Dissolved and precipitated oxalate

Transcription

1 Accepted 2005 Process liquors from bleach plants Dissolved and precipitated oxalate Using Ion Chromatography 0 Introduction In bleach plants of pulp mills with a high degree of system closure, there is a risk that calcium oxalate is deposited in bleaching stages having acidic or neutral environment. 1 Scope This SCAN-test Method specifies a procedure for the determination of dissolved oxalate and of precipitated calcium oxalate. It is intended for process control purposes in bleach plants of pulp mills. The detection limit of the method is approximately 0,5 mg/l. Note To be able to make mill balances for oxalate, a procedure applicable to pulp suspensions is decribed in Annex A. Precision data for pulp suspensions are however poor. For that reason, the procedure applicable to pulp suspensions is described in an informative Annex. 2 Definitions For the purpose of this Method, the following definitions apply: 2.1 Dissolved oxalate The content of oxalate ions, CO in the liquid phase of the sample. 2.2 Total oxalate The content of oxalate in the sample, i.e. the sum of dissolved oxalate, CO, and solid (precipitated) calcium oxalate, CaC2O4 (s). 3 Principle For determination of dissolved oxalate, the sample is filtered and acidified in connection with the sampling and the oxalate in the liquid phase is For determination of total oxalate, the sample is acidified using hydrochloric acid in order to dissolve solid calcium oxalate. The solution is filtered and the oxalate in the liquid phase is The end-determination is based on ion chromatography (IC). 4 Reagents 4.0 All chemicals and reagents must be of analytical grade. Distilled or deionised water must be used in the preparation of reagents.

2 Page Concentrated oxalate standard solution, c(c2o 4 ) = 1000 mg/l. Dissolve 1,523 g of Na2CO in water (4.0) and dilute to 1 litre. The solution is stable at least one month. 4.2 Oxalate standard solution. Dilute the concentrated standard solution (4.1) to between 2 mg/l and 8 mg/l C 2 O 4. The solution is stable only one day. 4.3 Hydrochloric acid, 1 mol/l. 4.4 Eluent solution, for ion chromatography. The composition of this solution is highly variable depending on the type of ion chromatography column used. Therefore, follow the recommendations given by the IC column supplier. 5 Apparatus Ordinary laboratory equipment and 5.1 Ion Chromatograph, with a conductivity detector and an anionic exchange column suitable for determination of oxalate. 5.2 Membrane filter, a syringe filter with a pore size of 0,2 μm or 0,45 μm. 6 Sampling and sample pre-treatment Carry out the sampling in such a way that representative samples are obtained. If total oxalate is to be determined, acidify the sample by using hydrochloric acid (4.3) to ph 2 at the sampling. A suitable volume of acid is usually 1 ml per 50 ml of filtrate. The acifidication is made for dissolving solid calcium oxalate, if present. If dissolved oxalate is to be determined, filter the hot sample through a membrane filter (5.2) at the sampling in order to remove any precipitated calcium oxalate. Then, acidify the sample using hydrochloric acid (4.3) to ph 2 to prevent precipitation of any calcium oxalate before the analysis. Note 1 The solubility of calcium oxalate will decrease with decreasing temperature. To avoid any change in the oxalate content before analysis, it is therefore important to perform the filtration and acidification at the sampling. filter (e.g. a paper filter or a glass fibre filter) before the membrane filtration. As soon as possible after sampling and sample pre-treatment, carry out the ion chromatography analysis using the procedure described in Clause 7. 7 Ion Chromatography Inject a sample of the oxalate standard solution (4.2) and check the retention time. Note Since sulphate and oxalate are eluting at close retention times, it is important to check the separation between them. Using the oxalate standard solution (4.2), make a calibration curve consisting of approx. three points, within the preferred concentration range. Filter an aliquot, 5 ml is sufficient, of the acidified sample through a membrane filter (5.2) to prevent any contamination of the column. Inject the filtered sample into the ion chromatograph (5.1). If necessary, dilute the filtered solution until the concentration of the sample falls within the range of the calibration curve. Samples of unknown origin should be diluted to avoid the column from being overloaded. 8 Calculation Identify the oxalate peak in the sample by comparing the retention time to that of the oxalate standard solution. Quantify the oxalate concentration by measuring the peak area and comparing it to the calibration curve. Calculate the concentration of oxalate in the sample from the expression: X 1 = C f [1] where X 1 C f is the concentration of oxalate, C 2 O 4, in the original sample, in milligram per litre; is the concentration of oxalate in the diluted sample, calculated from the calibration curve, in milligram per litre; is the dilution factor. Note The dilution, taking place in the acidification step, should be included in the dilution factor. Note 2 If the sample is diffucult to filter, a pre-filtration can be performed using a coarser

