NEW YORK, Sept 21 (Reuters) - Nathan Eldridge, a mergers and acquisitions (M&A) banker specializing in the industrial sector, has left Citigroup Inc after almost three decades to join private equity firm Platinum Equity LLC, people familiar with the matter said.

Eldridge will be a managing director in the Los Angeles headquarters of Platinum Equity and report to partner and global M&A co-head Jacob Kotzubei, the sources said.

Eldridge's areas of expertise include industrial, chemical, building products and services companies, according to his LinkedIn profile.

Platinum Equity, which has approximately $36 billion in assets under management, is in the process of raising a new $12 billion buyout fund, according to one source familiar with the process.

The firm landed some of its biggest deals ever last year, spending $7.2 billion to acquire electronics distributor Ingram Micro Inc from a unit of Chinese aviation and shipping conglomerate HNA Group, and $5.25 billion to snap up specialty chemicals group Solenis from Clayton, Dubilier & Rice and BASF SE. (Reporting by David Carnevali in New York; Editing by Emelia Sithole-Matarise)