77 GGJJTT Vol. 2, No. 1, September, 2017

Effects of Potassium Nitrate (Saltpetre) On Gold Cyanidation*1M. N. Aakyiir, 1H. Ebenezer and 1C. Owusu

1University of Mines and Technology, Box 237, Tarkwa, Ghana

Aakyiir, M. N., H. Ebenezer, and Owusu, C. (2017), “Effects of Potassium Nitrate (Saltpetre) on GoldCyanidation”, Ghana Journal of Technology, Vol. 2, No. 1, pp. 77 - 81.

Abstract

Experimental work and analysis have been carried out in this investigation to ascertain the effect of saltpetre in the Ghanaianmarket on gold dissolution rate and recovery. The locally produced saltpetre has been used in this study as a potentialoxidising agent and its performance compared with already existing oxidising agents (hydrogen peroxide and air) in goldcyanidation process. The cyanidation test was conducted on a free milling, oxide gold ore. The outcome of the leaching testshowed that saltpetre could be used as an oxidant for gold cyanidation. After using saltpetre as an oxidant in cyanideleaching, Au recovery of approximately 99% was realised whiles approximately 98% recovery was obtained from cyanideleaching involving hydrogen peroxide and atmospheric air as oxidants. Hydrogen peroxide (H2O2) however, proved to be abetter oxidant than saltpetre in terms of leaching kinetics. The slow leaching kinetics for the saltpetre when compared toH2O2 is attributed to the probably low purity of saltpetre. Saltpetre also proved to be a better pH modifier in terms ofalkalinity, compared to the normally used lime in the gold industry. The pH modifying ability of saltpetre despite its naturalpH of about 7 is attributed to its dissociation in water to produce potassium hydroxide (KOH), a good pH modifier.

Keywords: Saltpetre, Potassium Nitrate, Hydrogen Peroxide, Leaching, Gold Cyanidation

1 Introduction

Oxidising environment is generally required ingold cyanidation process to effect gold dissolution.The role of oxygen in cyanide leaching isextensively explained in literature; (Arslan et al.,2003; Marsden and House, 2006). Oxygen isknown to increase the gold dissolution rate in goldcyanidation process (Guzman et al., 1999; Kondoset al., 1995; Kondos et al., 1996). The currentmajor challenge of using air as an oxidant in goldcyanidation is that diffusion of oxygen across thepulp boundary layer is very slow and this reducesthe overall kinetics of the process (Pinnel, 1979).Experimental work carried out by Kydryk andKellog (1954) sought to use H2O2 as an oxygensupplier other than air as an oxidant owing to thefact that one of the most important conditions forfast dissolution of gold is the presence of readilydissolved oxygen in the pulp. Although H2O2 hasbeen successful in enhancing leaching kinetics ingold cyanidation, it has proven to be an expensivereagent. Currently, investigations are focused onthe use of an alternative and cheaper oxidisingagents to enhance gold leaching recoveries.Leaching is mostly applied to gold particles that aretoo fine to be recovered by gravity methods. Thedissolved metal should be stable in the aqueousphase and therefore metals that are not stable in theaqueous phase require a complexant to ensurestability. Cyanidation is a leaching process thatrequires the use of cyanide as a complexant inorder to ensure that gold ions remain stable in thewater solubility region (Ling and Korey, 1996).Gold, being a noble metal, is not easily affected bywater and acids. It requires a very strong oxidisingagent to move from the stable state to its ionicstates (Au+ and Au3+) (Krauskopf, 1951). Strong

oxidising agents have been tested and provenfeasible (Arslan et al., 2003; Esmkhani et al., 2013;Knoore et al., 1993) whiles others are underinvestigation. Hydrogen peroxide is known to themining industry as a powerful oxidant (Knoore andGriffiths, 1984). The cyanidation and oxidation areof prime importance in the conversion of gold towater soluble cyanides. It is established thatinsufficient transfer of oxygen from gaseous phaseinto ore pulp is often the cause for low levels ofdissolved oxygen and correspondingly slowextraction kinetics and low gold recoveries(Loroesch, 1990). Although very efficient ascompared to normal atmospheric air, concerns havebeen raised regarding high price of hydrogenperoxide and lime consumption. This has led toresearch in alternative oxidising agents that canoxidise gold effectively and economically. One ofthe most important nitrate salts commercially usedis potassium nitrate. It occurs naturally as thinwhite granular crusts or masses or in minuteneedle-form crystals, and as a thin coating on earth,walls and rocks. It is a transparent crystalline saltwith a cooling, rather sharp, saline taste. It has avery high solubility between 0.32x106 to 0.18x106

g/m3 in water depending on its purity. It is apowerful oxidant in the explosives industry Thehigh oxidising ability and solubility of potassiumnitrate could be advantageous in gold cyanidationhence its potential to be used in the miningindustry. In this investigation however, reagentgrade potassium nitrate is not used but rather,saltpetre, acquired from the Ghanaian market hasbeen used to ascertain its effectiveness in goldcyanidation once it is known to be majorly made upof potassium nitrate (MGRG, 2006).

