How can i detect metal using Image processing?

It's related to your system (camera) and its output. In another word, you need some samples first. There will be couple of approaches for what you are looking for. Classifying, and Clustering are mostly what you need. For some systems, it can be supper simple, by high pass or low pass filtering! But again, you need samples to discriminates the object you need from its arounds, intensity, colours or textures.

Dear Mehmood

Their are two type of images in my knowledge that helps detecting the metal with image processing.

1. Xray Image

2. Infrared(Thermal) Image.

Explanation: 

Metal are dense in nature as compared to other objects, so you can see darker

shade of metal in Xray and Infrared Image, on the basis of darker gray level of object (metal)

you can extract or detect this part as metal via image segmentation.

You can see the example of image here.