This is how we did in the paper here attached (section 2.1) :

The material was first soaked into 5 N hydrochloric acid and stirred for 24 h, next vacuum-filtered and acetone-washed, then soaked into 5 N hydrofluoric acid and stirred for 24 h, and again into 5 N hydrochloric acid, after filtering and washing ([36,37] and references therein). Such a way of doing allowed first the dissolution of the major part of the metallic oxides, followed by that of the silica, then finally that of the remaining oxides not available before the removal of the silica. This demineralisation process was rather soft for the carbonaceous material, by comparison with a treatment using an oxidising acid, such as nitric acid, which produces a number of new surface functions. The demineralised char was finally thoroughly washed with distilled water.

Alain

You can use either hydrochloride acid solution or nitric acid solution to remove calcium oxide species, it is dependent on what will be the application of the activated carbons. Hydrochloric acid is not recommended when the carbons will be used as catalysts. Choride ions could poison the catalyst and reduce its activity. Nitric acid is recommended, but effectively some organic groups are oxidized enough, and the chemistry of the surface should be analyzed. In all cases, destilled/deionized water should be used to wash (until ions free, following the washing solution by conductivimetry) the solids after acid treatment.

I used a modification of Alain's procedure to reduce ash content of coal from 16% to >0.1%

The as-received particulate coal (AR-C) was ground and sieved to 53–180 μm. The size-graded coal was demineralized using the following protocol: ca. 30 g of AR-C and 200 mL of 5N HCl was magnetically stirred at 55–65 °C for 1 h and filtered. The residue was stirred in 23N HF (250 mL) at 55–65 °C for 1 h and filtered. This residue was stirred in 12N HCl (200 mL) at 55–65 °C for 1 h and filtered. The filtered coal was rinsed with boiling double-distilled water until Cl anions could not be detected (using 0.1 M AgNO3 aqueous solution). The demineralized coal (H-C) was obtained after overnight drying at 100 °C in an oven.

http://www.sciencedirect.com/science/article/pii/S0920586107000247

There is a procedure for etching using HF. But you must exercise extra care since HF is extremely dangerous. The HF is very harsh on any inorganic material. The process proceed by way of vigorous stirring your sample in considerable amount of HF at a moderate temperature in a fume hood. For example 10 g of sample in 50 ml of HF at 70 oC for 2 to 5 h with vigorous stirring.