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Johannesburg – Impala Platinum, the world’s second-largest producer of the precious metal, plunged in Johannesburg trading after saying it will refinance about $400 million (R5.2 billion) of convertible bonds, which could dilute existing shareholders.

The existing bonds had an equity conver­sion price more than four times higher than Thursday's closing price of R41.46 a share.

The new bonds will convert to equity at a much lower level of 30 to 35 percent above the average price of the stock during the current offer, Johannesburg-based Impala said on Thursday.

Impala tumbled as much as 18 percent, the most since 2008, before rebounding to trade 9.7 percent lower at R37.53 a share at 10.07am in Johannesburg.

The miner expects “some period of volatility” in the shares while the new debt structure is digested by investors, a spokesperson said.

The bond offer, which will raise money in both dollars and rand, will extend the debt’s maturity from 2018 to 2022, Impala said.

The dollar debt will pay an interest rate of about 3.3 percent while the rand debt will pay about 6.4 percent.

Read also: Implats boss upbeat despite challenges

The aim is to push out the bond’s maturity date, “thereby significantly enhancing Implats’ short-to-medium-term liquidity,” the company said. Like other platinum-group metal miners, Impala is struggling with a metal price that has tumbled by 35 percent in the past four years.