An estimated $7 million worth of Royal Canadian Mint gold was disposed of in refinery slag, leaving as much $13 million in precious metals still unaccounted for, a mint employee told a group of visitors Tuesday.

During a public tour of the historic Sussex Drive plant, a mint tour guide volunteered to the group, which included a reporter, that an internal investigation has determined an estimated $7 million in gold was not captured in its refining process last year.

Senior mint officials later would not confirm or deny the young guide's remarks or say what happened to the resulting slag, including whether it may have ended up as garbage or was sold to recyclers.

But last week, mint chairman James Love said a huge demand for mint gold last year meant it didn't have time to re-refine, as it usually does, an estimated 90 tonnes of slag for residual gold.

"An estimate was made at the year end as to what the value of the gold in this slag would be, and it was thought that this could explain a significant portion of this reconciliation difference."

"The amount of gold that was determined to be in that slag was significantly higher than the estimate that was originally used," Love said in a media report.

The mint guide said each "batch" of slag is now thought to have contained about five ounces of gold, not the original estimate of about two ounces. The mint will not define what constitutes a "batch."

The federal government last week ordered the mint to call for an RCMP criminal probe, after a four-month external audit was unable to reconcile the unaccounted-for gold and other precious metals at the mint's Sussex Drive headquarters. Mint insiders tell the Citizen it could amount to as much as $20 million.

The RCMP continues to review the request for an investigation. The audit findings are expected to be made public soon, a mint spokeswoman said Tuesday.

The mint refines gold in five major stages, explained on its website.

Doré bars in purities ranging from five per cent to 95 per cent are melted in a furnace. Dip samples are taken from the molten gold to determine its purity.

Chlorine gas is then injected into the molten metal mix. All impurities and metals but gold float to the surface to form a slag of molten chloride. The resulting .995 fine gold (99.5 per cent pure) is poured into an anode mould.

Next, in a step that was not done with the 90 tonnes of slag in question, soda ash is added to the molten chloride slag recovered from chlorination. The reaction causes gold particles to collect in a silver-gold alloy "button" that settles at the bottom of the crucible.

The gold anode is placed in a bath containing a solution of hydrochloric acid and gold chloride. The anode is then subjected to an electric current. The anode dissolves, and the dissolved gold plates onto a titanium cathode. Impurities (mostly silver) fall to the bottom of the cell or form soluble chlorides. This process brings gold to .9999 purity. It is then cast into bars of various sizes or turned into granulation gold.

imacleod@thecitizen.canwest.com