The Chinese state media said on Wednesday that some of the victims, who were regularly abused, had toiled for more than seven years without any pay. Henan province, where the scandal originated, is already at the heart of a massive slavery scandal in 2007.
"In that case, they are being sheltered by the departments that rescued them," Zhang said.
The official China Daily newspaper, quoting a television channel that exposed the scandal, said the victims were mostly abducted and sold to factory bosses for 300 to 500 yuan (US$47 to US$78).
A police officer quoted by state media said that one factory supervisor accused of whipping the workers was just 14 years old.
This case coming out of China underscores the continuous efforts and difficulty faced by the Chinese government in eliminating slave labor in the world's second-largest economy, where standards of living are generally improving. The weakest in Chinese society, including those with mental disabilities, can be particularly vulnerable due to a lack of services.
According to the Wall Street Journal, the problem is particularly acute in the poor rural hinterlands of Henan and elsewhere in China, with many young people leaving their hometowns in search of work. In 2007, after a major push in which 45,000 police had fanned out across Henan and nearby Shanxi provinces in search of enslaved workers, authorities said they rescued more than 500 people from illegal brick kilns and coal mines.