarly, both 40 and 50 wt% EDTA-1.0 wt% ABF systems were able to hold 10,000 ppm of dissolved calcium ions with no sign of calcium fluoride precipitation. A different trend was noted when the ABF wt% was decreased to 0.5 wt%. Figure 5 shows the amount of dissolved calcium in different EDTA/0.5 wt% ABF mixtures as a function of added amount of calcium chloride salt at 25C and atmospheric pressure. Average pH in these solutions ranged from 4.0 to 4.3. The average maximum calcium level these solutions could hold was nearly 6,500 ppm. This is lower than the value noted with EDTA-1.0 wt% ABF mixtures. This is mainly attributed to difference in the solution pH values. At higher pH values, as the case in EDTA-1.0 wt% ABF mixtures, there are more chelation cites for calcium ions (AlKhaldi et al. 2011). All additional amounts of calcium ions above 6,500 ppm precipitated as calcium fluoride, photo 1. Figure 6 shows the amount of dissolved aluminum in different EDTA/0.5 wt% ABF mixtures as a function of added amount of aluminum chloride salt at 25C and atmospheric pressure. It is well known that solution containing both aluminum and fluoride ions will precipitate aluminum fluoride if the pH value exceeds 2.5 (Shuchart 1996, 1997). However, it is obvious from Figure 6 that all EDTA/0.5 wt% ABF mixtures were able to hold up to 5,000 ppm of aluminum ion without precipitating aluminum fluoride. Additionally, these mixtures had comparable performance. Initially it was thought that aluminum fluoride did not precipitate due to the ability of EDTA to chelate aluminum ions. However, different trend was noted in EDTA/ABF mixtures when the ABF amount was increased to 1.0 wt%. Figure 7 shows the amount of dissolved aluminum in different EDTA/1.0 wt% ABF mixtures as a function of added amount of aluminum chloride salt at 25C and atmospheric pressure. Initially, in these systems aluminum fluoride precipitated when aluminum ions were added, Photo 2. Then, no precipitation occurred in test tubes that contained more than 2,000 ppm of aluminum. It was also interesting to note that the aluminum fluoride precipitation re-dissolved when aluminum level was

4 IPTC 16969

above nearly 2,000 ppm. This trend indicated that there is a critical F/Al molar ratio above which aluminum fluoride precipitation will occur. From Figures 8 and 9, this critical ratio was found to be nearly 1.9. Conclusions The interaction between different EDTA-HF mixtures and calcium and aluminum ions was investigated. The effects of different parameters such as solution pH value, temperature and EDTA/ABF ratio were investigated. The following conclusions can be drawn:

1. All different EDTA/1.0 wt% ABF had comparable performance where they were able to hold up to 10,000 ppm of calcium ions with no calcium fluoride precipitation.

2. Calcium fluoride precipitated in EDTA/0.5 wt% ABF mixtures when the level of dissolved calcium ions exceeded 6,500 ppm

3. Aluminum fluoride precipitation was not noted in all EDTA/0.5 wt% ABF mixtures. This was because that the F/Al molar ratio did not exceed 1.9 in these mixtures.

4. Initially aluminum fluoride precipitation was noted in all EDTA/1.0 wt% ABF mixtures when the dissolved aluminum level was below 2,000 ppm

5. Aluminum fluoride precipitation did not occur or re-dissolved in all EDTA/1.0 wt% ABF mixtures when dissolved aluminum level was above 2,000 ppm, F/Al molar ratio was above 1.9.

