Dewatering: Thickening, Filtering, CCD, Water Treatment & Tailings Disposal

Dewatering: Thickening, Filtering, CCD, Water Treatment & Tailings Disposal 2017-03-23T09:42:05+00:00
  • To participate in the 911Metallurgist Forums, be sure to JOINLOGIN
  • Use Add New Topic to ask a New Question/Discussion about Thickening, Filtering or Tailings and Water.
  • OR Select a Topic that Interests you.
  • Use Add Reply = to Reply/Participate in a Topic/Discussion (most frequent).
    Using Add Reply allows you to Attach Images or PDF files and provide a more complete input.
  • Use Add Comment = to comment on someone else’s Reply in an already active Topic/Discussion.

Bridging Flocculation (13 replies)

Carl Jenkins
1 year ago
Carl Jenkins 1 year ago

Can it be possible for caustic soda (NaOH) to form bridging flocculation in a slurry?

Oberfuhrer
1 year ago
Oberfuhrer 1 year ago

What does the slurry consist of (particles, sizes, etc.) and what are you seeing that makes you think bridging vs. coagulation?

Carl Jenkins
1 year ago
Carl Jenkins 1 year ago

The slurry is actually an iron ore slurry with small amounts of NaOH (0.01 to 0.03 wt. %). Without NaOH, the slurry had a pseudo plastic behavior. However, adding NaOH up to 0.03 wt% made the slurry to transform from pseudoplasticity to dilatancy. Just need a reason for the transition. I know bridging flocculation is associated mainly with addition of polymers. Please any clue?

Oberfuhrer
1 year ago
Oberfuhrer 1 year ago

What pH of the slurry before & after NaOH addition?

Carl Jenkins
1 year ago
Carl Jenkins 1 year ago

Did not check for the pH! However, Na is monovalent and thus may not have strong effect on the electrical double layer of the iron ore particles in the slurry. Please can it be possible for me to have an idea of how NaOH can lead to increase in slurry viscosity at high shear rates?

Tarun Karakoti
1 year ago
Tarun Karakoti 1 year ago

Laboratory test may help in understanding the bridging flocculation.

Oberfuhrer
1 year ago
Oberfuhrer 1 year ago

pH will influence surface change. Is important wrt ZPC.
What % solids of slurry?
High dosage of caustic will cause double layer compression.
What other ions are present when NaOH is added?
What other solids, e.g., clays are present?

We need some basic knowledge of your system to help you?

Carl Jenkins
1 year ago
Carl Jenkins 1 year ago

You are correct. High dosage of caustic will cause double layer compression, which in turn will cause the iron ore slurry to have a pseudo plastic behavior with little or no yield, and reduced viscosity. What of low dosage of caustic? What will it do the slurry rheology? That's the challenge I am having. Will it reduce double layer repulsion? If yes, how! According to literature, low dosage of polymers can increase flocculation by bridging mechanism. Caustic is not a polymer and I guess it may not give rise to bridging effect at low dosage. How does low dosage of NaOH affect iron ore slurry rheology. The % solid of the slurry is 40 vol. % (72.68 wt. %). No other reagent was added to the iron ore except water and NaOH. Please help!

Can caustic soda (NaOH) cause bridging flocculation? I know polymers used as dispersants can do that.

I think I made a mistake. The EDL compression will lead to flocculation, which will in turn encourage dilatant flow behavior and increase in viscosity. Please ignore my mistake. At low dosage of NaOH, what will happen to the slurry?

Bill Fraser
1 year ago
Bill Fraser 1 year ago

You have to look at a bit more in your mixtures. Quite apart from ZPC and ion concentrations which all affect the system, there are possible chemical reactions which are pH and ion concentration dependent. If you have enough Caustic soda present, it will dissolve some of the silica minerals in the iron ore slurry, forming sodium silicate in solution. Subject to double layer conditions and solution ion concentrations and the charge on the mineral particles the minerals may flocculate and start to settle. As this is a mixture, and the solids settling separate from the liquid, the pH of the two components will change. If the solids settling out in the sludge have a lower pH (than the mixed slurry), then some of the silica from the dissolved sodium silicate will re-precipitate, giving the effect of a bridging flocculation by cementing the iron ore particles in the sludge. At the same time, the pH of the separated solution would rise. I have seen this behavior in sea water-slurries.

Carl Jenkins
1 year ago
Carl Jenkins 1 year ago

Thanks a lot for your write up; it's really going to help me with my work. No doubt, the iron ore has high silica content and therefore the caustic soda is likely to form sodium silicate when the caustic soda is high enough. Please what is likely to happen to the stability of the suspension/slurry at low dosage of caustic soda?

Alan Carter
1 year ago
Alan Carter 1 year ago
1 like by johnclark

I worked at Outotec for 12 years doing flocculation and thickening test work. I can't profess to know / remember the ins and outs of the physical chemistry - suffice to say, the presence of Ca ions (typically ex Lime) was normally beneficial to flocculation (coagulation of slimes) whereas the presence of Na ions, especially from Caustic, tended to cause slimes /clay dispersion and major issues with getting good flocculation, particularly good clarity. It is, of course, situation dependent, but caustic causing flocculation and bridging does not align with my experience.

Carl Jenkins
1 year ago
Carl Jenkins 1 year ago

Do you by chance know the mechanism behind the dispersion of the iron ore slurry by NaOH?

Alan Carter
1 year ago
Alan Carter 1 year ago
1 like by johnclark

If your target is dispersion, both silicates and iron oxides should be negatively charged at alkaline pH. Is your target to disperse so you can remove the silicates? Or are you pumping the slurry? If you are seeing dilatancy in your rheology, this may relate to a bimodal size distribution.

johnclark
9 months ago
johnclark 9 months ago
1 like by David

There is an article ( http://www.yokogawa.com/ca/technical-library/resources/application-notes/ph-in-iron-ore-slurry-naoh-solution/ ) which measured pH vs ZP for "iron ore slurry" and is showing high negative ZP at alkaline pHs. Different iron minerals will have their own unique pH vs ZP, and again differing based on plant water chemistry. The "mechanism" appears to be: many minerals have an affinity for OH ions (even at pH=7.0, where [OH] and [H] ions are equal concentration, the mineral preference is typically to adsorb (or other mechanism) the OH ion). A similar mechanism applies to soil particles. If lime is used to elevate pH, typically the Ca ion is stronger competition for the OH ion and as we see in many tailings slurries, the lime is an effective "settling aid".   

Please join and login to participate and leave a comment.