You must have retained a copy, of your offer letter & appointment letter.You have to study the terms of service/termination/resignation.
If you have verbally informed your superiors and HR about the unhealthy conditions of firm and safety issues, and later resigned after feeling these won't improve, you should have mentioned about these and your discussions with superiors in your resignation letter, and that you were told that acceptance shall be sent to you,once the CEO returns. Did your superiors mention how they ensure healthy conditions and safety?
Now you should reply to their communication by giving detail of the facts and incidences that made you feel about the unhealthy conditions of firm and safety issues, your discussions and that being a female you can not work under such conditions.
The company is under obligation to provide a safe and healthy working environment, in the company premises, to all employees male or female, and won't agree that the conditions in their company are unhealthy and there are safety issues.
Company can ask to you to remit notice pay.You getting exit without remitting notice pay shall set a wrong precedence for other employees.
It shall be appropriate to take up the matter with your superiors and HR and convince them and avoid litigation.You may settle the matter amicably and give and obtain communication suitable to both employer and emplyee.HR can issue you the acceptance ,relieving lettre,NOC and later fairly good reference check.If there is any company property issued to you , you may submit it under receipt.