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2 Resources and Methods Used

About 5 kg free milling, oxide gold ore was usedfor this investigation. Sodium cyanide (NaCN) andlime (CaO) were obtained from the mineralsengineering laboratory of the University of Minesand Technology, Tarkwa-Ghana. These wererespectively used as complexant and pH modifierduring leaching studies. The potassium nitrate(saltpetre) used in this investigation was obtainedfrom the local producers. It is normally producedfrom raw materials such as the shells of Parkiabiglobosa (dawadawa), corncob and dry maizestalk. Hydrogen peroxide was obtained from theminerals engineering department of the Universityof Mines and Technology. Laboratory size Jaw,cone and roll crushers were used for size reductionof the ore. A laboratory size ball mill was also usedin grinding the samples to the required size for thevarious test works in this study.

2.1 Sample Preparation

The gold ore sample as received contained about2.5% moisture hence was dried in the laboratoryoven. A dry sample was thus used for thisinvestigation. Sample contamination was avoidedby thoroughly blowing off any residual samples onthe laboratory equipment. The Jones’ riffle samplerwas used to divide the bulk sample into sub-samples of 1 kg. Grindability test was thenconducted on the ore to determine the time requiredto grind 1 kg of material to 80% passing 106 µm.Grindability test established the time required togrind 1 kg of material to 80% passing 106 µm to be8 min. The particle size obtained from grinding wasused for this investigation as the sample is alreadybeing processed by a mining company in Ghanaand is producing higher recoveries. 500 g wassampled from each ground sample for the majortest works.

2.2 Leaching Test

Leaching tests were conducted on the 0.5 kgsampled ores. All leaching tests were conducted at50% pulp density, 250 ppm cyanide concentrationand 10.5-11 pH range. Acid digestion (aqua-regia)was performed on the resulting tailings to estimatethe residual gold. Three two (2) litre capacitybottles were obtained and used for the bottle rollleaching tests. The leaching bottles were labelledA, B and C. The labels A, B and C respectivelyrepresent the leaching test works done usingatmospheric air, saltpetre and hydrogen peroxide asoxidising agents. The sampled 0.5 kg of the groundsamples were transferred into each of the threebottles and pulped at 50% density. 333.3 ml of tapwater was used in the case of C due to theadditional use of hydrogen peroxide in that test.

166.7 ml of hydrogen peroxide was thus transferredinto leach bottle C to make up for the 50% pulpdensity and to attain a concentration of 2%hydrogen peroxide in the leach bottle. The bottleswith the various contents were placed on rollers for5 min to ensure a uniform mixture of the pulp. Thenatural pH of the pulp was measured using a TPSpH meter and was recorded to be in the range of 9and 9.1 for the three samples. Also, 10 g ofsaltpetre and 2% strength of hydrogen peroxidewere dosed in leach bottles B and C, respectively.Bottles B and C were allowed to roll for a further 5min to ascertain the effect of saltpetre andhydrogen peroxide on pH. The pH values were thusrecorded to be 10.71 and 8.41 respectively. Therecorded pH values of bottle A and C weremodified to between 10.5 and 11 with the additionof lime. No lime was used in test B once the pHrecorded after saltpetre addition already fell in thedesired pH range. Each bottle was then placed onthe rollers as shown in Fig. 1 and leached for 24hours. This residence time was chosen since it iscurrently the residence time used in leaching oresof the mine from which the ore was obtained.

Fig. 1 Leaching of Various Gold Ore Samples

Whilst leaching, samples were taken at designatedtime intervals of 2, 4, 8, 12, 16 and 24 h to monitorthe performance of each oxidising agent in theleaching of the mineral of interest. About 50 ml ofsolution samples was obtained from each test. Thevarious solution samples were then analysed forgold concentrations using a VARIAN AA240FSAtomic Absorption Spectrometer (AAS). The pHof samples A and C dropped below 10.5 hence thepH of samples A and C was raised to above 10.5using lime after solution samples were taken. Thedrop in pH could be attributed to other processfactors that usually govern the actual pH conditionsapplied, such as the dissolution rate of other oreconstituents (copper, iron, tellurium, antimony, andarsenic minerals), which can negatively affect gold

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leaching and pH modification and precipitation ofsolution species (Ramli and Osman, 2015). The pHof sample B however, did not drop below 10.5hence no lime was used in sample B during theentire leaching process. Cyanide concentration atthe 4th hour was topped up from 180 ppm to 250ppm. This was to ensure that there was enoughfree cyanide to complex the dissolved gold.

