The company, formerly known as Ortac Resources, upped its interest from 14% by acquiring 35% of Terra Metals' shares in Zamsort in exchange for 102.1 million new ordinary shares of no par value.
It has the option to raise its interest in the project even further to 54.34% through a convertible loan note it holds that is convertible to an additional 5.34% interest in Zamsort.
Arc said it had started talks with the Zambian Competition and Consumer Protection Commission to increase its stake in Zamsort to over 50%, although this would be subject to it not triggering any obligations under AIM Rule 14 of the AIM Rules for Companies.
The company plans to start production of copper and cobalt hydroxide cake from Zamsort within six months, as it has 10,000 tonnes of screened ore grading 2.08% copper and 0.29% cobalt stockpiled on site and is about 75% complete with the construction of a commercial-scale demonstration plant.
Arc executive chairman Nick von Schirnding said the increased interest made Arc the largest shareholder in Zamsort and he did not think there'd be a shortage of potential strategic partnerships options.
"We consider Zamsort to be one of the most exciting polymetallic assets in the Zambian Copperbelt. The combination of near-term production from the plant, as well as the exploration potential, is very exciting," he said.
Meanwhile, Arc also raised £2.5 million (US$3.4 million) before expenses through a share placement of 104.2 million new ordinary shares at 2.4p per share to fund an exploration programme at Zamsort and complete the resource upgrade at the Casa projects.
The company owns a 84.7% stake in Casa Mining, which holds a 71.25% interest in the Misisi gold project in the DRC's South Kivu region. This gives Arc a 49.9% stake in the included Akyanga project.
Zamsort has an existing non-JORC code compliant near-surface estimated copper-cobalt oxide resource of 16.59 million tonnes at 0.94% copper and a historical exploration target of 150Mt.
Arc's nomad and broker SP Angel said the raised funds would help it advance exploration and development works at its portfolio of assets.
"Critically, Arc Minerals are now able to drive the project forward with the support of a number of smaller minority shareholders," it said.