Apparatus for treating materials with plasma, comprising a microwave generator, a waveguide and a tubular torch comprising an upstream section (30.1) having fixing and centering means, a central section (30.2), a downstream section (30.3) and at least one inner tube, an intermediate tube and an outer tube arranged coaxially, each having an upstream end and a downstream end, with said outer tube extending from said upstream section (30.1) at least up to said central section (30.2) of said torch and defines a plasma containment chamber in said central section (30.2) of the torch; wherein: - the longitudinal axis of said torch is aligned with the electric field operatively generated in said waveguide; - said inner tube, said intermediate tube and said outer tube of said torch are individually removably mounted at their upstream ends by means of the fixing and centering means in said upstream section (30.1) of said torch; - said waveguide is connected to said microwave generator and to a hollow body of said central section (30.2) of the torch, said hollow body having a cavity in which there are defined an upstream side and a downstream side, each provided with a circular hole for housing said outer tube of the torch, said plasma containment chamber being defined in the outer tube; - the downstream end of said intermediate tube is aligned with said upstream side of said cavity of said hollow body, or extends for a portion (d) beyond said side upstream side of said cavity; - the downstream end of said inner tube is aligned with the downstream end of said intermediate tube, or is arranged upstream of the downstream end of said intermediate tube; - said downstream section (30.3) of said torch comprises a secondary treatment zone defined in said outer tube downstream of said plasma containment chamber, ending with an exhaust opening, whereby said material to be treated is operatively introduced into said inner tube and said plasma is formed in said plasma containment chamber from a gas introduced into said intermediate tube and subjected to the electromagnetic field generated by said microwaves. - the longitudinal axis of said torch is aligned with the electric field operatively generated in said waveguide; - said inner tube, said intermediate tube and said outer tube of said torch are individually removably mounted at their upstream ends by means of the fixing and centering means in said upstream section (30.1) of said torch; - said waveguide is connected to said microwave generator and to a hollow body of said central section (30.2) of the torch, said hollow body having a cavity in which there are defined an upstream side and a downstream side, each provided with a circular hole for housing said outer tube of the torch, said plasma containment chamber being defined in the outer tube; - the downstream end of said intermediate tube is aligned with said upstream side of said cavity of said hollow body, or extends for a portion (d) beyond said side upstream side of said cavity; - the downstream end of said inner tube is aligned with the downstream end of said intermediate tube, or is arranged upstream of the downstream end of said intermediate tube; - said downstream section (30.3) of said torch comprises a secondary treatment zone defined in said outer tube downstream of said plasma containment chamber, ending with an exhaust opening,