2.3 Acid Digestion of Tailings

After the 24th hour sampling, the tailings of eachsample was washed thoroughly with tap water toget rid of residual cyanide and gold. The washedsamples were transferred into clean pans and driedin an oven at 105 °C. Aqua regia (1:3 portions ofHNO3 and HCl, respectively) test was performedon 50 g of each of the tailings sample on a hotplate for 10 minutes. Filtrates of the digestedsamples (A, B and C) were analysed using the AASfor gold concentrations. Fig. 2 summarises themajor works carried out in this investigation.

Fig. 2 Summary of Experimental Work

3 Results and Discussion

3.1 Effect of Saltpetre (Potassium Nitrate)on Leaching

Fig. 3 shows gold recovery as a function of time forthe various oxidising agents. The results show ageneral trend of an increase in gold recovery withtime for all oxidising agents. The results also showthat the use of any of the oxidising agents after 24hours leaching time results in approximately thesame gold recovery. The maximum recovery ofgold after 24 hours were calculated to be 98.61%,98.55% and 97.64%, respectively, for the saltpetre,H2O2 and atmospheric air. The ability to recoverabout 98% of gold when saltpetre is used suggeststhat saltpetre can be used as an alternative

oxidising agent during gold cyanidation process.The very high amount of the gold dissolving insolution with respect to the total gold confirms thatthe ore used in the investigation is a free millingore.

Kinetics study for the various oxidising agents wasalso indicated (Fig.3) to ascertain the effectivenessor performance of saltpetre in relation to H2O2 andair purging systems. It was shown that the rate ofgold dissolution using H2O2 as oxidising agent isfaster compared to when saltpetre or atmosphericair is used. Fig. 3 shows that at about 16 hours ofleaching, 97% of gold recovery was recorded forH2O2 whereas 90% and 72%, respectively, wererecorded for the saltpetre and air. The golddissolution rate for the saltpetre ranged betweenthat of H2O2 and atmospheric air. The high rate ofdissolution of gold in the case of H2O2 is as a resultof the availability of readily dissolved oxygen forgold dissolution. The efficiency of saltpetre as anoxidising agent is directly related to the rate atwhich saltpetre dissolved in solution. The faster itsdissolution rate, the greater the oxidising ability. Inother words it is asserted that the locally acquiredsaltpetre was not made up of 100% KNO3 (MGRG,2006) hence rendering the saltpetre with a degreeof impurity. The reagent hence was sparinglysoluble. Additionally, because the saltpetre wasintroduced as a solid, as compared to hydrogenperoxide and atmospheric air, it will take sometime to dissolve in solution. This accounts for therelatively slow kinetics of potassium nitrate(saltpetre).

Fig. 3 Au Recovery as a Function of Time forVarious Oxidants

3.2 Effect of Potassium Nitrate on LimeConsumption and pH

Fig. 4 shows that saltpetre has a significant effecton pH and lime consumption. Saltpetre proved tobe a pH modifier as it provided the needed pH

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range during the entire leaching process. The pH ofsaltpetre is naturally neutral but when dissolved inwater results in the formation of KOH (equation 1).The KOH is thought of to be the reagent causingthe increase in pH from 9.01 to above 10.5 at theinitial stage.

KNO3(s) + H2O(l) = KOH (l) +HNO3(l) (1)

The pH dropped slightly at the end of the 24 hoursto 10.68, requiring no lime addition. As comparedto lime as a typical pH modifier, saltpetre proved tobe a good and stable pH modifier. This emanatesfrom the fact that the pH modified with limedropped significantly, which was not the case inusing saltpetre as pH modifier. Without saltpetreaddition, the amount of lime consumed at the endof the entire 24 hours of leaching was 0.8 g/L. Theuse of H2O2 further increased the lime consumptionto 1.1 g/L which clearly confirms that saltpetre hasthe advantage of being used as an oxidising agentas well as a pH modifier. Fig. 4 shows theassociated lime consumption using each of theoxidising agents.

Fig. 4 Total Lime Consumption during Leaching

4 Conclusions

The performance of saltpetre (KNO3) in goldcyanidation as against air and hydrogen peroxide asoxidants was investigated. It was comprehendedfrom the investigation that the use of saltpetre canbe used as an alternative oxidant. Comparatively,H2O2 proved to be better than saltpetre in terms ofthe leaching kinetics. From the results, potassiumnitrate will be a potential replacement of hydrogenperoxide if residence time is relatively longer.Hydrogen peroxide will certainly be of preferencefor shorter leaching times. Notwithstanding this, itis clear that the slow kinetics of leaching whenusing saltpetre does not pose any challenge to theoverall recovery of gold at the end of leaching. It isenvisaged that a purer form of potassium nitrate or

saltpetre would overcome the slow kineticschallenge during gold dissolution which canpotentially result in the cut down of residence timeduring gold cyanidation. The results also show thatapart from the oxidising potential of saltpetre, itcould be used as a pH modifier during goldcyanidation. The pH modifying ability of saltpetreresults in no lime usage in gold cyanidation whichwould potentially offset the huge cost of lime ingold processing plants where lime is used as pHmodifier.

