Guernsey-based investment manager Better Capital PCC, which specialises in turning around distressed businesses, acquired ATH's debt from existing lenders HSBC and Clydesdale Bank.
As a result ATH, headed by chief executive Alistair Black, warned there will have to be "a comprehensive review and restructuring" of the business.
That places further uncertainty over the jobs of its 300 workers, with the majority of those at mines in Ayrshire, Fife and Dumfries & Galloway.
ATH has been struggling with the price of coal, which plunged 28% on international markets in the first half of the year.
The fall in prices has been attributed to an oversupply of coal in North America due to a mild winter and power companies turning to cheaper fuel from the rapidly growing shale gas sector.
Better Capital, founded by venture capitalist Jon Moulton, said its Becap Fund has committed around £15 million through a special purpose vehicle to take on the debts of ATH and its subsidiary Aardvark TMC.
It is thought ATH had in the region of £18 million of debt.
Mr Moulton, best known for leading a bid to buy MG Rover from BMW in 2000, previously ran Alchemy Partners which was once the largest shareholder in ATH and oversaw its flotation on the AIM in 2004.
Nick Sanders, head of portfolio at Better Capital LLP, the London arm of the business, said: "As things stand at the moment the entire debt [of ATH] has been sold to us.
"If you go back historically ATH was backed by Alchemy Partners, a London-based private equity fund, which was headed by Jon Moulton.
"It is a business he knows. He knows the individuals and the industry so it was based on that that the fund wanted to invest in buying the debt."
Executives at ATH, which reported a half-year pre-tax loss of £7.1m, have been working with accountancy firm Deloitte since October regarding a restructuring or sale of the business.
However, the review announced yesterday is in addition to the process which has been going on over the past few weeks.
Mr Sanders said: "As the debt holder we are working with the company to look at all the options going forward.
"We don't know what is going to come out of the review. It is too early to say. We are undertaking the review.
"Nobody likes uncertainty but the business has been in a very uncertain position for some time.
"The review has to be done properly but we want it to be done quickly as well."
Industry sources suggested the review may be completed in weeks rather than months.
Better Capital's funds have interests in businesses including fashion retailer Jaeger, stationery wholesaler Spicers, luxury boat-maker Fairline and publisher Reader's Digest.
ATH, which has its headquarters in Doncaster, indicated its new lender is prepared to inject new capital in the "restructured business".
The drop in ATH's share price is potentially another expensive blow for Swedish activist investor Peter Gyllenhammar who has a 20% stake.
His Leeds Group vehicle has already seen its stake in Dawson International disappear this year due to the insolvency of the textiles firm.
ATH shares, which were trading above 40p at the start of the year, finished the day down 0.4p, or 48.5%, at 0.42p.