Acknowledgments The authors wish to acknowledge Saudi Aramco for granting permission to present and publish this paper. Special thanks go to the Chemistry and Advanced Instruments Units of the R&D Center for their analysis of different solutions and solids. References Ali, A.H.A., Frenier, W.W., Xiao, Z. et al. 2002. Chelating Agent-Based Fluids for Optimal Stimulation of High-Temperature Wells. Paper SPE 77366 presented at the SPE Annual Technical Conference and Exhibition, San Antonio, Texas, 29 September-2 October. Al-Dahlan, M.N., Nasr-El-Din, H.A., and Al-Qahtani, A.A., 2001. Evaluation of Retarded HF Acid Systems. Paper SPE 65032, presented at the SPE International Symposium on Oil Field Chemistry, Houston, Texas, 13-16 February. Al-Khaldi, M.H., et al., 2011. New Insights into the Removal of Calcium Sulfate Scale. SPE paper 144158 presented at the European Formation Damage Conference held in Noordwijk, The Netherlands, 7–10 June. Al-Harbi, B.G., Al-Khaldi, M.H., and Al-Dossary, K.A., 2011. Interactions of Organic-HF Systems with Aluminosilicates: Lab Testing and Field Recommendations. SPE paper 144100 presented at the European Formation Damage Conference held in Noordwijk, The Netherlands, 7–10 June. Al-Harbi, B.G., Al-Dahlan, M.N. and Al-Khaldi, M.H. 2012. Aluminum and Iron Precipitation during Sandstone Acidizing using Organic-HF Acids. Paper 151871, presented at the SPE Formation Damage Control Conference, Lafayette, LA, 10-11 February. Al-Mohammad, A.M., et al., 2011. Acidizing Induced-Damage in Sandstone Injector Wells: Lab Testing and a Case History. SPE paper 144007 presented at the European Formation Damage Conference held in Noordwijk, The Netherlands, 7–10 June. Brady, J.L., Gantt, L.L., Fife, D.M., Rich, D.A and Almond, S.W., 1989. Cement Solubility in Acids. Paper SPE 18986 presented at the SPE Joint Rocky Mountain Regional/Low Permeability Reservoirs Symposium and Exhibition, Denver, Mar. 6-8. Crowe, C.W., 1986. Precipitation of Hydrated Silica from Spent Hydrofluoric Acid – How Much of a Problem is it?. JPT, 1234 - 1244. Frenier, W.W., Wilson, D., Crump, D. and Jones, L. 2000. Use of Highly Acid-Soluble Chelating Agents in Well Stimulation Services. Paper SPE 63242 presented at the SPE Annual Technical Conference and Exhibition, Dallas, TX, 1-4 October. Frenier, W.W., Brady, M., Al-Harthy, S., Arangath, R., Chan, K.S., Flamant, N. and Samuel, M. 2004. SPE P&F, 189-199.

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Fredd, C.N. and Fogler, H.S. 1998. The Influence of Chelating Agents on the Kinetics of Calcite Dissolution. J. Col. Interface. Sci, 187-197.

