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Speciation Analysis of Colloidal Silica in Acidic Conditions Ewa Cukrowska, Levi Ochieng, Hlanganani Tutu, Luke Chimuka

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Speciation Analysis of Colloidal Silica in Acidic Conditions Ewa Cukrowska, Levi Ochieng, Hlanganani Tutu, Luke Chimuka Speciation Analysis of Colloidal Silica in Acidic Conditions Ewa Cukrowska, Levi Ochieng, Hlanganani Tutu, Luke Chimuka School of Chemistry, University of the Witwatersrand, P. Bag X3, 2050 Johannesburg, South Africa [email protected] Abstract Dissolved silica speciation is of great importance in mining as it interferes in the various metals extraction/removal processes especially in electro-winning. Analytical methods have been devised to identify species in colloidal silica in acidic conditions. Total sil- ica was determined aft er microwave acid digestion by ICP-AES. Monosilicic acid was determined using molybdic method; alpha and beta isomers were quanti ed. Silicon dioxide was determined a er precipitation as potassium uorosilicate by titrimetric method. Various interfering ion species which can co-precipitate with silicon dioxide were investigated. Keywords: colloidal silica, acidic conditions, speciation Introduction fuse to form interlinking chains. e aggrega- Silicon is the second most common element tion of subunits due to van der Waals forces found in the earths crust and mantle. It is most results in a so then hard gel. abundant in the form of silicates and occurs Most literature on silica analysis focus throughout the mineral realm complexed to on the determination of monomeric form of metals such as magnesium, calcium, iron and silica using colorimetric technique where a aluminium (Ning 2002; Holleman-Wiberg molybdate solution reacts with monomeric 1995). silicic acid to produce silicomolybdic acid Many di erent techniques have been as proposed by Dienert and Wandenbulkcke adapted to extract minerals from ores. e (Govett 1961; Roa 1992). rst step of processes is the milling or crush- Silicic acid is a tetravalent weak acid with ing of ores followed by a dissolution step. A a strong tendency to eliminate water during method of metal extraction is electro-win- condensation reactions with other silicic acid ning, once the sample is completely dissolved. molecules (Ning 2002; Holleman-Wiberg Silica dissolves from the ore samples, 1995). e acid is most stable at a low pH of both under acidic or alkali conditions. Vari- 1 - 3 and least stable at pH 5 - 6. e solu- ous species of silica and its complexes un- bility of silicic acid increases in the presence dergo polymerization to form colloidal sus- of acids such as hydro uoric acid (HF) and pensions. e colloidal silica then interferes reacts with it to form silico ouride anions with electrodes, complexes with metal ions (Govett 1961). Polymerization of silicic acid and metal extraction processes cannot occur results in large polymers that form a colloidal (Ning 2002). solution. e molecules condense to produce ere are two types of colloidal systems a Si-O-Si bond between two molecules. Si- formed by silica. Silica sols are poly-silicic licic acid polymerization is strongly depen- acids in spheroid, amorphous forms. When dant on pH (Ning 2002). Below pH 3.2, the sodium silicate (Na4SiO4) is acidi ed a sol polymerization reactions result in linear or is formed. Addition of a base causes depro- openly branched polysilicic acids. Above pH tonation and a negative overall charge of the 3.2, the polymerization results in cyclic or molecules that prevent bonding and results cross-linked polymer arrangements. Polym- in a sol. Under very specifi c conditions, a sol erization is rapid in alkaline or neutral envi- can convert itself to a gel (Holleman-Wiberg ronments and in acidic pH it is much slower. 1995; Govett 1961). If the pH is acidic or if a e condensations eventually result in a 3-D base does not stabilize the silica, the subunits structure with the repeating units of (SiO2)n. Wolkersdorfer, Ch.; Sartz, L.; Weber, A.; Burgess, J.; Tremblay, G. (Editors) 103 02_Water treatment process BOOK.indb 103 9/3/18 11:58 AM 11th ICARD | IMWA | MWD Conference – “Risk to Opportunity” Monomeric as well as polymeric silica hydroxide was dissolved in 1 L of deionised species exists in dissolved ore samples. It is water. imperative that their concentration is deter- Preparation of molybdate acidic reagent. A mined. It is therefore not substantial to just mixture of 100 mL of ammonium molybdate know the total concentrations of silica but solution, 500 mL of deionised water and 200 more importantly, its speciation (Holleman- mL of 1.5% (H2SO4) were mixed to give a Wiberg 1995). concentration of 0.0707M MoO4-2, 0.375M e objective of this study was to develop SO4-2 and 0.148M NH+4, with a pH of 1.2. methods for the analysis of various silica