HF Neutralization Procedure EXP396

During EXP396, palynologists used the following method to process silica rich sediment

1. Sediment (~10 grams)
2. Add small amounts of HCl (10 or 30%) at a time until the reaction with carbonate is over. Fill up with water.
3. Centrifuge (2500 rpm, 5-10 min) and decant.
4. On the vortex make sediment loose and add small quantity of HF (38-40%) and let react. Add more HF until reaction is over (in total about 50-70 ml HF) and put on shaker for 2 hours (not heated).
5. Add water until bottle is filled and centrifuge (this step I have never done, so I guess also 5-10 min at 2500 rpm?) and decant
6. Make sediment loose again from bottom of the bottle on vortex (if it is under the fume hood otherwise do not).
7. Add HCl to disaggregate silica gel up to less than half bottle
8. Fill up the bottle with water

The Q here if we want to centrifuge again or we go upstairs to the paleo lab with this bottle (still containing some acid).

Sample through 250- and 10-micron sieve and residue collected for slides.

Due the high concentration of HCL used in the sample process, the HF test paper turned yellow even in the Acid Eater diluted (1:1) waste solution. We could not determined if the solution has been neutralized for -F. Therefore, we adopted the following procedure

1. Place 2 (if space allowed) 1L Nalgene bottles in the HF hood. Each bottle filled with Acid Eater up to half full. Scientist(s) pour HF/HCl waste solution directly into these bottles. Acid Eater should change color to indicate the neutralization of -F
2. When the above bottle is full, entire bottle is transferred into a 9L plastic carboy
3. When the 9L carboy is filled about half full, we moved them into another hood.
4. Calcite sand was added slowly into the 9L waste carboy. Caution: this action cause foaming, add calcite little at a time and very slowly to allow the foaming reaction to subside.
5. Test pH of the waste solution frequently, treat the solution as -F still exist. 
6. When pH is above 5, take a small aliquot out. Add a few drop of concentrated HCl to reduce the pH back to 1.  Test the aliquot with HF test paper. If paper doesn't change color, the waste solution now can be treated as -F neutralized. (We have found that the Acid Eater does neutralize free F, we just cannot proof it with high concertation of HCl, as HF test paper turn yellow)
7. Solution can be pour down the chemical sink, and the calcite sand can be disposed off overboard to prevent clogging the drain line.
8. Carboy should then be rinsed and re-used for the next waste batch.