Can I get a 24-hour free transit VOA at Beijing Capital International Airport?

I am from South Africa and on my way to visit Mongolia at the beginning of July (already got visa approval letter for Mongolia - visa to be stamped on arrival in Ulan Batar). Now, I will be flying from Hong Kong to Beijing on the evening of 5th of July and flying out for Ulan Batar in the morning on the 6th of July. Can I leave the airport and stay in one of the airport hotels close by and return to airport for my onwards flight?