spe- cies in colloidal matrix under acidic condi- Alpha Monosilicic acid analysis tions and to apply the developed methods to A mixture of 20 mL of molybdate acidic re- carry out silica speciation in electro-winning agent with no more than 5 mL of sample so- processes. lution was prepared containing about 0.01 to 1.0 mg of SiO2 and the total volume was then Experimental diluted to 25 mL with deionised water. e Determination of silica in this study followed absorption was measured at a wavelength of the following procedure: standard sample 410 nm. Fresh reagent mixtures were made preparation, determination of total silica con- daily (Govett 1961). tent (ICP-AES), monomeric silica determina- Beta Monosilicic acid analysis tion (Molybdate reagent) and SiO2 determi- nation by titrimetric method. Sample solution was diluted to silica concen- tration of between 0.5 - 4 mg/L. All dilutions Preparation of colloidal silica stan- preformed were with 1.5 % H2SO4. 10 mL of dard samples each sample was then into 50 mL asks. 2 mL Analytical grade silica gel was milled to a ne of a 10% w/v solution of ammonium molyb- powder using Fritsch Planetary mono mill date was added. e solutions were mixed GmbH ‘Pulverisette 6’ at a speed of 130 rpm and allowed to react for 20 minutes. 2 mL of for 6 hours, sulphuric acid solution of known 8% w/v solution of tartaric acid was added pH was added and the mixture shaken for 16 to the reagent mixture to prevent ferric (III) hours to prepare colloidal solutions of known ion reduction, followed by 1 mL of reducing concentration (Saccone et al. 2006) . agent (4-amino-3-hydroxy-1-naphthalene- sulfonic acid/sodium metabisul te/sodium Acid digestion and determination of sul te solution). e sample was allowed to total silica by ICP-AES stand for 20 minutes to react before reading Acid digestion was used to convert samples the absorbance on a UV-Vis spectrometer into solution for determination of total sili- ((UV- 1201) at 820 nm). ca (Multiwave 3000 digestion Anton Paar Determination of silica dioxide (SiO ) GmbH). 0.1 g of ground sample masses 2 were weighed into reaction vessels and 5 mL To a 1 mL of sample, 5 mL of 20% calcium chloride and 1 g of sodium uoride was added; HNO3, 5 mL HF and 0.5 mL HCl added. Th e digested samples are then placed in a 100 mL the solution was stirred. Solid potassium chlo- volumetric ask and mixed with boric acid in ride was added until an excess of 2 - 3 g over the ration of 6 mL : 1 mL of hydro uoric acid saturation point was attained. e precipitate [Roa 1992; May and Rowe (1965). Total silica was ltered and washed with a solution con- was determined by spectral analysis using taining 70 g of potassium chloride in 1 L of Spectro ICP-AES Genesis. 1:1 ethanol-water to ensured e ective removal of excess acid. e washed precipitate, lter Determination of soluble silica paper were transferred to a 1 L beaker con- (monosilicic acid complexation). taining 500 mL of boiled deionised water. e Preparation of a stable solution of ammonium solution was titrated with 0.15M sodium hy- molybdate. 100 g of (NH )6Mo O .4H O and droxide and 1% phenolphthalein (Lewis-Russ 4 7 24 2 A, Ranville J and Kashuba AT (1991). 47 g of concentrated (28% NH3) ammonium 104 Wolkersdorfer, Ch.; Sartz, L.; Weber, A.; Burgess, J.; Tremblay, G. (Editors) 02_Water treatment process BOOK.indb 104 9/3/18 11:58 AM 11th ICARD | IMWA | MWD Conference – “Risk to Opportunity” Results and discussion 1.8 -2.0. ey were le to stand and sampled Total silica at the top, middle and bottom for analysis to assess the stability of the colloidal solutions e concentration of Si determined for the and to get a representative fraction of the col- prepared colloidal solution is shown in table loid samples. e means and percentages of 1. e measurements were done in triplicate monosilicic acids were calculated (table 2) and taken at a wavelength of 251.612 nm. for beta and alpha monosilicic acid values, Percentage of silica in the solution was de- respectively. termined as 99.4% with a good precision of e values obtained are all time depen- 0.15 % RSD. dant as the beta isomer is continually con- Monomeric silicates verting into the stable alpha isomer. e two silicic acids di er due to variation in their Monomeric silicates were quanti ed using ionic charges. UV-Vis spectroscopy. Th e acidic molybdate Beta molybdosilicic acid predominates reagent forms two compounds when react- during maximum colour formation at pH ing with the Monosilicic acid; Molybdenum 1.2 -1.4 and below. Both beta and alpha acids heteropolyacid (MHA) (SiMo O )-4 and 12 40 are formed at pH 1.8 2.0. e absorbance heteropoly anion (SiMo O )-5 complexes. 12 40 of alpha acid increases with time at any pH ese ions are commonly known as the alpha value. e time dependence is minimized (MHA) and beta (anion) forms.
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