Reference method for measuring releases of mercury from mercury cell chlor-alkali plants: section 6


6.1 Sources Confined to Ducts or Stacks

  1. Preparation of Sampling Train
    Wash all glass components carefully with cleaning solution. Rinse well with distilled water. Add 200 mL and 150 mL of absorbing solution to the first and second impingers, respectively. Leave the third impinger empty to buffer against acid or water carry over. Place 200 g or silica gel in the fourth impinger to protect the pump and the dry test meter. Weigh the impingers to within 0.5 g and record this information on the Moisture Analysis Data Sheet (Figure 4). Assemble the train as shown in Figure 1. Leak check the sampling train at the sampling site by plugging the inlet to the first impinger and pulling 250 mm Hg (3.3 x 104 Pa) of vacuum. A leakage rate not greater than 1% of the sampling rate is acceptable.
  2. Sample Collection
    Using a barometer, measure the atmospheric pressure to within 2.5 mm Hg (3.3 x 102 Pa). Place the probe near the centre of the duct or stack. Turn on the pump and sample at a rate proportional to the stack gas velocity, maintaining the flow rate in the sample train between 1 to 3 L/min. Take readings at least every 15 minutes and when significant changes in stack conditions necessitate adjustments in sample flow rate. The dry gas meter outlet must be vented to the outside when sampling the hydrogen stack. Record the data required for each run in the table shown in Figure 5. When the desired sampling time has elapsed, turn off the pump and record the final readings. Remove the probe from the stack and disconnect it from the sampling train.

NOTE: Refer to Section 5 on Reagents for details on cleaning solutions and reagent preparation.

Stack Gas Velocity and Volunrelric Flow Rate

The average stack gas velocity and average volumetric flow rate are determined according to the procedures specified in Reference Method EPS 1/RM/8, Method B. When applying this method, the dry molecular weight of the hydrogen stream is assumed to be 2.016. Similarly, if the content of the gas stream is substantially air, assume a dry molecular weight of 28.96.

6.2 Sampling Within the Cell Room Ventilation System

Preparation of Collection Train

Prepare the sample collection train according to part (a) of Section 6.1.

Sample Collection

The diversity and uniqueness of the ventilation systems which exist in the chlor-alkali industry preclude the preparation of an acceptable sample collection procedure to cover all systems. The system designed to sample each ventilation system must be approved by the appropriate regulatory agency. Once the sample collection program has been approved, sample collection is conducted according to the sample collection method (b) of Section 6.1.

Cell Room Air Velocity and Volumetric Flow Rate

The velocity and hence the volumetric flow rate of the cell room ventilation air is determined by using the procedures and apparatus specified in Reference Method EPS 1/RM/8, Method B, providing that the exiting air is confined to a duct and the air velocity exceeds 3 m/s.

If the ventilation system is relatively unconfined and the air velocity is less than 3 m/s (10 ft/s), a low-flow measuring device such as a vane anemometer must be used. The total number of traverse points and the frequency of traversing must be approved by the appropriate regulatory agency.

The volumetric flow rate is calculated from the average of these velocity measurements and the total ventilation outlet area.

Figure 4: Moisture Analysis Data Sheet

Moisture Analysis Data Sheet

Figure 5: Mercury Sampling Data Sheet

Mercury Sampling Data Sheet

6.3 Sample Recovery and Analysis

Samples

Transport the impinger train to the analytical laboratory taking all practical precautions to avoid sample contamination or loss. Determine the change in moisture in each impinger and record the results on the Moisture Analysis Data Sheet (Figure 4). Rinse the probe and connector to the inlet of the impinger train with fresh acid permanganate absorbing solution. This rinse may be added to either the first or second impinger. Reduce the excess permanganate in each of the first and second impingers immediately after sampling by slowly adding hydroxylamine hydrochloride directly to each impinger and gently mixing until the solutions turn clear. Transfer the reduced contents of each impinger to a 500-mL volumetric flask. Rinse the impingers (except silica gel), probe, and connecting glassware with distilled water adding the washings to the volumetric flask and diluting to the mark with distilled water.

Note: Atomic absorption must be completed within 30 minutes of the addition of hydroxylamine hydrochloride.

Standards

To prepare the standard curve, add 80 mL of acid permanganate to a 100-mL volumetric flask, and 1 mL of the 10 ppm mercury standard. Next, add enough hydroxylamine to decolorize the solution and make up to volume with distilled water.

Caution: Heat is generated and gas is evolved when the contents of the flask are mixed.

Invert slowly and mix carefully. Use this standard within ½ h of preparation to calibrate the atomic absorption unit. A 1-mL aliquot contains 100 ng of mercury, a 2-mL aliquot contains 200 ng, etc. Five points, each representing the mean of duplicate determinations, covering the range 0 to 500 ng/mL produce a linear response.

Prepare a reagent blank by following the same analytical procedure, but omit the addition of the mercury. The absorbance of a reagent blank using a 10-mL aliquot should be < 0.008. The following description of the aeration procedure applies to both standards and samples.

Pipet an aliquot (usually 1 mL) of sample into the sample bottle used for aeration and dilute to 100 mL with water. Rapidly add 5 mL of stannous chloride reagent using a graduated cylinder or dispenser. Immediately close the analysis system by placing the aerator in the sample bottle. Aerate the sample at 3 L/min and record the maximum absorbance at 253.7 mm. Between samples, run a reagent blank to ensure that the analysis system has been purged of mercury by the circulating air.

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