Acknowledgement

The authors are most grateful to the MineralsEngineering Department of the University of Minesand Technology, Tarkwa for making availablereagents and equipment in the Minerals ProcessingLaboratory. The availability of these resources ledto the success of this investigation and the authorsexpress their heartfelt appreciation to theDepartment.

References

Arslan, F., Ozdamar, D. and Muduroglu, M.,(2003), "Cyanidation of Turkish Gold-SilverOre and the use of Hydrogen Peroxide", TheEuropean Journal of Mineral Processing andEnvironmental Protection, 3(3), pp. 309-315.

Esmkhani, R., Ghobadi, B., Amirkhani, A. andRezadust, S., (2013), "The Effect of IncreasingCapacity on Gold Recovery and Optimizationof Cyanidation Parameters in Aghdarreh GoldOre Plant", Australian Journal of Basic andApplied Sciences, 2(7), pp. 702-708.

Guzman, L., Segarra, M., Chimenos, J.M.,Fernandez, M.A. and Espiell, F., (1999), "GoldCyanidation using Hydrogen Peroxide",Hydrometallurgy, 52(1), pp. 21-35.

Knoore, H., Loroesch, J., Gos, S. and Stoll, M. Z.A., (1993), "Process for Leaching PreciousMetals with Hydrogen Peroxide and CyanideLeaching Solution", Australian Journal ofBasic and Applied Sciences, pp. 1-23.

Knoore, H. and Griffiths, A., (1984), "CyanideDetoxification with Hydrogen Peroxide usingthe Degusaa Process", Tucson, pp. 1-11.

Kondos, P.D., Deschenes, G. and Morrison, R.M.,(1995), "Process Optimization Studies in GoldCyanidation", Hydrometallurgy, 39(1-3), pp.235-250.

Kondos, P.D., Griffith, W.F. and Jara, J.O., (1996),"The use of Oxygen in Gold Cyanidation",

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Canadian Metallurgical Quarterly, 35(1), pp.39-45.

Krauskopf, K.B., (1951), "The Solubility of Gold",Economic Geology, 46(8), pp. 858-870.

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Loroesch, J., (1990), "Peroxide-Assisted Leach:Three Years of Increasing Success", RandolGold Forum, pp. 215-219.

Marsden, J. O. and House, I. (2006), "TheChemistry of Gold Extraction", SME, Colorado-USA, (2nd ed.), 651 pp.

Medieval Gunpowder Research Group (MGRG),(2006), "Saltpetre from India", A Galathea 3Project, Report No. 6, 17 pp.

Pinnel, M.R., (1979), "Diffusion-related Behaviourof Gold in Thin Film Systems", GoldBulletin, 12(2), pp. 62-71.

Ramli, S.C.S. and Osman, R.M., (2015), “Meetingthe challenge of Penjom Gold Mine’s geologyin the recovery of fine gold in carbonaceousores”, Bulletin of the Geological Society ofMalaysia, 61, pp. 1-9.

AuthorsMathias N. Aakyiir holds a BSc Degreein Minerals Engineering from theUniversity of Mines and Technology,Tarkwa, Ghana. He is currently ateaching assistant (National Service) inthe Minerals Engineering Department ofthe University of Mines and Technologyand has interest in research workspertaining to mineral processing

including ferrous metallurgy, flotation and high temperatureprocesses in metallurgy.

Hayford Ebenezer is a fourth yearMinerals Engineering Student of theUniversity of Mines and Technology,Tarkwa, Ghana. His final year projectwork was based on the use of saltpetrein leaching where he studied its effectson leaching with various combinationsof existing oxidising agents as well as

the pH modifying ability of saltpetre.

Clement Owusu holds a PhD inMinerals and Materials Engineeringfrom the University of South Australia(UniSA) and a Bachelor of ScienceHonours Degree in MineralsEngineering from the University ofMines and Technology (UMaT),Tarkwa, Ghana. He is a Lecturer and a

Research Fellow in the Minerals Engineering Department,UMaT, Tarkwa, Ghana. His current research interest includesoxygen optimisation in gold processing, fine particle flotation ofsulphide minerals, surface chemistry of minerals, statisticalanalysis and modeling of metallurgical data. He is a member ofthe Institute of Chemical Engineers (IChemE), Australia.