Effective BaCl2 mixing in 55 gallon drum

[Paul1984]

Hello, I am new to these forums and really hope there is someone out there who can help me.

 

I am working on a project where we are mixing BaCl2 (actually BaCl2*2H2O) with river water. The river water contains an extremely high concentration of sulfate (around 5000 ppm) and we want to reduce it to around 10 ppm or less. So we are mixing it with barium chloride (after baking the BaCl2 for 48 hours at 200 degrees C to remove the dihydrate) in a 55 gallon drum to form barium sulphate precipitate which eventually settles out of solution. The total volume of water is around 130 liters and we are using a 1350 RPM motor with a 3.5" diameter standard propeller.

 

We have to do this in two phases as our equipment cannot accurately measure high concentrations of sulfate. We will first add enough barium chloride to target a 90% removal and then, during the second phase, target 100% removal of the remaining sulfate. Normally this means that we go from 5000 ppm down to 30 ppm and then down to less than 10 ppm (it has traditionally been around 3 ppm). This was all fine when we were mixing with a smaller motor, mixing blade, and quanityt. We used to just mix 5 gallons of water but now we have stepped things up.

 

The issue is that we are getting ineffective mixing with out current setup and we cannot find where the problem is located. The BaCl2 is extremely soluble and the barium sulphate precipitate isn't. We are mixing the water for 2 hours and then letting everything settle out overnight. This is all done in a standard 55 gallon stainless steel drum. The pH of the water is around 1.18 and there are other aspects but they don't reall matter for this discussion. We are using barium chloride as we are not concerned with the final pH of 1.18 and we don't want to remove any of the NOM in the water.

 

Historical tests have shown us that BaCl2 is the best chemical to use for this. We were previously using barium hydroxide but that served two purposes: removing the sulfate and raising the pH. That was when the water was going to be shipped off for health affects studies. The subjects needed water with a pH of 6.8-8.5. This isn't the case anymore as we are directly freeze drying the water after we remove the sulftate. We need to remove the sulfate as the freeze drying process would essentially produce sulfuric acid sludge with high amounts of NOM. We have experimented for approximately 4 years regarding the best methods to remove the sulfate, without affecting the NOM of the river water, and using barium precipitation (more specifically, BaCl2) is the best method. We are just getting inefficient mixing.

 

We have tried stepping down to a smaller motor with a larger prop, using mutliple small motors with small and large props, using a large prop with the large 1350 RPM motor, etc. We still end up having to complete this procedure at least 3 times. The issue is that we are going down from 5000 ppm to 30 ppm just fine with the first phase of mixing. The second phase of mixing is only bringing us down to around 15 ppm and the final stage is producing a final concentration of around 10.

 

We are using an appropriate formula to determine the correct BaCl2 mass that needs to be added to our volume of water.

 

Is there a better method that can be used for mixing water other than standard props? We don't have the money to buy a vortex reactor or anything like that. This essentially needs to be a solution that we can solve with $200 or less.

[John Cuthber]

This might be a stupid question, how much does a cement mixer hold?

[Paul1984]

Actually a cement mixer would not work. We have already tried something very similar to that in the past and it ended up not mixing the barium compound efficiently into the water. We are under the solubility (35 g/mL of water) of barium chloride with our experiments so that isn't an issue.

 

It just seems as if there is something wrong with our mixing apparatus. Just to give an update: I ended up modifying our current mixing setup and the end results were the same. I fashioned a round stainless steel dome, inverted it, and attached it to the inside bottom of the stainless steel drum. I also took a 3" PVC pipe, cut it in half, and placed that along the inside edge of the drum to serve as a baffle. I was following the designs of professional mixing reactors (which would likely do the job) that we simply cannot afford. This is the U.S. government we are talking about; they have the money for a $1500 executive chair but lack the funds for a $1200 mixing reactor.

[John Cuthber]

Another cheap and cheerful way to mix liquids is to blow air through them. A stream of bubbles flowing up through a drain pipe in the barrel might do the job.

Of course, anything will do the job if you give it long enough.

[Paul1984]

I have been busy trying various other techniques. Bubbles would not work for mixing as that would release a large amount of the NOM in the water to be released. It would also cause too much foam to form and eventually flow out of the top of our 55 gallon drum. We tried this years ago with a few standard "air" lines in our lab all in a 2 L glass flask. It was filled with 1 L of water and we added our BaCl2. A large amount of foam was formed that ended up pouring out the top of the flask.

 

So far nothing I have tried seems to be working. I cut a 3" PVC pipe in half and created a "baffle" going down the side of the 55 gallon drum. That caused a less turbulent vortex to form but it did not help in our mixing needs. I also tried a “flow” mixing setup where we used multiple pumps with inlets located at the very bottom with outlets at various heights throughout the sides of the drum. The flexible tubing was angled so that the water would flow in a circular pattern. 8 different pumps were used with each one experiencing a maximum flowrate of 10 l/m. It appears to cause a nice mixing pattern as the precipitate at the bottom was continually being moved with the pumps. However, the results were just as the same as before and we hand to go through three different mixing steps again.

 

We have also tried targeting a higher initial sulfate removal of 95% instead of 90% (as the first phase seems to be the most effective). Again, we had to go through three different stages. We cannot target 100% sulfate removal with the first step as our instruments cannot accurately measure high sulfate levels. The lab technician will take out 5000-6000 ppm sulfate concentration samples and dilute them down to approximately 3 ppm. Their instrument will then report results of 3.1, 3.2, and 3.3 ppm for our triplicate samples. That is a small variation and definitely “good enough” to call the sample at 3.2 ppm. However, that variation becomes rather large by the time dilution factors are taken into account where 0.1 ppm suddenly turns into 200 ppm. We also cannot add too much barium as that produces an undesirable end product that cannot be used.

 

Lastly, I have also installed a concave bottom in the drum. I tried mixing with the rigged baffle and without it. The results were again the same as we had to go through three different stages. I am really at my wits end here as I have tried everything I can think of.