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Problems with zinc precipitation using MHL

We are a zinc recycling company and we are trying to research some information on a water treatment facility. We currently have stormwater discharging to a local creek at approximately 20 ppm Zinc. We need this down to 0.5ppm. We have been doing this with Liquid Magnesium Hydroxide(MHL). The problem is that it takes about 3 days to settle. We have tried a few flocculants but have yet to find anything that will settle in 30 minutes. The MHL forms a very fine dispersed particle. Any help would be greatly appreciated.

Leanne Adams
- Melbourne, Victoria, Australia
2002

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You're actually missing a unit of measure in your inquiry, Ms. Adams. It's best to talk about a settling rate rather than a settling time. Like one inch per minute, or one meter in 30 minutes, or something like that.

That way, the settling rate is something that can be discussed in chemistry terms, and your actual settling problem can be assessed in engineering terms. You may have a settling problem that can be improved with co-precipitants, better poly electrolytes, a faster acting neutralizing agent, etc., but it's tough to tell from your description.

Take a beaker [beakers on eBay or Amazon [affil links] of the waste, stir it up in the lab, and record the elevation of the top edge of the solids as a function of time as it settles. Then get back to us with the settling rate. Thanks.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2002

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I work at a pretreatment plant that precipitates zinc. What is presenty being used is polymer,calcium, and aluminum chlorhydrate. Caustic and acid are used to control pH. Sometimes the zinc leaving the plant is less than 0.01.
Your're influent zinc may be higher than ours. I think our influent zinc ranges more in the 1.0 - 10.0 range.

Sincerely,

Gary Long
- White House, TN
January 5, 2008