Gdanski, R.D., 1985. AlCl3 Retards HF Acid for More Effective Stimulation. OGJ, October, 111-115. Gdanski, R.D. and Shuchart, C.E., 1996. Newly Discovered Equilibrium Controls HF Stoichiometry. JPT, 48(2), 145-149. Gidley, J.L., 1985. Acidizing Sandstone Formations: A Detailed Examination of Recent Experience. Paper SPE 14164, presented at the SPE Annual Technical Conference and Exhibition, Las Vegas, Nevada, 22-26 September. Lullo, D.G. 1996. A New Acid for True Stimulation of Sandstone Reservoirs. Paper SPE 37015, Presented at the SPE Annual Technical Conference and Exhibition, Denver, CO, 6-9 October. Martell, A. E., Smith, R. M., 1976. Critical Stability Constants, Vol. 1–4. Plenum Press: New York, USA. Mahmoud, M.A., Nasr-El-Din, H.A. and De Wolf, C.A. 2011. Removing Formation Damage and Stimulation of Deep Illitiec-Sandstone Reservoirs using Green Fluids. Paper SPE 147395 presented at the SPE Annual Technical Conference and Exhibition, Denver, Colorado, 30 October-2 November. Mahmoud, M.A., Nasr-El-Din, H.A. and De Wolf, C.A. 2011. Novel Environmentally Friendly Fluids to Remove Carbonate Minerals from Deep Sandstone Formations. Paper SPE 143301 presented at the SPE European Formation Damage Conference, Noordwijk, The Netherlands, 7-10 June. Parkinson, M., Munk, T., Brookley, J., Caetano, A., Albuquerque, M., Cohen, D., and Reekie, M. 2010. Stimulation of Multilayered-Carbonate-Content Sandstone Formations in West Africa Using Chelant-Based Fluids and Mechanical Diversion. Paper SPE 128043 presented at the SPE International Symposium and Exhibition on Formation Damage, Lafayette, Louisiana, 10-12 February. Permyakov, E. and Kretsinger, R., 2011. Calcium Binding Proteins. John Wiley & Sons Inc.: New Jersey, USA, 85-90. Perrin, D., 1987. Dissociation Constants of Organic Bases in Aqueous Solution, Butterworths, London, UK. Rae, P and Lullo, G. 2003. Matrix Acid Stimulation-A Review of the State of the Art. Paper SPE 82260 presented at the SPE European Formation Damage Conference, the Hague, the Netherlands 13-14 May. Serjeant, E. P., and Dempsey, B., 1979. Ionization Constants of Organic Acids in Aqueous Solution. Pergamon Press: Oxford, USA. Shuchart, C.E., 1995. HF Acidizing Returns Analyses Provide Understanding of HF Reactions. Paper SPE 30099, presented at the SPE European Formation Damage Symposium, the Hague, Netherlands, May 15-16. Shuchart, C.E., 1997. Chemical Study of Organic-HF Blends Leads to Improved Fluids. Paper SPE 37281, presented at the SPE International Symposium, Houston, 18-21 February. Shuchart, C.E. and Gdanski, R.D., 1996. Improved Success in Acid Stimulations with a New Organic-HF System. Paper SPE 36907, presented at the European Petroleum Conference, 22-24 October. Simon, D.E. and Anderson, M.S., 1990. Stability of Clay Minerals in Acid. Paper SPE 19422, presented at the Formation Damage Control Symposium, Lafayette, February 22-23. Smith, C.F. and Hendrickson, A.R., 1965. Hydrofluoric Acid Stimulation of Sandstone Reservoirs. JPT, 17(2), 215-222. Taylor, K.C., Nasr-El-Din H.A. and Al-Alawi, M., 1999. Systematic Study of Iron Control Chemicals Used During Well Stimulation SPEJ 4, 19-24 Thomas, R.L. and Crowe, C.W., 1978. Matrix Treatment Employs New Acid System for Stimulation and Control of Fines Migration in Sandstone Formation. Paper SPE 7566 presented at the SPE Annual AIME, Houston, TX, 1-3 October. Thomas, R.L., Nasr-El-Din, H.A., Lynn, J.D., Mehta, S. and Zaidi, S.R., 2001. Precipitation During the Acidizing of a HT/HP Illitic Sandstone Reservoir in Eastern Saudi Arabia: A Laboratory Study. Paper SPE 71690, presented at the SPE Annual

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Technical Conference and Exhibition, New Orleans, Louisiana, 30 September-3 October. Yokel, R.A., 2002. Aluminum Chelation Principles and Recent Advances. Coordination Chemistry Reviews, 228, 97-113.

Table 1: The formation constants of EDTA with different ions.

Ion Formation constant,

log KF* at 25C

Calcium, Ca2+ 10.70 Magnesium, Mg2+ 8.69

Iron (III), Fe3+ 25.7 * Martell and Smith 1976.

Table 2: Different on-step acid formulations used in this study.

EDTA ABF

Stock solution Total solution

weight, g wt% Weight, g wt% Weight, g H2O, g

1 120 30% 36 1% 1.2 82.8

2 120 30% 36 0.50% 0.6 83.4

3 120 40% 48 1% 1.2 70.8

4 120 40% 48 0.50% 0.6 71.4

5 120 50% 60 1% 1.2 58.8

6 120 50% 60 0.50% 0.6 59.4

IPTC 16969 7

Figure 1: Distribution of EDTA species as a function of solution pH value at 25C (Serjant et al. 1979, Perrin et al. 1981).

