just replace the sand. in a 180g tank you do not want to be starting off with sand already laden with phosphates.
you can not cook sand in the same matter as you can LR unless you are able to keep the sand suspended all of the time. the problem is that the bacterial turgor can not get a mechanical advantage against the other grains of sand allowing the bacterial flock from pushing up to the surface to be removed. since the bacterial flock (which is phosphate laden) can not be removed in a timely manner. the phosphates that were removed by the turgor will quickly reabsorb phosphates again from the bacterial flock that the sand is again sitting in since the flock can not be removed quickly enough. in LR the bacterial flock is removed constantly by the flow in the cooking bucket. since the bucket only has LR in it the flock then settles to the bottom of the bucket where it is removed when you change the water.
theoretically, if you have the time, you MIGHT be able to cook sand. it would require you to vacuum the sand out every day minimum. you would end up with a lot less sand from all of the vacuuming. the more the sand blows around in the bucket the better.
sand is cheap. why spend the effort for a $5 bag of 40lb sand? heck you can get a truckload of sand for about $100 and do more tanks than you could ever want.
you can not check for phosphates that way. the sand itself absorbs the phosphates. this is the problem. since the sand is sitting in the very stuff you are trying to remove from the system (the phosphates) the sand will just uptake the phosphates from the water before you can test the water. the readings will always read 0. if you want to test if it is working you would need to take samples of the sand itself, crush it to oblivion then test the resulting powder, make sure to test the water first to make sure you are not already starting with phosphates. in this matter you will have an idea on if it is working.
G~