Your comments have been passed onto me to respond. Sorry it has taken me so long to do this.
Although you are factually correct that much of the ash from the Sheffield incinerator is used this is because at the present moment in time there is a market for it. Are you confident that this market will remain for the next 25 years or the lifetime of the incinerator? We are not.
The toxic element of the ash only goes to prove that incinerators do not solve the problem they concentrate it. Would you be comfortable living in an area in the knowledge that lorries passing your house contain this concentrated toxic material? We are not.
In order to take out the toxic materials the filtration systems at incinerators have to be working perfectly all the time. Are you confident that this will be the case over 25 years or the lifetime of the incinerator? We are not.
Veolia run the Sheffield plant. They are a French multi national company who have a poor accident record, a poor safety record and have also shown to manage their plants inadequately when it comes to ensuring pollutants do not exceed EU limits. They are also widely criticized for investing heavily in the occupied West Bank in Israel/Palestine.
In short incineration is a high risk strategy operated by massive companies who care about profits not people and in practice have held Councils over a barrel when it comes to negotiating changes to contracts when circumstances change.