3 Page 3 9 Report The test report shall include reference to this SCAN-test Method and the following particulars: (a) date and place of testing; (b) precise identification of the sample; (c) the results, dissolved oxalate and/or precipitated oxalate, given with two significant figures; (d) any departure from the procedure described in this Method and any other circumstances that may have affected the result Reproducibility The same samples were analysed at different laboratories. The following results, mean oxalate and coefficient of variation ( between laboratories), were obtained: CO, mg/l laboratories D 0 filtrate 26 12,7 5 D 0 filtrate + precipitated oxalate , Precision In order to determine the precision of the method one filtrate from a D 0 stage was analysed. Both the disssolved oxalate and, after addition of solid calcium oxalate, the total oxalate were 10.1 Repeatability The samples were analysed repeatedly in the same laboratory. The following results, mean oxalate and coefficient of variation ( within lab), were obtained: CO, mg/l duplicates D 0 filtrate 3,2 6 D 0 filtrate + precipitated oxalate 115 4,1 6

4 Page 4 Annex A (informative) Pulp suspensions Dissolved and precipitated oxalate A.0 Introduction This procedure for pulp suspensions is described to make it possible to carry out oxalate balances in a pulp mill. A.1 Scope This Method is applicable to pulp suspensions. The detection limit is approximately 50 mg/kg dry pulp. A.4 Reagents A.4.0 The same reagents as described in Clause 4 of the main procedure should be used. A.4.1 Acidic cation exchange resin, e.g. Amberlite IR-120 (H + ). Note Other strongly acidic resins may be used. A.5 Apparatus A.5.0 The same equipment as described in Clause 5 in the main procedure should be used. A.6 Sampling and sample-pretreatment Run the procedure in duplicate. Weigh to the nearest 0,1 gram a sample corresponding to approx. 1 g of dry pulp. Add water until the total weight of the suspension is 100 g. The dry matter content of the suspension should not exceed 1. Weigh at the same time a separate sample for determination of dry matter content as described in ISO 638. To the pulp suspension, add approx. 10 g of a strong acidic cat ion exchange resin (A.4.1). The amount of the resin must be sufficient to lower the ph of the suspension to approx. ph 2. If necessary, a small amount of hydrochloric acid (4.3) may be added to adjust the ph. Let the sample stand for 60 min while stirring. A.7 Chromatography Follow the procedure described in Clause 7 in the main part of this Method. Note To remove fibres from pulp samples, first filter through a glass fibre filter before the membrane filtration. A.8 Calculation Calculate the concentration of oxalate in the pulp sample from the expression: 100 C W f X 2 = [A.1] M T where X 2 is the oxalate content, CO, in the pulp, in milligram per kilogram dry pulp; C is the oxalate concentration in the diluted sample, calculated from the calibration curve, in milligram per litre; W is the total weight of the pulp suspension (pulp and added water), in gram; f is the dilution factor; M is the mass of the wet sample taken to analysis, in gram; T is the concentration of the pulp suspension, as a percentage. A.11 Precision In order to determine the precision of the method, one pulp sample from a washing filter was analysed. A.11.1 Repeatability The sample was analysed repeatedly in the same laboratory. The following result, mean oxalate and coefficient of variation ( within lab), was obtained: CO, mg/kg duplicates Pulp ,2 10 A.11.2 Reproducibility The same sample was analysed at different laboratories. The following result, mean oxalate and coefficient of variation ( between laboratories), was obtained: CO,mg/kg laboratories Pulp ,6 5

5 Page 5 SCAN-test Methods are issued and recommended by KCL, PFI and STFI-Packforsk for the pulp, paper and board industries in Finland, Norway and Sweden. Distribution: Secretariat, Scandinavian Pulp, Paper and Board Testing Committee, Box 5604, SE Stockholm, Sweden.