Alkaline pH neutralization

[piolom]

Has anyone any experience with neutralizing alkaline wastewaters of 11 - 12 pH for low flows with carbon dioxide or other acidifying techniques?

 

[vintage70]

What are the specific characteristics of the wastewater?
Be careful what you use to neutralize. Remember heat of mixing. Avoid strong acids; consider weak acids.

 

[saxon]

piolom, Carbon dioxide is a good choice for certain types of alkaline neutraization dependent on the chemical causing the alkalinity as well as its concentration. As for acids, they too are dependent of the chemical causing the alklinity, as well as the type of neutrlizaton salts that will be generated. Without specific data on the waste stream including the mass flow, this is as detailed as I can get.

Hope this helps.
saxon

 

[piolom]

The wastewater has alkalinity of 150 - 300 ppm as CaCO3 and is alkaline due to passing thru limestone filter

 

[piolom]

By the way, we are looking for a simple approach that avoids acids such as sulfuric, addition. Flows are 500 gpd and 50,000 gpd

 

[saxon]

piolom, The basic acid/base neutralization reaction follows teh following form:
HX (acid)+BOH (base)>>>BX (salt)+H2O

CO2 may be an effective reactant in your case. Although the amount required for 50,000 gpd flow may be a little high. You'll need to run a titration on the waste water/co2 reactant to see what the curve looks like and the required quantities. In fact, if you check with Carbonic at the following site they will work up the curves for you, do a feasibility study, and design a system for you. That site is:

http://www.carbonic.com/page 6 carb web.htm

BTW, you'll need to somehow separate the salt that is generated from the neutralization reaction.

Hope this helps.
saxon

 

[piolom]

Thanks Saxon

 

[kenvlach]

For neutralizing an alkaline stream containing this much Ca, consider using hydrochloric acid. This avoids precipitation of solids. CaCl₂ is highly soluble whereas carbonates, sulfates, etc. are not.

 

[piolom]

kenvlach

Could you help me remember to estimate quantitties of HCl needed to neutralize? The concern we have with HCl is hazardous gas.

 

[kenvlach]

It would be best to do a titration to determine total alkalinity, but a neutralization calculation based upon pH is quite simple:
For pH 12, [OH^-] = 10^-2 = 0.01 moles/L
Hence, neutralization will require [H⁺] = 10^-2 = 0.01 moles/L:

OH^- + H⁺ = H₂O

As the source for H⁺, can use technical grade hydrochloric acid (37 wt%, SG 1.19).
Since HCl has FW of 36.46, the hydrochloric acid contains [HCl] = [H⁺] = 12 moles/L.
Hence, to supply 0.01 mole H⁺ requires 0.01/12 = 0.000833 L of hydrochloric acid.

To neutralize 1 m³ of alkaline solution would require 0.833 L of hydrochloric acid of this concentration. In order to greatly reduce the HCl(g) pressure, I would use hydrochloric acid of lower concentration, e.g., 30 wt%, if available. For 30 wt% HCl, 1.03 L would be required for neutralization of 1 m³ of alkaline solution.

 

[kenvlach]

As mentioned above, P_HCl decreases as the hydrochloric acid concentration decreases.
Perry’s Chemical Engineers’ Handbook gives vapor pressures for aqueous solutions of HCl. At 25^oC, the HCl partial pressure varies with solution concentration as follows:
Wt, % P_HCl, Torr
28 7.05
30 15.1
32 32.5
34 68.5
36 277
38 515

1 Torr = 1/760 atm = 133.3 Pa

 

[piolom]

thanks kenlach

 

[bimr]

Here is another site for you to check out:

http://www.digital-analysis.com/NeutralizationChemicals.htm

BTW, it is suggested that you retain someone with experience in this area as there are various hazards that can be remediated somewhat through proper design.

While CO2 seems to be somewhat non-hazardous, you should note that
1. It is a toxic gas being handled at a hazardous pressure.
2. As you add CO2, it will be generating solid waste (sludge). The CO2 reacts with the calcium in the water to form CaCO3.
3. You will need equipment for CO2 dilution and mixing. Probably more expensive than using acid.

The most commonly used acid used for neutralizing for this application is H2SO4.

A few questions for you? Why are you running this through a limestone filter? I don't believe that a limestone filter is capable of putting out a pH of 11-12. Limestone filters are usually called neutralizing filters for a reason.