The bipolar transistor is a current amplifier, so the op amp doesn't have to supply all the valve current.
The voltage divider is just a scale factor that converts the input voltage range into an output current range.
Op amps require capacitors at the power supply terminals for stability.
First of all i wanna thank you all for your replies. But i need to ask one more thing.
Im convinced in using the circuit proposed by jremington, but as i lack of experience in using transistor i have some difficulties.
I think TIP120 should be good for my project. It can bear both 24Vdc and 85mA for my load.
Since arduino due dac can output from 0.55V to 2.75V, to have just 1mA flowing in the voltage divider R1+R2=275ohm. And using a 1ohm resistor as negative feedback, when the valve is fully on I will have on this resistor 85mV. So R2=85ohm and R1=275-85=190ohm. Is it right?
Now what bothers me is the reisistor between opamp output and transistor base. I tried searching in the internet but my doubts remain. Some say i should watch the Vbe_sat, others say that it should be automatically regulated by the opamp and circuit design. Can you give me a hand on this?
Oh boy 
If you can't learn from theory I suggest that you start practicing your ideas. Then the circuit will show how wrong you are, and we can try to help you out.
DrDiettrich:
Oh boyIf you can't learn from theory I suggest that you start practicing your ideas. Then the circuit will show how wrong you are, and we can try to help you out.
Why this answer?
Ive just decided to use the design you proposed and you leave me there? Did i say something very stupid?
Please, im a mechanical engineer and i dont know much about electronics. But im not stupid, if you explain i will try to understand.
Also i dont wanna burn components if you guys explain my mistakes
Your voltage divider calculations are off by a factor of 10, please check.
The base resistor only shall protect the opamp output from a short.
The circuit still is missing a snubber.
Does your opamp work with zero input voltage, or do you power it also by a negative supply voltage?
DrDiettrich:
Your voltage divider calculations are off by a factor of 10, please check.The base resistor only shall protect the opamp output from a short.
The circuit still is missing a snubber.
Does your opamp work with zero input voltage, or do you power it also by a negative supply voltage?
oops my bad. R1+R2=2750, R2=85ohm and R1=2750-R2. If for snubber you mean a flyback diode I will use one (maybe 1N4001) to prevent transient in the inductive load.
So the base resistor is there just to prevent short? I can use whatever value, just a very high one? Then 1k will do the job right?
About the opamp, i think that when i input 0V it outputs 0V and the transistor is closed, so nothing move. I thought i'd be supplying it with the same power supply of the valve, about 30V.
Why should i power it with negative power supply?
See reply #12