NO. If you were below any minimum calcium value then the pH would be too low and the cyanide would escape as HCN, with Deadly results for the plant operators.

Are you talking about Calcium on carbon or calcium in the eluate? Which plant are you on?

I assume you are asking if you can omit the acid wash prior to elution if the calcium concentration on the loaded carbon is below a certain level.

The effect of not acid washing will depend on the quality of water you are using for elution and the elution method (Zadra or AARL). The elution efficiency achieved from non-acid-washed carbon is very sensitive to water quality. The elution efficiency will decrease with increasing ionic strength of the water used for elution.

Work carried by Mintek at Grootvlei in 1981 showed a barren carbon grade of 60g/t Au from the elution of a loaded carbon containing 1.9% Ca. The barren carbon grade increased to 250 g/t when the Ca content of the loaded carbon increased to 5% I don't recommend that you remove acid washing prior to elution.

Even with good quality elution water, acid washing prior to elution will improve elution efficiency help maintain good carbon activity. The solution tail obtained from a CIL/CIP plant is proportional to the barren carbon loading. i.e. the lower the barren carbon loading, the lower your solution tail grade will be. Any savings on acid will be off-set by the increased gold solution losses due to increased barren carbon grades.

I talk of calcium on loaded carbon before elution. It is zadra method that we use with softener water and when there is problem on this system we use raw water, in generally I think that, there is no problem on water quality. Our barren is in the range of [50-120]g/t

The results of our calcium percentage which question me, the percentage is about 0.65 and after acid wash it is about 0.35. Sometimes I ask me if it real results.

When you apply the acid washing before the gold stripping, you have to calculate the efficiency of this process, it could be higher than 80%, if this value is lower than 50%, you have to review the acid concentration and the reaction time with the CaCO3, in the loaded carbon (there is a relation between the CaCO3 content and the acid volume). During the gold stripping by the pressure and temperature effect (45 psi and 130°C), the CaCO3 in the loaded carbon is increased.

Please check, the cyanide concentration (ppm) and sodium hydroxide volume (m3), that you are using in the gold stripping, is your process ZADRA or AARL?

It’s necessary to do acid washing because the CaCO3 can crack the carbon in the thermal regeneration and also increased the grade of barren solution in the adsorption circuit. Typical value to do acid washing is 25 kg de CaCO3/ ton carbon.

Your % calcium on carbon values is fine. It is important to acid wash carbon to control calcium. You don't have to acid wash every cycle if your % calcium is below 1%. All you have to worry about is the % calcium on carbon. When calcium precipitates in the carbonate form onto the exterior surface of carbon it can also occlude fine ore particles. All of this impedes kinetic gold adsorption. Normal calcium loadings on carbon have no effect on elution that is why plants successfully acid wash either before or after elution /regeneration. Regen temperatures are too low for calcium to react with carbon.

As above said it is not mandatory to do a carbon strip on every cycle. Careful accounting will identify when it is necessary. But unless acid washing is done on every cycle then there is a risk that it may be omitted more often than intended, which is why I advised against it.

I often did carbon kinetic testing to see if acid washing improved loading was needed. When you say your "barren" is 50-120g/t is this the residual gold on carbon? In which case it is "better than average", or barren solution, in which case you need to look at reducing the loss!

50-120g/t are the value of residual gold on carbon. I have also think to do analyse of carbon activity after acid wash to see improvement, what do you think?

Several comments & reiterations on above contributions:

If your Loaded Carbon (LC) & Stripped Carbon (SC) are indeed <1-wt% Ca, you have nothing to worry about; and it appears that you have excellent SC residual Au levels at 1.4 to 4-troz/st C;
To determine if your reported carbon Ca loadings are accurate, digest in aqua regia, filter & AAS for Ca;
Determine C activity of LC & SC (freundlich isotherm) for each batch;
HCl washing (4 to 5-wt%) of every other batch may be resorted to ONLY if above are performed over a period of time & consistent results enable you to make that decision;
Although most operators will tell you that H+ washing LC is not an issue, we (GoldFields Chimney Creek, NV, 1990) had serious issues w/FINE loaded carbon loss that in-place filters couldn't accommodate;
H+ washing SC is fine as long as you fresh water wash the acidified C to minimize chlorides otherwise reactivation kiln corrosion becomes an issue.

Hi everybody! Is really HCl the unique acid suitable for carbon decalc? Did anybody ever heard about sulfamic acid in this role?