Figure 2: Distribution of HF species as a function of solution pH value at 25C (Serjant et al. 1979, Perrin et al. 1981).

0.00

0.20

0.40

0.60

0.80

1.00

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0

pH

HEDTA3-EDTA4-

0.00

0.20

0.40

0.60

0.80

1.00

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0

pH

HF

H4EDTA

H3EDTA-

H2EDTA2-

F-

pH = 0.65 Onset of free F-

8 IPTC 16969

Figure 3: Octahedral geometry of 1:1 metal-EDTA ion complex (Permyakov and Kretsinger 2011).

Figure 4: Dissolved calcium ions in different EDTA/1.0 wt% ABF mixtures as a function of added amount of calcium chloride salt at 25C and atmospheric pressure.

0

2000

4000

6000

8000

10000

12000

0 0.1 0.2 0.3 0.4 0.5 0.6

Dis

solv

ed C

a2+, p

pm

Added amount of CaCl2.6H2O, g

30 wt% EDTA

40 wt% EDTA

50 wt% EDTA

Average pH = 4.5-4.7

Line of theoriticalCa concentration

IPTC 16969 9

Figure 5: Dissolved calcium ions in different EDTA/0.5 wt% ABF mixtures as a function of added amount of calcium chloride salt at 25C and atmospheric pressure.

Figure 6: Dissolved aluminum ions in different EDTA/0.5 wt% ABF mixtures as a function of added amount of aluminum chloride salt at 25C and atmospheric pressure.

0

1000

2000

3000

4000

5000

6000

7000

8000

0 0.1 0.2 0.3 0.4 0.5 0.6

Dis

solv

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a2+, p

pm

Added amount of CaCl2.6H2O, g

30 wt% EDTA40 wt% EDTA50 wt% EDTA

Average pH = 4.0-4.3Line of theoriticalCa concentration

0

1000

2000

3000

4000

5000

6000

0 0.1 0.2 0.3 0.4 0.5 0.6

Dis

solv

ed A

l3+, p

pm

Added amount of AlCl3.6H2O, g

30 wt% EDTA40 wt% EDTA50 wt% EDTA

Line of theoriticalAl concentration

10 IPTC 16969

Figure 7: Dissolved aluminum ions in different EDTA/1.0 wt% ABF mixtures as a function of added amount of aluminum chloride salt at 25C and atmospheric pressure.

Figure 8: Precipitation of aluminum fluoride as function of F/Al molar ratio in different EDTA/1.0 wt% ABF.

0

1000

2000

3000

4000

5000

6000

0 0.1 0.2 0.3 0.4 0.5 0.6

Dis

solv

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l3+, p

pm

Added amount of AlCl3.6H2O, g

30 wt% EDTA40 wt% EDTA50 wt% EDTA

Line of theoriticalAl concentration

0

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9

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solv

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l3+, p

pm

Added amount of AlCl3.6H2O, g

50 wt% EDTA‐1.0 wt% ABF

F/Al for 50 wt% EDTA‐1.0 wt% ABF

Line of theoriticalAl concentration

F/Al moral ratio = 1.9

IPTC 16969 11

Figure 9: Precipitation of aluminum fluoride as function of F/Al molar ratio in different EDTA/0.5 wt% ABF.

Photo 1: XRD analysis of solid material precipitated in different EDTA/0.5 wt% ABF mixtures after adding calcium chloride salt.

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0

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Dis

solv

ed A

l3+, p

pm

Added amount of AlCl3.6H2O, g

50 wt% EDTA‐0.5 wt%ABF

F/Al for 50 wt% EDTA‐0.5 wt% ABF

Line of theoriticalAl concentration

F/Al moral ratio = 1.9

Filtered solid sample

CaF2

NaCl

12 IPTC 16969

Photo 2: ESEM analysis of solid material precipitated in different EDTA/1.0 wt% ABF mixtures after adding aluminum chloride salt.

AlF3