The answer is "it depends", what is your standard water pH, what other chemicals are invilved (reagents, flocculents, etc.), what particle size range a are tailings in, and a lot more.

Settling rates are based on water and particle surface chemistry, and pH has a role in that. But, each operation is different and the effect of adding or reducing the pH will vary. In some cases adding CaO (up to a point) will improve settling, or hinder it. This is a condition that you need to experiment with at your site.

There is often an optimum pH point for any site with poorer performance above and below that. It also is dependent on if and what flocculants you are using.

You really need to sit in a laboratory and play with the factors, no blanket statement will be adequate.

Correct, the usage of Cao might result in formulation of gypsum which also hinders settling ability, therefore a test will provide further details regarding the relationship.

I think the lab should be used for this diagnosis and hence the results will give you what you want to know or see.pH definitely have an impact on settling  I want to say that overdose may hinder settling because of the particle sizes of the CaO. Hence the settling rate is surely to be affected by the pH and again the lab is key to concluding on our inputs

You need to conduct laboratory settling test work to determine the effect of pH.

Hi, look at the charge range of the flocculant and coagulants being used. For eg, magnafloc 333 is a neutral floc which works within a pH of 6 to 8 at worst . You then get flocculants which tip either way of the scale and are designed particularly to work within the chemistry confines of high or low pH. Lab scale cylinder tests will help greatly . Consult the industry leaders like senmin.

I'd suspect that you are operating a CIP plant. My observation is why put CaO into a thickener to control pH when you are controlling pH for the process requirements. I'd think you should decide on the process requirements and then look to a reputable supplier of flocculants to help you decide on the best floc for the process chemistry. You don't mention whether the thickener is for a pre-leach or tailings application. If it is for tailings then the addition of the lime is to my mind wasting money as the process is complete at this time and you are adding another reagent prior to tailings disposal.

I think CaO should be added on the density control pump box feeding the process (leach/CIL). Thus will be a best solution to the thickener settling rate.

Yes, Ca being divalent, acts as a coagulant. Floculant is normally required for optimum settling rates. Best!

I am agree, in a cyanide leach circuit in my previous job in thickening stage we added CaO in slurry form for slime control, like a coagulant and this worked fine

Might I suggest people read what the early workers in cyanidation wrote. Dorr, Hamilton and Park will give you an indication of what cyanidation is about. Lime is added for pH modification and has flocclating properties. The use of synthetic flocculants help to accelerate the settling and improve the clarity of the overflow. An e-book of Hamilton is available for $US10

Helena sums it pretty well. You could be creating a calcium carbonate factory on the thickener underflow piping with the dP through the pump and the remaining pipe. Some water analysis and bench scale getting could provide some interesting data. Increasing your water hardness more notably free calcium rarely provides any benefit in the recirculating water. Keep your particle surface charges close to zero will reduce your chemical consumption, unless you're using the thickener as some type of hydro separator.

Several operations that I am aware of have the issue that was mentioned, calcium carbonate buildup and hence scale. To the point that even large lines can almost close up in just a few years. This becomes even more of an issue as people tighten up there water systems to use less fresh water (i.e greater recycling).

It is one of those damned if you damned if you don't issues, and finding an alternative to lime addition maybe important in the long term.

For Pre-Detox Tailings we tried settling test using CaO at different pH (from 10.5 to 11.60); it worked well in terms of visual clarity between pH 11.3 to 11.60. In actual operation the CaO consumption and its purity are the main issues.

As mentioned by other folks, the thickener overflow typically needs to be low in suspended solids. The settling rates you mention in the thickener are typically calculated using Stoke’s equation (and the retention time provided by the thickener) and then confirmed by settling tests of the thickener feed slurry. The settling rate determines the thickener area, or if the settling rates are too slow, settling aids may have to be used. For many ores in the mining industry, particle surface charge (Zeta potential) increases in negativity as pH increases. For example, using quartz mineral, at pH = 7.0, quartz particles typically have a surface charge of minus 50 mv; but, for particles to agglomerate, the surface charge must be approximately +/- 10 mv, and the particles then agglomerate and settle quite effectively. The agglomeration of particles is caused by the universal van der Waals attractive force between particles. Of course, in the mill solutions, things are more complex, but generally when lime is used to elevate pH the calcium ion has a greater effect on the particle charge compared to the OH^- and H⁺ ions (which at pH=7.0, are 10^-7 Molar). If lime is added at (assume) 1.0 pound lime/tonne ore, it increases pH, but the calcium ion typically lowers particle surface charge below the minus 10 mv threshold which allows the particles to agglomerate and settle. If this system does not work effectively enough for conditions in your mill environment, then typically a suitable flocculant can be used to achieve more rapid settling and clarification. If potassium hydroxide, for example, were used instead of calcium hydroxide, the potassium ion would have negligible effect on particle surface charge and settling rate.