CRL - certificate revocation

I've looked through the docs but I can't seem to find a way to revoke a given certificate

This does not appear to work, in the sense that i messes up the crl.pem file and nothing works any more.

def revoke_certificates(certificates):
    # load files
    try:
        with open(settings.CERT_CA_FILE) as ca_file:
            ca = crypto.load_certificate(crypto.FILETYPE_PEM, ca_file.read())
        with open(settings.CERT_CA_KEY_FILE) as ca_key_file:
            ca_key = crypto.load_privatekey(crypto.FILETYPE_PEM, ca_key_file.read())
    except IOError as e:
        log.error(e)
        raise

    with open(settings.CERT_REVOKE_FILE, 'r') as f:
        crl = crypto.load_crl(crypto.FILETYPE_PEM, f.read())
        for cert in certificates:
            x509 = crypto.load_certificate(crypto.FILETYPE_PEM, cert)
            revoked = crypto.Revoked()
            revoked.set_serial(str(x509.get_serial_number()))
            crl.add_revoked(revoked)
        crl_text = crl.export(ca, ca_key)
    with open(settings.CERT_REVOKE_FILE, 'a') as f:
        f.write(crl_text)

Whay am I doing wrong?
Does any one have an example to which you could point me to?